Security-Enhanced Linux - User Guide

Red Hat Enterprise Linux 6
Security-Enhanced Linux
User Guide
Security-Enhanced Linux
Draft
Red Hat Enterprise Linux 6 Security-Enhanced Linux
User Guide
Edition 2.0
Author
Copyright © 2010 Red Hat, Inc.
The text of and illustrations in this document are licensed by Red Hat under a Creative Commons
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at http://creativecommons.org/licenses/by-sa/3.0/. In accordance with CC-BY-SA, if you distribute this
document or an adaptation of it, you must provide the URL for the original version.
Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert,
Section 4d of CC-BY-SA to the fullest extent permitted by applicable law.
Red Hat, Red Hat Enterprise Linux, the Shadowman logo, JBoss, MetaMatrix, Fedora, the Infinity
Logo, and RHCE are trademarks of Red Hat, Inc., registered in the United States and other countries.
Linux® is the registered trademark of Linus Torvalds in the United States and other countries.
All other trademarks are the property of their respective owners.
1801 Varsity Drive
Raleigh, NC 27606-2072 USA
Phone: +1 919 754 3700
Phone: 888 733 4281
Fax: +1 919 754 3701
PO Box 13588 Research Triangle Park, NC 27709 USA
The SELinux User Guide assists users and administrators in managing and using Security-Enhanced
Linux®.
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Preface
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1. Document Conventions ................................................................................................... v
1.1. Typographic Conventions ...................................................................................... v
1.2. Pull-quote Conventions ........................................................................................ vii
1.3. Notes and Warnings ............................................................................................ vii
2. We Need Feedback! ..................................................................................................... viii
2.1. Technical Review Requests ................................................................................ viii
1. Trademark Information
1
2. Introduction
2.1. Benefits of running SELinux ..........................................................................................
2.2. Examples .....................................................................................................................
2.3. SELinux Architecture ....................................................................................................
2.4. SELinux on Other Operating Systems ...........................................................................
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3. SELinux Contexts
7
3.1. Domain Transitions ....................................................................................................... 8
3.2. SELinux Contexts for Processes ................................................................................... 9
3.3. SELinux Contexts for Users ........................................................................................ 10
4. Targeted Policy
4.1. Confined Processes ...................................................................................................
4.2. Unconfined Processes ................................................................................................
4.3. Confined and Unconfined Users ..................................................................................
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5. Working with SELinux
5.1. SELinux Packages .....................................................................................................
5.2. Which Log File is Used ..............................................................................................
5.3. Main Configuration File ...............................................................................................
5.4. Enabling and Disabling SELinux ..................................................................................
5.4.1. Enabling SELinux ............................................................................................
5.4.2. Disabling SELinux ............................................................................................
5.5. SELinux Modes ..........................................................................................................
5.6. Booleans ....................................................................................................................
5.6.1. Listing Booleans ..............................................................................................
5.6.2. Configuring Booleans .......................................................................................
5.6.3. Booleans for NFS and CIFS .............................................................................
5.7. SELinux Contexts - Labeling Files ...............................................................................
5.7.1. Temporary Changes: chcon ..............................................................................
5.7.2. Persistent Changes: semanage fcontext ............................................................
5.8. The file_t and default_t Types .....................................................................................
5.9. Mounting File Systems ...............................................................................................
5.9.1. Context Mounts ...............................................................................................
5.9.2. Changing the Default Context ...........................................................................
5.9.3. Mounting an NFS File System ..........................................................................
5.9.4. Multiple NFS Mounts .......................................................................................
5.9.5. Making Context Mounts Persistent ....................................................................
5.10. Maintaining SELinux Labels .....................................................................................
5.10.1. Copying Files and Directories .........................................................................
5.10.2. Moving Files and Directories ..........................................................................
5.10.3. Checking the Default SELinux Context ............................................................
5.10.4. Archiving Files with tar ...................................................................................
5.10.5. Archiving Files with star .................................................................................
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5.11. Other Tools ............................................................................................................... 45
5.12. Information Gathering Tools ...................................................................................... 46
6. Confining Users
6.1. Linux and SELinux User Mappings ..............................................................................
6.2. Confining New Linux Users: useradd ...........................................................................
6.3. Confining Existing Linux Users: semanage login ...........................................................
6.4. Changing the Default Mapping ....................................................................................
6.5. xguest: Kiosk Mode ....................................................................................................
6.6. Booleans for Users Executing Applications ..................................................................
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7. sVirt
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7.1. Security and Virtualization ........................................................................................... 56
7.2. sVirt Labelling ............................................................................................................ 56
8. Troubleshooting
8.1. What Happens when Access is Denied .......................................................................
8.2. Top Three Causes of Problems ...................................................................................
8.2.1. Labeling Problems ...........................................................................................
8.2.2. How are Confined Services Running? ...............................................................
8.2.3. Evolving Rules and Broken Applications ............................................................
8.3. Fixing Problems .........................................................................................................
8.3.1. Linux Permissions ............................................................................................
8.3.2. Possible Causes of Silent Denials ....................................................................
8.3.3. Manual Pages for Services ..............................................................................
8.3.4. Permissive Domains ........................................................................................
8.3.5. Searching For and Viewing Denials ..................................................................
8.3.6. Raw Audit Messages .......................................................................................
8.3.7. sealert Messages ............................................................................................
8.3.8. Allowing Access: audit2allow ............................................................................
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9. Further Information
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9.1. Contributors ............................................................................................................... 75
9.2. Other Resources ........................................................................................................ 75
A. Revision History
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Preface
The Red Hat Enterprise Linux 6 SELinux User Guide is for people with minimal or no experience
with SELinux. Although system administration experience is not necessary, content in this guide is
written for system administration tasks. This guide provides an introduction to fundamental concepts
and practical applications of SELinux. After reading this guide you should have an intermediate
understanding of SELinux.
Thank you to everyone who offered encouragement, help, and testing - it is most appreciated. Very
special thanks to:
• Dominick Grift, Stephen Smalley, and Russell Coker for their contributions, help, and patience.
1
• Karsten Wade for his help, adding a component for this guide to Red Hat Bugzilla , and sorting out
web hosting on http://docs.fedoraproject.org/.
2
• The Fedora Infrastructure Team for providing hosting.
• Jens-Ulrik Petersen for making sure the Red Hat Brisbane office has up-to-date Fedora mirrors.
1. Document Conventions
This manual uses several conventions to highlight certain words and phrases and draw attention to
specific pieces of information.
3
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts set. The
Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not,
alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes
the Liberation Fonts set by default.
1.1. Typographic Conventions
Four typographic conventions are used to call attention to specific words and phrases. These
conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight
keycaps and key combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current
working directory, enter the cat my_next_bestselling_novel command at the
shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a keycap, all presented in mono-spaced bold
and all distinguishable thanks to context.
Key combinations can be distinguished from keycaps by the hyphen connecting each part of a key
combination. For example:
Press Enter to execute the command.
3
https://fedorahosted.org/liberation-fonts/
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Press Ctrl+Alt+F1 to switch to the first virtual terminal. Press Ctrl+Alt+F7 to
return to your X-Windows session.
The first paragraph highlights the particular keycap to press. The second highlights two key
combinations (each a set of three keycaps with each set pressed simultaneously).
If source code is discussed, class names, methods, functions, variable names and returned values
mentioned within a paragraph will be presented as above, in mono-spaced bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for
directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialog box text;
labeled buttons; check-box and radio button labels; menu titles and sub-menu titles. For example:
Choose System → Preferences → Mouse from the main menu bar to launch Mouse
Preferences. In the Buttons tab, click the Left-handed mouse check box and click
Close to switch the primary mouse button from the left to the right (making the mouse
suitable for use in the left hand).
To insert a special character into a gedit file, choose Applications → Accessories
→ Character Map from the main menu bar. Next, choose Search → Find… from the
Character Map menu bar, type the name of the character in the Search field and click
Next. The character you sought will be highlighted in the Character Table. Doubleclick this highlighted character to place it in the Text to copy field and then click the
Copy button. Now switch back to your document and choose Edit → Paste from the
gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific
menu names; and buttons and text found within a GUI interface, all presented in proportional bold and
all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or
variable text. Italics denotes text you do not input literally or displayed text that changes depending on
circumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name at
a shell prompt. If the remote machine is example.com and your username on that
machine is john, type ssh john@example.com.
The mount -o remount file-system command remounts the named file
system. For example, to remount the /home file system, the command is mount -o
remount /home.
To see the version of a currently installed package, use the rpm -q package
command. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version and
release. Each word is a placeholder, either for text you enter when issuing a command or for text
displayed by the system.
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Pull-quote Conventions
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and
important term. For example:
Publican is a DocBook publishing system.
1.2. Pull-quote Conventions
Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and presented thus:
books
books_tests
Desktop
Desktop1
documentation
downloads
drafts
images
mss
notes
photos
scripts
stuff
svgs
svn
Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows:
package org.jboss.book.jca.ex1;
import javax.naming.InitialContext;
public class ExClient
{
public static void main(String args[])
throws Exception
{
InitialContext iniCtx = new InitialContext();
Object
ref
= iniCtx.lookup("EchoBean");
EchoHome
home
= (EchoHome) ref;
Echo
echo
= home.create();
System.out.println("Created Echo");
System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
}
}
1.3. Notes and Warnings
Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.
Note
Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note
should have no negative consequences, but you might miss out on a trick that makes your
life easier.
Important
Important boxes detail things that are easily missed: configuration changes that only
apply to the current session, or services that need restarting before an update will apply.
Ignoring a box labeled 'Important' won't cause data loss but may cause irritation and
frustration.
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Warning
Warnings should not be ignored. Ignoring warnings will most likely cause data loss.
2. We Need Feedback!
If you find a typographical error in this manual, or if you have thought of a way to make this manual
better, we would love to hear from you! Please submit a report in Bugzilla: http://bugzilla.redhat.com/
against the product Red Hat Enterprise Linux.
When submitting a bug report, be sure to mention the manual's identifier: doc-SELinux_User_Guide
and version number: 6.
If you have a suggestion for improving the documentation, try to be as specific as possible when
describing it. If you have found an error, please include the section number and some of the
surrounding text so we can find it easily.
2.1. Technical Review Requests
All review requests are classified into one of the following five categories:
New Content
content documented for the first time — an entirely new feature, procedure, or concept. For
example: "Section now describes the new procedure for creating bootable USB devices."
Correction
a factual error previously present in the text has been corrected. For example: "Section previously
stated (incorrectly) that IPv4 and IPv6 were both supported; section now states that IPv6 has
never been supported."
Clarification
material that was already factually correct but is now better explained. Clarifications are usually in
response to reader feedback that the previous content was confusing or misleading in some way.
For example: "Paths described in Example 1.2.3 now better reflect the directory structure of an
actual installed system."
Obsoletion
a description of a feature or a procedure has been dropped. Material might be obsolete because
of a feature that is no longer supported, a known issue that has been corrected, or hardware that
is now obsolete. For example, "Section no longer describes how to update kernel modules using a
floppy disk."
Verification
a request to check a fact, procedure, or whether material should be obsoleted. For example,
"Section describes how to connect to a generic iSCSI storage device. Please verify this on your
hardware" or "Section still describes how to update kernel modules using a LS-120 SuperDisk;
please verify that we still need to tell readers about this obsolete hardware."
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Trademark Information
Linux® is the registered trademark of Linus Torvalds in the U.S. and other countries.
UNIX is a registered trademark of The Open Group.
Type Enforcement is a trademark of Secure Computing, LLC, a wholly owned subsidiary of McAfee,
Inc., registered in the U.S. and in other countries. Neither McAfee nor Secure Computing, LLC, has
consented to the use or reference to this trademark by the author outside of this guide.
Apache is a trademark of The Apache Software Foundation.
MySQL is a trademark or registered trademark of MySQL AB in the U.S. and other countries.
Other products mentioned may be trademarks of their respective corporations.
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Introduction
Security-Enhanced Linux (SELinux) is an implementation of a mandatory access control mechanism
in the Linux kernel, checking for allowed operations after standard discretionary access controls are
checked. It was created by the National Security Agency and can enforce rules on files and processes
in a Linux system, and on their actions, based on defined policy.
When using SELinux, files, including directories and devices, are referred to as objects. Processes,
such as a user running a command or the Mozilla® Firefox® application, are referred to as subjects.
Most operating systems use a Discretionary Access Control (DAC) system that controls how subjects
interact with objects, and how subjects interact with each other. On operating systems using DAC,
users control the permissions of files (objects) that they own. For example, on Linux® operating
systems, users could make their home directories world-readable, giving users and processes
(subjects) access to potentially sensitive information, with no further protection over this unwanted
action.
Relying on DAC mechanisms alone is fundamentally inadequate for strong system security. DAC
access decisions are only based on user identity and ownership, ignoring other security-relevant
information such as the role of the user, the function and trustworthiness of the program, and the
sensitivity and integrity of the data. Each user typically has complete discretion over their files, making
it difficult to enforce a system-wide security policy. Furthermore, every program run by a user inherits
all of the permissions granted to the user and is free to change access to the user's files, so minimal
protection is provided against malicious software. Many system services and privileged programs run
with coarse-grained privileges that far exceed their requirements, so that a flaw in any one of these
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programs could be exploited to obtain further system access.
The following is an example of permissions used on Linux operating systems that do not run SecurityEnhanced Linux (SELinux). The permissions and output in these examples may differ slightly from
your system. Use the ls -l command to view file permissions:
$ ls -l file1
-rwxrw-r-- 1 user1 group1 0 2009-08-30 11:03 file1
In this example, the first three permission bits, rwx, control the access the Linux user1 user (in this
case, the owner) has to file1. The next three permission bits, rw-, control the access the Linux
group1 group has to file1. The last three permission bits, r--, control the access everyone else
has to file1, which includes all users and processes.
Security-Enhanced Linux (SELinux) adds Mandatory Access Control (MAC) to the Linux kernel, and is
enabled by default in Red Hat Enterprise Linux. A general purpose MAC architecture needs the ability
to enforce an administratively-set security policy over all processes and files in the system, basing
decisions on labels containing a variety of security-relevant information. When properly implemented,
it enables a system to adequately defend itself and offers critical support for application security by
protecting against the tampering with, and bypassing of, secured applications. MAC provides strong
separation of applications that permits the safe execution of untrustworthy applications. Its ability to
limit the privileges associated with executing processes limits the scope of potential damage that
can result from the exploitation of vulnerabilities in applications and system services. MAC enables
"Integrating Flexible Support for Security Policies into the Linux Operating System", by Peter Loscocco and Stephen Smalley.
This paper was originally prepared for the National Security Agency and is, consequently, in the public domain. Refer to the
original paper [http://www.nsa.gov/research/_files/selinux/papers/freenix01/index.shtml] for details and the document as it was
first released. Any edits and changes were done by Murray McAllister.
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information to be protected from legitimate users with limited authorization as well as from authorized
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users who have unwittingly executed malicious applications.
The following is an example of the labels containing security-relevant information that are used on
processes, Linux users, and files, on Linux operating systems that run SELinux. This information is
called the SELinux context, and is viewed using the ls -Z command:
$ ls -Z file1
-rwxrw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0
file1
In this example, SELinux provides a user (unconfined_u), a role (object_r), a type
(user_home_t), and a level (s0). This information is used to make access control decisions. With
DAC, access is controlled based only on Linux user and group IDs. It is important to remember that
SELinux policy rules are checked after DAC rules. SELinux policy rules are not used if DAC rules deny
access first.
Linux and SELinux Users
On Linux operating systems that run SELinux, there are Linux users as well as SELinux users.
SELinux users are part of SELinux policy. Linux users are mapped to SELinux users. To avoid
confusion, this guide uses "Linux user" and "SELinux user" to differentiate between the two.
2.1. Benefits of running SELinux
• All processes and files are labeled with a type. A type defines a domain for processes, and a type
for files. Processes are separated from each other by running in their own domains, and SELinux
policy rules define how processes interact with files, as well as how processes interact with each
other. Access is only allowed if an SELinux policy rule exists that specifically allows it.
• Fine-grained access control. Stepping beyond traditional UNIX® permissions that are controlled at
user discretion and based on Linux user and group IDs, SELinux access decisions are based on all
available information, such as an SELinux user, role, type, and, optionally, a level.
• SELinux policy is administratively-defined, enforced system-wide, and is not set at user discretion.
• Reduced vulnerability to privilege escalation attacks. One example: since processes run in domains,
and are therefore separated from each other, and because SELinux policy rules define how
processes access files and other processes, if a process is compromised, the attacker only has
access to the normal functions of that process, and to files the process has been configured to have
access to. For example, if the Apache HTTP Server is compromised, an attacker can not use that
process to read files in user home directories, unless a specific SELinux policy rule was added or
configured to allow such access.
• SELinux can be used to enforce data confidentiality and integrity, as well as protecting processes
from untrusted inputs.
However, SELinux is not:
"Meeting Critical Security Objectives with Security-Enhanced Linux", by Peter Loscocco and Stephen Smalley. This paper was
originally prepared for the National Security Agency and is, consequently, in the public domain. Refer to the original paper [http://
www.nsa.gov/research/_files/selinux/papers/ottawa01/index.shtml] for details and the document as it was first released. Any
edits and changes were done by Murray McAllister.
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Examples
• antivirus software.
• a replacement for passwords, firewalls, or other security systems.
• an all-in-one security solution.
SELinux is designed to enhance existing security solutions, not replace them. Even when running
SELinux, it is important to continue to follow good security practices, such as keeping software up-todate, using hard-to-guess passwords, firewalls, and so on.
2.2. Examples
The following examples demonstrate how SELinux increases security:
• The default action is deny. If an SELinux policy rule does not exist to allow access, such as for a
process opening a file, access is denied.
• SELinux can confine Linux users. A number of confined SELinux users exist in SELinux policy.
Linux users can be mapped to confined SELinux users to take advantage of the security rules and
mechanisms applied to them. For example, mapping a Linux user to the SELinux user_u user,
results in a Linux user that is not able to run (unless configured otherwise) set user ID (setuid)
applications, such as sudo and su, as well as preventing them from executing files and applications
in their home directory - if configured, this prevents users from executing malicious files from their
home directories.
• Process separation is used. Processes run in their own domains, preventing processes from
accessing files used by other processes, as well as preventing processes from accessing other
processes. For example, when running SELinux, unless otherwise configured, an attacker can not
compromise a Samba server, and then use that Samba server as an attack vector to read and write
to files used by other processes, such as databases used by MySQL®.
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• SELinux helps limit the damage made by configuration mistakes. Domain Name System (DNS)
servers often replicate information between each other in what is known as a zone transfer.
Attackers can use zone transfers to update DNS servers with false information. When running the
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Berkeley Internet Name Domain (BIND) as a DNS server in Red Hat Enterprise Linux, even if an
administrator forgets to limit which servers can perform a zone transfer, the default SELinux policy
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prevents zone files from being updated via zone transfers, by the BIND named daemon itself, and
by other processes.
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• Refer to the Red Hat® Magazine article, Risk report: Three years of Red Hat Enterprise Linux 4 ,
for exploits that were restricted due to the default SELinux targeted policy in Red Hat® Enterprise
Linux® 4.
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• Refer to the LinuxWorld.com article, A seatbelt for server software: SELinux blocks real-world
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exploits , for background information about SELinux, and information about various exploits that
SELinux has prevented.
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• Refer to James Morris's SELinux mitigates remote root vulnerability in OpenPegasus blog post for
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information about an exploit in OpenPegasus that was mitigated by SELinux as shipped with Red
Hat Enterprise Linux 4 and 5.
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http://www.tresys.com/
http://www.tresys.com/innovation.php
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The Tresys Technology website has an SELinux Mitigation News section (on the right-hand side),
that lists recent exploits that have been mitigated or prevented by SELinux.
2.3. SELinux Architecture
SELinux is a Linux security module that is built into the Linux kernel. SELinux is driven by loadable
policy rules. When security-relevant access is taking place, such as when a process attempts to
open a file, the operation is intercepted in the kernel by SELinux. If an SELinux policy rule allows the
operation, it continues, otherwise, the operation is blocked and the process receives an error.
SELinux decisions, such as allowing or disallowing access, are cached. This cache is known as the
Access Vector Cache (AVC). Caching decisions decrease how often SELinux policy rules need to be
checked, which increases performance. Remember that SELinux policy rules have no effect if DAC
rules deny access first.
2.4. SELinux on Other Operating Systems
Refer to the following for information about running SELinux on other Linux distributions:
• Hardened Gentoo: http://www.gentoo.org/proj/en/hardened/selinux/selinux-handbook.xml.
• Debian: http://wiki.debian.org/SELinux.
• Ubuntu: https://wiki.ubuntu.com/SELinux and https://help.ubuntu.com/community/SELinux.
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• Fedora: http://fedoraproject.org/wiki/SELinux and the Fedora SELinux FAQ .
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SELinux Contexts
Processes and files are labeled with an SELinux context that contains additional information, such as
an SELinux user, role, type, and, optionally, a level. When running SELinux, all of this information is
used to make access control decisions. In Red Hat Enterprise Linux, SELinux provides a combination
of Role-Based Access Control (RBAC), Type Enforcement® (TE), and, optionally, Multi-Level Security
(MLS).
The following is an example showing SELinux context. SELinux contexts are used on processes,
Linux users, and files, on Linux operating systems that run SELinux. Use the ls -Z command to view
the SELinux context of files and directories:
$ ls -Z file1
-rwxrw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0
file1
SELinux contexts follow the SELinux user:role:type:level syntax:
SELinux user
The SELinux user identity is an identity known to the policy that is authorized for a specific set of
roles, and for a specific MLS range. Each Linux user is mapped to an SELinux user via SELinux
policy. This allows Linux users to inherit the restrictions placed on SELinux users. The mapped
SELinux user identity is used in the SELinux context for processes in that session, in order to
define what roles and levels they can enter. Run the semanage login -l command as the
Linux root user to view a list of mappings between SELinux and Linux user accounts:
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
unconfined_u
unconfined_u
system_u
s0-s0:c0.c1023
s0-s0:c0.c1023
s0-s0:c0.c1023
Output may differ slightly from system to system. The Login Name column lists Linux users, and
the SELinux User column lists which SELinux user the Linux user is mapped to. For processes,
the SELinux user limits which roles and levels are accessible. The last column, MLS/MCS Range,
is the level used by Multi-Level Security (MLS) and Multi-Category Security (MCS). Levels are
briefly discussed later.
role
Part of SELinux is the Role-Based Access Control (RBAC) security model. The role is an attribute
of RBAC. SELinux users are authorized for roles, and roles are authorized for domains. The role
serves as an intermediary between domains and SELinux users. The roles that can be entered
determine which domains can be entered - ultimately, this controls which object types can be
accessed. This helps reduce vulnerability to privilege escalation attacks.
type
The type is an attribute of Type Enforcement. The type defines a domain for processes, and a type
for files. SELinux policy rules define how types can access each other, whether it be a domain
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accessing a type, or a domain accessing another domain. Access is only allowed if a specific
SELinux policy rule exists that allows it.
level
The level is an attribute of MLS and Multi-Category Security (MCS). An MLS range is a pair of
levels, written as lowlevel-highlevel if the levels differ, or lowlevel if the levels are identical (s0-s0
is the same as s0). Each level is a sensitivity-category pair, with categories being optional. If there
are categories, the level is written as sensitivity:category-set. If there are no categories, it is written
as sensitivity.
If the category set is a contiguous series, it can be abbreviated. For example, c0.c3 is the same
as c0,c1,c2,c3. The /etc/selinux/targeted/setrans.conf file maps levels (s0:c0)
to human-readable form (ie. CompanyConfidential). Do not edit setrans.conf with a text
editor: use semanage to make changes. Refer to the semanage(8) manual page for further
information. In Red Hat Enterprise Linux, targeted policy enforces MCS, and in MCS, there is
just one sensitivity, s0. MCS in Red Hat Enterprise Linux supports 1024 different categories: c0
through to c1023. s0-s0:c0.c1023 is sensitivity s0 and authorized for all categories.
1
MLS enforces the Bell-La Padula Mandatory Access Model , and is used in Labeled Security
Protection Profile (LSPP) environments. To use MLS restrictions, install the selinux-policy-mls
package, and configure MLS to be the default SELinux policy. The MLS policy shipped with
Red Hat Enterprise Linux omits many program domains that were not part of the evaluated
configuration, and therefore, MLS on a desktop workstation is unusable (no support for the X
2
Window System); however, an MLS policy from the upstream SELinux Reference Policy can be
built that includes all program domains.
3.1. Domain Transitions
A process in one domain transitions to another domain by executing an application that has the
entrypoint type for the new domain. The entrypoint permission is used in SELinux policy, and
controls which applications can be used to enter a domain. The following example demonstrates a
domain transition:
1. A user wants to change their password. To do this, they run the passwd application. The /usr/
bin/passwd executable is labeled with the passwd_exec_t type:
$ ls -Z /usr/bin/passwd
-rwsr-xr-x root root system_u:object_r:passwd_exec_t:s0 /usr/bin/passwd
The passwd application accesses /etc/shadow, which is labeled with the shadow_t type:
$ ls -Z /etc/shadow
-r--------. root root system_u:object_r:shadow_t:s0
/etc/shadow
2. An SELinux policy rule states that processes running in the passwd_t domain are allowed to read
and write to files labeled with the shadow_t type. The shadow_t type is only applied to files that
1
2
http://en.wikipedia.org/wiki/Bell-LaPadula_model
http://oss.tresys.com/projects/refpolicy
8
Draft
SELinux Contexts for Processes
are required for a password change. This includes /etc/gshadow, /etc/shadow, and their
backup files.
3. An SELinux policy rule states that the passwd_t domain has entrypoint permission to the
passwd_exec_t type.
4. When a user runs the /usr/bin/passwd application, the user's shell process transitions to the
passwd_t domain. With SELinux, since the default action is to deny, and a rule exists that allows
(among other things) applications running in the passwd_t domain to access files labeled with
the shadow_t type, the passwd application is allowed to access /etc/shadow, and update the
user's password.
This example is not exhaustive, and is used as a basic example to explain domain transition. Although
there is an actual rule that allows subjects running in the passwd_t domain to access objects labeled
with the shadow_t file type, other SELinux policy rules must be met before the subject can transition
to a new domain. In this example, Type Enforcement ensures:
• the passwd_t domain can only be entered by executing an application labeled with the
passwd_exec_t type; can only execute from authorized shared libraries, such as the lib_t type;
and can not execute any other applications.
• only authorized domains, such as passwd_t, can write to files labeled with the shadow_t type.
Even if other processes are running with superuser privileges, those processes can not write to files
labeled with the shadow_t type, as they are not running in the passwd_t domain.
• only authorized domains can transition to the passwd_t domain. For example, the sendmail
process running in the sendmail_t domain does not have a legitimate reason to execute passwd;
therefore, it can never transition to the passwd_t domain.
• processes running in the passwd_t domain can only read and write to authorized types, such as
files labeled with the etc_t or shadow_t types. This prevents the passwd application from being
tricked into reading or writing arbitrary files.
3.2. SELinux Contexts for Processes
Use the ps -eZ command to view the SELinux context for processes. For example:
1. Open a terminal, such as Applications → System Tools → Terminal.
2. Run the /usr/bin/passwd command. Do not enter a new password.
3. Open a new tab, or another terminal, and run the ps -eZ | grep passwd command. The
output is similar to the following:
unconfined_u:unconfined_r:passwd_t:s0-s0:c0.c1023 13212 pts/1 00:00:00 passwd
4. In the first tab/terminal, press Ctrl+C to cancel the passwd application.
In this example, when the /usr/bin/passwd application (labeled with the passwd_exec_t type) is
executed, the user's shell process transitions to the passwd_t domain. Remember: the type defines a
domain for processes, and a type for files.
9
Chapter 3. SELinux Contexts
Draft
Use the ps -eZ command to view the SELinux contexts for running processes. The following is a
truncated example of the output, and may differ on your system:
system_u:system_r:dhcpc_t:s0
system_u:system_r:sshd_t:s0-s0:c0.c1023
system_u:system_r:gpm_t:s0
system_u:system_r:crond_t:s0-s0:c0.c1023
system_u:system_r:kerneloops_t:s0
system_u:system_r:crond_t:s0-s0:c0.c1023
1869
1882
1964
1973
1983
1991
?
?
?
?
?
?
00:00:00
00:00:00
00:00:00
00:00:00
00:00:05
00:00:00
dhclient
sshd
gpm
crond
kerneloops
atd
The system_r role is used for system processes, such as daemons. Type Enforcement then
separates each domain.
3.3. SELinux Contexts for Users
Use the id -Z command to view the SELinux context associated with your Linux user:
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
In Red Hat Enterprise Linux, Linux users run unconfined by default. This SELinux context shows that
the Linux user is mapped to the SELinux unconfined_u user, running as the unconfined_r role,
and is running in the unconfined_t domain. s0-s0 is an MLS range, which in this case, is the same
as just s0. The categories the user has access to is defined by c0.c1023, which is all categories (c0
through to c1023).
10
Chapter 4.
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Draft
Targeted Policy
Targeted policy is the default SELinux policy used in Red Hat Enterprise Linux. When using targeted
policy, processes that are targeted run in a confined domain, and processes that are not targeted
run in an unconfined domain. For example, by default, logged in users run in the unconfined_t
domain, and system processes started by init run in the initrc_t domain - both of these domains
are unconfined.
Unconfined domains (as well as confined domains) are subject to executable and writeable memory
checks. By default, subjects running in an unconfined domain can not allocate writeable memory and
1
execute it. This reduces vulnerability to buffer overflow attacks . These memory checks are disabled
by setting Booleans, which allow the SELinux policy to be modified at runtime. Boolean configuration
is discussed later.
4.1. Confined Processes
Almost every service that listens on a network is confined in Red Hat Enterprise Linux. Also,
most processes that run as the Linux root user and perform tasks for users, such as the passwd
application, are confined. When a process is confined, it runs in its own domain, such as the httpd
process running in the httpd_t domain. If a confined process is compromised by an attacker,
depending on SELinux policy configuration, an attacker's access to resources and the possible
damage they can do is limited.
The following example demonstrates how SELinux prevents the Apache HTTP Server (httpd)
from reading files that are not correctly labeled, such as files intended for use by Samba. This is an
example, and should not be used in production. It assumes that the httpd and wget packages are
installed, the SELinux targeted policy is used, and that SELinux is running in enforcing mode:
1. Run the sestatus command to confirm that SELinux is enabled, is running in enforcing mode,
and that targeted policy is being used:
$ /usr/sbin/sestatus
SELinux status:
SELinuxfs mount:
Current mode:
Mode from config file:
Policy version:
Policy from config file:
enabled
/selinux
enforcing
enforcing
24
targeted
SELinux status: enabled is returned when SELinux is enabled. Current mode:
enforcing is returned when SELinux is running in enforcing mode. Policy from config
file: targeted is returned when the SELinux targeted policy is used.
2. As the Linux root user, run the touch /var/www/html/testfile command to create a file.
3. Run the ls -Z /var/www/html/testfile command to view the SELinux context:
-rw-r--r--
1
root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/testfile
http://en.wikipedia.org/wiki/Buffer_overflow
11
Chapter 4. Targeted Policy
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By default, Linux users run unconfined in Red Hat Enterprise Linux, which is why the testfile
file is labeled with the SELinux unconfined_u user. RBAC is used for processes, not files. Roles
do not have a meaning for files - the object_r role is a generic role used for files (on persistent
storage and network file systems). Under the /proc/ directory, files related to processes may use
2
the system_r role. The httpd_sys_content_t type allows the httpd process to access this
file.
4. As the Linux root user, run the service httpd start command to start the httpd process.
The output is as follows if httpd starts successfully:
# /sbin/service httpd start
Starting httpd:
[
OK
]
5. Change into a directory where your Linux user has write access to, and run the wget http://
localhost/testfile command. Unless there are changes to the default configuration, this
command succeeds:
--2009-11-06 17:43:01-- http://localhost/testfile
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: `testfile'
[ <=>
] 0
--.-K/s
in 0s
2009-11-06 17:43:01 (0.00 B/s) - `testfile' saved [0/0]
6. The chcon command relabels files; however, such label changes do not survive when the file
system is relabeled. For permanent changes that survive a file system relabel, use the semanage
command, which is discussed later. As the Linux root user, run the following command to change
the type to a type used by Samba:
chcon -t samba_share_t /var/www/html/testfile
Run the ls -Z /var/www/html/testfile command to view the changes:
-rw-r--r--
root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/testfile
7. Note: the current DAC permissions allow the httpd process access to testfile. Change into
a directory where your Linux user has write access to, and run the wget http://localhost/
testfile command. Unless there are changes to the default configuration, this command fails:
--2009-11-06 14:11:23-- http://localhost/testfile
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
12
Draft
Unconfined Processes
2009-11-06 14:11:23 ERROR 403: Forbidden.
8. As the Linux root user, run the rm -i /var/www/html/testfile command to remove
testfile.
9. If you do not require httpd to be running, as the Linux root user, run the service httpd stop
command to stop httpd:
# /sbin/service httpd stop
Stopping httpd:
[
OK
]
This example demonstrates the additional security added by SELinux. Although DAC rules allowed
the httpd process access to testfile in step 7, because the file was labeled with a type that the
httpd process does not have access to, SELinux denied access.
An error similar to the following is logged to /var/log/audit/audit.log:
type=AVC msg=audit(1220706212.937:70): avc: denied { getattr } for pid=1904 comm="httpd"
path="/var/www/html/testfile" dev=sda5 ino=247576 scontext=unconfined_u:system_r:httpd_t:s0
tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1220706212.937:70): arch=40000003 syscall=196 success=no exit=-13
a0=b9e21da0 a1=bf9581dc a2=555ff4 a3=2008171 items=0 ppid=1902 pid=1904 auid=500 uid=48
gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=1 comm="httpd" exe="/
usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
Also, an error similar to the following is logged to /var/log/httpd/error_log:
[Wed May 06 23:00:54 2009] [error] [client 127.0.0.1] (13)Permission denied: access to /
testfile denied
4.2. Unconfined Processes
Unconfined processes run in unconfined domains, for example, init programs run in the unconfined
initrc_t domain, unconfined kernel processes run in the kernel_t domain, and unconfined Linux
users run in the unconfined_t domain. For unconfined processes, SELinux policy rules are applied,
but policy rules exist that allow processes running in unconfined domains almost all access. Processes
running in unconfined domains fall back to using DAC rules exclusively. If an unconfined process is
compromised, SELinux does not prevent an attacker from gaining access to system resources and
data, but of course, DAC rules are still used. SELinux is a security enhancement on top of DAC rules it does not replace them.
The following example demonstrates how the Apache HTTP Server (httpd) can access data
intended for use by Samba, when running unconfined. Note: in Red Hat Enterprise Linux, the httpd
process runs in the confined httpd_t domain by default. This is an example, and should not be used
in production. It assumes that the httpd, wget, dbus and audit packages are installed, that the SELinux
targeted policy is used, and that SELinux is running in enforcing mode:
13
Chapter 4. Targeted Policy
Draft
1. Run the sestatus command to confirm that SELinux is enabled, is running in enforcing mode,
and that targeted policy is being used:
$ /usr/sbin/sestatus
SELinux status:
SELinuxfs mount:
Current mode:
Mode from config file:
Policy version:
Policy from config file:
enabled
/selinux
enforcing
enforcing
24
targeted
SELinux status: enabled is returned when SELinux is enabled. Current mode:
enforcing is returned when SELinux is running in enforcing mode. Policy from config
file: targeted is returned when the SELinux targeted policy is used.
2. As the Linux root user, run the touch /var/www/html/test2file command to create a file.
3. Run the ls -Z /var/www/html/test2file command to view the SELinux context:
-rw-r--r--
root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/test2file
By default, Linux users run unconfined in Red Hat Enterprise Linux, which is why the test2file
file is labeled with the SELinux unconfined_u user. RBAC is used for processes, not files. Roles
do not have a meaning for files - the object_r role is a generic role used for files (on persistent
storage and network file systems). Under the /proc/ directory, files related to processes may use
3
the system_r role. The httpd_sys_content_t type allows the httpd process to access this
file.
4. The chcon command relabels files; however, such label changes do not survive when the file
system is relabeled. For permanent changes that survive a file system relabel, use the semanage
command, which is discussed later. As the Linux root user, run the following command to change
the type to a type used by Samba:
chcon -t samba_share_t /var/www/html/test2file
Run the ls -Z /var/www/html/test2file command to view the changes:
-rw-r--r--
root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/test2file
5. Run the service httpd status command to confirm that the httpd process is not running:
$ /sbin/service httpd status
httpd is stopped
If the output differs, run the service httpd stop command as the Linux root user to stop the
httpd process:
14
Draft
Unconfined Processes
# /sbin/service httpd stop
Stopping httpd:
[
OK
]
6. To make the httpd process run unconfined, run the following command as the Linux root user to
change the type of /usr/sbin/httpd, to a type that does not transition to a confined domain:
chcon -t unconfined_exec_t /usr/sbin/httpd
7. Run the ls -Z /usr/sbin/httpd command to confirm that /usr/sbin/httpd is labeled with
the unconfined_exec_t type:
-rwxr-xr-x
root root system_u:object_r:unconfined_exec_t /usr/sbin/httpd
8. As the Linux root user, run the service httpd start command to start the httpd process.
The output is as follows if httpd starts successfully:
# /sbin/service httpd start
Starting httpd:
[
OK
]
9. Run the ps -eZ | grep httpd command to view the httpd running in the unconfined_t
domain:
$ ps -eZ | grep httpd
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
unconfined_u:system_r:unconfined_t
7721
7723
7724
7725
7726
7727
7728
7729
7730
?
?
?
?
?
?
?
?
?
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
httpd
httpd
httpd
httpd
httpd
httpd
httpd
httpd
httpd
10. Change into a directory where your Linux user has write access to, and run the wget http://
localhost/test2file command. Unless there are changes to the default configuration, this
command succeeds:
--2009-05-07 01:41:10-- http://localhost/test2file
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 0 [text/plain]
Saving to: `test2file.1'
[ <=>
]--.-K/s
in 0s
2009-05-07 01:41:10 (0.00 B/s) - `test2file.1' saved [0/0]
15
Chapter 4. Targeted Policy
Draft
Although the httpd process does not have access to files labeled with the samba_share_t type,
httpd is running in the unconfined unconfined_t domain, and falls back to using DAC rules,
and as such, the wget command succeeds. Had httpd been running in the confined httpd_t
domain, the wget command would have failed.
11. The restorecon command restores the default SELinux context for files. As the Linux root user,
run the restorecon -v /usr/sbin/httpd command to restore the default SELinux context
for /usr/sbin/httpd:
# /sbin/restorecon -v /usr/sbin/httpd
restorecon reset /usr/sbin/httpd context system_u:object_r:unconfined_notrans_exec_t:s0>system_u:object_r:httpd_exec_t:s0
Run the ls -Z /usr/sbin/httpd command to confirm that /usr/sbin/httpd is labeled with
the httpd_exec_t type:
$ ls -Z /usr/sbin/httpd
-rwxr-xr-x root root system_u:object_r:httpd_exec_t
/usr/sbin/httpd
12. As the Linux root user, run the /sbin/service httpd restart command to restart httpd.
After restarting, run the ps -eZ | grep httpd to confirm that httpd is running in the confined
httpd_t domain:
# /sbin/service httpd restart
Stopping httpd:
Starting httpd:
# ps -eZ | grep httpd
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
unconfined_u:system_r:httpd_t
[
[
8880
8882
8883
8884
8885
8886
8887
8888
8889
?
?
?
?
?
?
?
?
?
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
OK
OK
]
]
httpd
httpd
httpd
httpd
httpd
httpd
httpd
httpd
httpd
13. As the Linux root user, run the rm -i /var/www/html/test2file command to remove
test2file.
14. If you do not require httpd to be running, as the Linux root user, run the service httpd stop
command to stop httpd:
# /sbin/service httpd stop
Stopping httpd:
16
[
OK
]
Draft
Confined and Unconfined Users
The examples in these sections demonstrate how data can be protected from a compromised
confined-process (protected by SELinux), as well as how data is more accessible to an attacker from a
compromised unconfined-process (not protected by SELinux).
4.3. Confined and Unconfined Users
Each Linux user is mapped to an SELinux user via SELinux policy. This allows Linux users to inherit
the restrictions on SELinux users. This Linux user mapping is seen by running the semanage login
-l command as the Linux root user:
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
unconfined_u
unconfined_u
system_u
s0-s0:c0.c1023
s0-s0:c0.c1023
s0-s0:c0.c1023
In Red Hat Enterprise Linux 6, Linux users are mapped to the SELinux __default__ login by default
(which is mapped to the SELinux unconfined_u user). The following defines the default-mapping:
__default__
unconfined_u
s0-s0:c0.c1023
The following example demonstrates adding a new Linux user, and that Linux user being mapped to
the SELinux unconfined_u user. It assumes that the Linux root user is running unconfined, as it
does by default in Red Hat Enterprise Linux 6:
1. As the Linux root user, run the /usr/sbin/useradd newuser command to create a new Linux
user named newuser.
2. As the Linux root user, run the passwd newuser command to assign a password to the Linux
newuser user:
# passwd newuser
Changing password for user newuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
3. Log out of your current session, and log in as the Linux newuser user. When you log in,
pam_selinux maps the Linux user to an SELinux user (in this case, unconfined_u), and sets up the
resulting SELinux context. The Linux user's shell is then launched with this context. Run the id Z command to view the context of a Linux user:
[newuser@localhost ~]$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
17
Chapter 4. Targeted Policy
Draft
4. Log out of the Linux newuser's session, and log in with your account. If you do not want the Linux
newuser user, run the /usr/sbin/userdel -r newuser command as the Linux root user to
remove it, along with the Linux newuser's home directory.
Confined and unconfined Linux users are subject to executable and writeable memory checks, and
are also restricted by MCS (and MLS, if the MLS policy is used). If unconfined Linux users execute
an application that SELinux policy defines can transition from the unconfined_t domain to its own
confined domain, unconfined Linux users are still subject to the restrictions of that confined domain.
The security benefit of this is that, even though a Linux user is running unconfined, the application
remains confined, and therefore, the exploitation of a flaw in the application can be limited by policy.
Note: this does not protect the system from the user. Instead, the user and the system are being
protected from possible damage caused by a flaw in the application.
The following confined SELinux users are available in Red Hat Enterprise Linux 6:
User
Domain
X Window
System
su and sudo
Execute
Networking
in home
directory and /
tmp/
guest_u
guest_t
no
no
optional
no
xguest_u
xguest_t
yes
no
optional
only Firefox
user_u
user_t
yes
no
optional
yes
staff_u
staff_t
yes
only sudo
optional
yes
Table 4.1. SELinux User Capabilities
• Linux users in the guest_t, xguest_t, and user_t domains can only run set user ID (setuid)
applications if SELinux policy permits it (such as passwd). They can not run the su and /usr/bin/
sudo setuid applications, and therefore, can not use these applications to become the Linux root
user.
• Linux users in the guest_t domain have no network access, and can only log in via a terminal
(including ssh; they can log in via ssh, but can not use ssh to connect to another system).
• The only network access Linux users in the xguest_t domain have is Firefox connecting to web
pages.
• Linux users in the xguest_t, user_t and staff_t domains can log in via the X Window System
and a terminal.
• By default, Linux users in the staff_t domain do not have permissions to execute applications
with /usr/bin/sudo. These permissions must be configured by an administrator.
By default, Linux users in the guest_t and xguest_t domains can not execute applications
in their home directories or /tmp/, preventing them from executing applications (which inherit
users' permissions) in directories they have write access to. This helps prevent flawed or malicious
applications from modifying files users' own.
By default, Linux users in the user_t and staff_t domains can execute applications in their
home directories and /tmp/. Refer to Section 6.6, “Booleans for Users Executing Applications” for
information about allowing and preventing users from executing applications in their home directories
and /tmp/.
18
Draft
Chapter 5.
Draft
Working with SELinux
The following sections give a brief overview of the main SELinux packages in Red Hat Enterprise
Linux; installing and updating packages; which log files are used; the main SELinux configuration file;
enabling and disabling SELinux; SELinux modes; configuring Booleans; temporarily and persistently
changing file and directory labels; overriding file system labels with the mount command; mounting
NFS file systems; and how to preserve SELinux contexts when copying and archiving files and
directories.
5.1. SELinux Packages
In Red Hat Enterprise Linux, the SELinux packages are installed by default, in a full installation, unless
they are manually excluded during installation. If performing a minimal installation in text mode, the
policycoreutils-python package will not be installed by default. Also, by default, SELinux targeted
policy is used, and SELinux runs in enforcing mode. The following is a brief description of the main
SELinux packages:
policycoreutils-python: provides utilities such as semanage, audit2allow, audit2why and chcat,
for operating and managing SELinux.
policycoreutils: provides utilities such as restorecon, secon, setfiles, semodule,
load_policy, and setsebool, for operating and managing SELinux.
policycoreutils-gui: provides system-config-selinux, a graphical tool for managing SELinux.
selinux-policy: provides the SELinux Reference Policy. The SELinux Reference Policy is a complete
SELinux policy, and is used as a basis for other policies, such as the SELinux targeted policy. Refer
1
to the Tresys Technology SELinux Reference Policy page for further information. The selinuxpolicy-devel package provides development tools, such as /usr/share/selinux/devel/
policygentool and /usr/share/selinux/devel/policyhelp, as well as example policy files.
selinux-policy-policy: provides SELinux policies. For targeted policy, install selinux-policy-targeted.
For MLS, install selinux-policy-mls.
setroubleshoot-server: translates denial messages, produced when access is denied by SELinux, into
detailed descriptions that are viewed with sealert (which is provided by this package).
2
setools-console: this package provides the Tresys Technology SETools distribution , a number of tools
and libraries for analyzing and querying policy, audit log monitoring and reporting, and file context
3
management . The setools package is a meta-package for SETools. The setools-gui package provides
the apol, seaudit, and sediffx tools. The setools-console package provides the seauditreport, sechecker, sediff, seinfo, sesearch, findcon, replcon, and indexcon command
4
line tools. Refer to the Tresys Technology SETools page for information about these tools.
libselinux-utils: provides the avcstat, getenforce, getsebool, matchpathcon,
selinuxconlist, selinuxdefcon, selinuxenabled, setenforce, togglesebool tools.
mcstrans: translates levels, such as s0-s0:c0.c1023, to an easier to read form, such as
SystemLow-SystemHigh. This package is not installed by default.
1
http://oss.tresys.com/projects/refpolicy
http://oss.tresys.com/projects/setools
Brindle, Joshua. "Re: blurb for fedora setools packages" Email to Murray McAllister. 1 November 2008. Any edits or changes in
this version were done by Murray McAllister.
4
http://oss.tresys.com/projects/setools
2
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To install packages in Red Hat Enterprise Linux, as the Linux root user, run the yum install
package-name command. For example, to install the mcstrans package, run the yum install
mcstrans command. To upgrade all installed packages in Red Hat Enterprise Linux, run the yum
update command.
5.2. Which Log File is Used
In Red Hat Enterprise Linux 6, the dbus, setroubleshoot-server and audit packages are installed if
packages are not removed from the default package selection.
SELinux denial messages, such as the following, are written to /var/log/audit/audit.log by
default:
type=AVC msg=audit(1223024155.684:49): avc: denied { getattr } for pid=2000 comm="httpd"
path="/var/www/html/file1" dev=dm-0 ino=399185 scontext=unconfined_u:system_r:httpd_t:s0
tcontext=system_u:object_r:samba_share_t:s0 tclass=file
May 7 18:55:56 localhost setroubleshoot: SELinux is preventing httpd (httpd_t) "getattr"
to /var/www/html/file1 (samba_share_t). For complete SELinux messages. run sealert -l
de7e30d6-5488-466d-a606-92c9f40d316d
In Red Hat Enterprise Linux 6, setroubleshootd no longer constantly runs as a service, however it
is still used to analyze the AVC messages. Two new programs act as a method to start setroubleshoot
when needed: sedispatch and seapplet. sedispatch runs as part of the audit subsystem, and
via dbus, sends a message when an AVC denial occurs, which will go straight to setroubleshootd
if it is already running, or it will start setroubleshootd if it is not running. seapplet is a tool which
runs in the system's toolbar, waiting for dbus messages in setroubleshootd, and will launch the
notification bubble, allowing the user to review the denial.
Starting Daemons Automatically
To configure the auditd and rsyslogd daemons to automatically start at boot, run the following
commands as the Linux root user:
/sbin/chkconfig --levels 2345 auditd on
/sbin/chkconfig --levels 2345 rsyslog on
Use the service service-name status command to check if these services are running, for
example:
$ /sbin/service auditd status
auditd (pid 1318) is running...
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Main Configuration File
If the above services are not running (service-name is stopped), use the service servicename start command as the Linux root user to start them. For example:
# /sbin/service auditd start
Starting auditd:
[
OK
]
5.3. Main Configuration File
The /etc/selinux/config file is the main SELinux configuration file. It controls the SELinux mode
and the SELinux policy to use:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#
enforcing - SELinux security policy is enforced.
#
permissive - SELinux prints warnings instead of enforcing.
#
disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
#
targeted - Targeted processes are protected,
#
mls - Multi Level Security protection.
SELINUXTYPE=targeted
SELINUX=enforcing
The SELINUX option sets the mode SELinux runs in. SELinux has three modes: enforcing,
permissive, and disabled. When using enforcing mode, SELinux policy is enforced, and SELinux
denies access based on SELinux policy rules. Denial messages are logged. When using
permissive mode, SELinux policy is not enforced. SELinux does not deny access, but denials are
logged for actions that would have been denied if running SELinux in enforcing mode. When using
disabled mode, SELinux is disabled (the SELinux module is not registered with the Linux kernel),
and only DAC rules are used.
SELINUXTYPE=targeted
The SELINUXTYPE option sets the SELinux policy to use. Targeted policy is the default policy.
Only change this option if you want to use the MLS policy. To use the MLS policy, install the
selinux-policy-mls package; configure SELINUXTYPE=mls in /etc/selinux/config; and
reboot your system.
Important
When systems run with SELinux in permissive or disabled mode, users have permission
to label files incorrectly. Also, files created while SELinux is disabled are not labeled. This
causes problems when changing to enforcing mode. To prevent incorrectly labeled and
unlabeled files from causing problems, file systems are automatically relabeled when
changing from disabled mode to permissive or enforcing mode.
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5.4. Enabling and Disabling SELinux
Use the /usr/sbin/getenforce or /usr/sbin/sestatus commands to check the status
of SELinux. The getenforce command returns Enforcing, Permissive, or Disabled. The
getenforce command returns Enforcing when SELinux is enabled (SELinux policy rules are
enforced):
$ /usr/sbin/getenforce
Enforcing
The getenforce command returns Permissive when SELinux is enabled, but SELinux policy rules
are not enforced, and only DAC rules are used. The getenforce command returns Disabled if
SELinux is disabled.
The sestatus command returns the SELinux status and the SELinux policy being used:
$ /usr/sbin/sestatus
SELinux status:
SELinuxfs mount:
Current mode:
Mode from config file:
Policy version:
Policy from config file:
enabled
/selinux
enforcing
enforcing
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targeted
SELinux status: enabled is returned when SELinux is enabled. Current mode: enforcing
is returned when SELinux is running in enforcing mode. Policy from config file: targeted
is returned when the SELinux targeted policy is used.
5.4.1. Enabling SELinux
On systems with SELinux disabled, the SELINUX=disabled option is configured in /etc/selinux/
config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#
enforcing - SELinux security policy is enforced.
#
permissive - SELinux prints warnings instead of enforcing.
#
disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of these two values:
#
targeted - Targeted processes are protected,
#
mls - Multi Level Security protection.
SELINUXTYPE=targeted
Also, the getenforce command returns Disabled:
$ /usr/sbin/getenforce
Disabled
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Enabling SELinux
To enable SELinux:
1. Use the rpm -qa | grep selinux, rpm -q policycoreutils, and rpm -qa | grep
setroubleshoot commands to confirm that the SELinux packages are installed. This guide
assumes the following packages are installed: selinux-policy-targeted, selinux-policy, libselinux,
libselinux-python, libselinux-utils, policycoreutils, policycoreutils-python, setroubleshoot,
setroubleshoot-server, setroubleshoot-plugins. If these packages are not installed, as the Linux
root user, install them via the yum install package-name command. The following packages
are optional: policycoreutils-gui, setroubleshoot, selinux-policy-devel, and mcstrans.
After installing the setroubleshoot-server package, use the /sbin/chkconfig --list
setroubleshoot command to confirm that setroubleshootd starts when the system is
5
running in runlevel 3, 4, and 5:
$ /sbin/chkconfig --list setroubleshoot
setroubleshoot 0:off
1:off
2:off
3:on
4:on
5:on
6:off
If the output differs, as the Linux root user, run the /sbin/chkconfig --levels 345
setroubleshoot on command. This makes setroubleshootd automatically start when the
system is in runlevel 3, 4, and 5.
2. Before SELinux is enabled, each file on the file system must be labeled with an SELinux context.
Before this happens, confined domains may be denied access, preventing your system from
booting correctly. To prevent this, configure SELINUX=permissive in /etc/selinux/config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#
enforcing - SELinux security policy is enforced.
#
permissive - SELinux prints warnings instead of enforcing.
#
disabled - No SELinux policy is loaded.
SELINUX=permissive
# SELINUXTYPE= can take one of these two values:
#
targeted - Targeted processes are protected,
#
mls - Multi Level Security protection.
SELINUXTYPE=targeted
3. As the Linux root user, run the reboot command to restart the system. During the next boot, file
systems are labeled. The label process labels all files with an SELinux context:
*** Warning -- SELinux targeted policy relabel is required.
*** Relabeling could take a very long time, depending on file
*** system size and speed of hard drives.
****
Each * character on the bottom line represents 1000 files that have been labeled. In the above
example, four * characters represent 4000 files have been labeled. The time it takes to label all
files depends upon the number of files on the system, and the speed of the hard disk drives. On
modern systems, this process can take as little as 10 minutes.
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4. In permissive mode, SELinux policy is not enforced, but denials are still logged for actions that
would have been denied if running in enforcing mode. Before changing to enforcing mode, as the
Linux root user, run the grep "SELinux is preventing" /var/log/messages command
as the Linux root user to confirm that SELinux did not deny actions during the last boot. If SELinux
did not deny actions during the last boot, this command does not return any output. Refer to
Chapter 8, Troubleshooting for troubleshooting information if SELinux denied access during boot.
5. If there were no denial messages in /var/log/messages, configure SELINUX=enforcing in /
etc/selinux/config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#
enforcing - SELinux security policy is enforced.
#
permissive - SELinux prints warnings instead of enforcing.
#
disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of these two values:
#
targeted - Targeted processes are protected,
#
mls - Multi Level Security protection.
SELINUXTYPE=targeted
6. Reboot your system. After reboot, confirm that the getenforce command returns Enforcing:
$ /usr/sbin/getenforce
Enforcing
7. As the Linux root user, run the /usr/sbin/semanage login -l command to view the
mapping between SELinux and Linux users. The output should be as follows:
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
unconfined_u
unconfined_u
system_u
s0-s0:c0.c1023
s0-s0:c0.c1023
s0-s0:c0.c1023
If this is not the case, run the following commands as the Linux root user to fix the user mappings. It is
safe to ignore the SELinux-user username is already defined warnings if they occur, where
username can be unconfined_u, guest_u, or xguest_u:
1. /usr/sbin/semanage user -a -S targeted -P user -R "unconfined_r
system_r" -r s0-s0:c0.c1023 unconfined_u
2. /usr/sbin/semanage login -m -S targeted -s "unconfined_u" -r s0s0:c0.c1023 __default__
3. /usr/sbin/semanage login -m -S targeted -s "unconfined_u" -r s0s0:c0.c1023 root
4. /usr/sbin/semanage user -a -S targeted -P user -R guest_r guest_u
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Disabling SELinux
5. /usr/sbin/semanage user -a -S targeted -P user -R xguest_r xguest_u
Important
When systems run with SELinux in permissive or disabled mode, users have permission
to label files incorrectly. Also, files created while SELinux is disabled are not labeled. This
causes problems when changing to enforcing mode. To prevent incorrectly labeled and
unlabeled files from causing problems, file systems are automatically relabeled when
changing from disabled mode to permissive or enforcing mode.
5.4.2. Disabling SELinux
To disable SELinux, configure SELINUX=disabled in /etc/selinux/config:
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
#
enforcing - SELinux security policy is enforced.
#
permissive - SELinux prints warnings instead of enforcing.
#
disabled - No SELinux policy is loaded.
SELINUX=disabled
# SELINUXTYPE= can take one of these two values:
#
targeted - Targeted processes are protected,
#
mls - Multi Level Security protection.
SELINUXTYPE=targeted
Reboot your system. After reboot, confirm that the getenforce command returns Disabled:
$ /usr/sbin/getenforce
Disabled
5.5. SELinux Modes
SELinux has three modes:
• Enforcing: SELinux policy is enforced. SELinux denies access based on SELinux policy rules.
• Permissive: SELinux policy is not enforced. SELinux does not deny access, but denials are logged
for actions that would have been denied if running in enforcing mode.
• Disabled: SELinux is disabled. Only DAC rules are used.
Use the /usr/sbin/setenforce command to change between enforcing and permissive
mode. Changes made with /usr/sbin/setenforce do not persist across reboots. To change
to enforcing mode, as the Linux root user, run the /usr/sbin/setenforce 1 command. To
change to permissive mode, run the /usr/sbin/setenforce 0 command. Use the /usr/sbin/
getenforce command to view the current SELinux mode.
Persistent mode changes are covered in Section 5.4, “Enabling and Disabling SELinux”.
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5.6. Booleans
Booleans allow parts of SELinux policy to be changed at runtime, without any knowledge of SELinux
policy writing. This allows changes, such as allowing services access to NFS file systems, without
reloading or recompiling SELinux policy.
5.6.1. Listing Booleans
For a list of Booleans, an explanation of what each one is, and whether they are on or off, run the
semanage boolean -l command as the Linux root user. The following example does not list all
Booleans:
# /usr/sbin/semanage boolean -l
SELinux boolean
ftp_home_dir
directories
xen_use_nfs
xguest_connect_network
Description
-> off
Allow ftp to read and write files in the user home
-> off
-> on
Allow xen to manage nfs files
Allow xguest to configure Network Manager
The SELinux boolean column lists Boolean names. The Description column lists whether the
Booleans are on or off, and what they do.
In the following example, the ftp_home_dir Boolean is off, preventing the FTP daemon (vsftpd)
from reading and writing to files in user home directories:
ftp_home_dir
directories
-> off
Allow ftp to read and write files in the user home
The getsebool -a command lists Booleans, whether they are on or off, but does not give a
description of each one. The following example does not list all Booleans:
$ /usr/sbin/getsebool -a
allow_console_login --> off
allow_cvs_read_shadow --> off
allow_daemons_dump_core --> on
Run the getsebool boolean-name command to only list the status of the boolean-name
Boolean:
$ /usr/sbin/getsebool allow_console_login
allow_console_login --> off
Use a space-separated list to list multiple Booleans:
$ getsebool allow_console_login allow_cvs_read_shadow allow_daemons_dump_core
allow_console_login --> off
allow_cvs_read_shadow --> off
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Configuring Booleans
allow_daemons_dump_core --> on
5.6.2. Configuring Booleans
The setsebool boolean-name x command turns Booleans on or off, where boolean-name is a
Boolean name, and x is either on to turn the Boolean on, or off to turn it off.
The following example demonstrates configuring the httpd_can_network_connect_db Boolean:
1. By default, the httpd_can_network_connect_db Boolean is off, preventing Apache HTTP
Server scripts and modules from connecting to database servers:
$ /usr/sbin/getsebool httpd_can_network_connect_db
httpd_can_network_connect_db --> off
2. To temporarily enable Apache HTTP Server scripts and modules to connect to database servers,
run the setsebool httpd_can_network_connect_db on command as the Linux root user.
3. Use the getsebool httpd_can_network_connect_db command to verify the Boolean is
turned on:
$ /usr/sbin/getsebool httpd_can_network_connect_db
httpd_can_network_connect_db --> on
This allows Apache HTTP Server scripts and modules to connect to database servers.
4. This change is not persistent across reboots. To make changes persistent across reboots, run the
setsebool -P boolean-name on command as the Linux root user:
# /usr/sbin/setsebool -P httpd_can_network_connect_db on
5. To temporarily revert to the default behavior, as the Linux root user, run the setsebool
httpd_can_network_connect_db off command. For changes that persist across reboots,
run the setsebool -P httpd_can_network_connect_db off command.
5.6.3. Booleans for NFS and CIFS
By default, NFS mounts on the client side are labeled with a default context defined by policy for NFS
file systems. In common policies, this default context uses the nfs_t type. Also, by default, Samba
shares mounted on the client side are labeled with a default context defined by policy. In common
policies, this default context uses the cifs_t type.
Depending on policy configuration, services may not be able to read files labeled with the nfs_t or
cifs_t types. This may prevent file systems labeled with these types from being mounted and then
read or exported by other services. Booleans can be turned on or off to control which services are
allowed to access the nfs_t and cifs_t types.
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The setsebool and semanage commands must be run as the Linux root user. The setsebool -P
command makes persistent changes. Do not use the -P option if you do not want changes to persist
across reboots:
Apache HTTP Server
To allow access to NFS file systems (files labeled with the nfs_t type):
/usr/sbin/setsebool -P httpd_use_nfs on
To allow access to Samba file systems (files labeled with the cifs_t type):
/usr/sbin/setsebool -P httpd_use_cifs on
Samba
To export NFS file systems:
/usr/sbin/setsebool -P samba_share_nfs on
FTP (vsftpd)
To allow access to NFS file systems:
/usr/sbin/setsebool -P allow_ftpd_use_nfs on
To allow access to Samba file systems:
/usr/sbin/setsebool -P allow_ftpd_use_cifs on
Other Services
For a list of NFS related Booleans for other services:
/usr/sbin/semanage boolean -l | grep nfs
For a list of Samba related Booleans for other services:
/usr/sbin/semanage boolean -l | grep cifs
Note
These Booleans exist in SELinux policy as shipped with Red Hat Enterprise Linux 6. They
may not exist in policy shipped with other versions of Red Hat Enterprise Linux or other
operating systems.
Refer to the SELinux Managing Confined Services Guide:http://docs.fedoraproject.org/selinuxmanaging-confined-services-guide for more information relating to SELinux Booleans.
5.7. SELinux Contexts - Labeling Files
On systems running SELinux, all processes and files are labeled in a way that represents securityrelevant information. This information is called the SELinux context. For files, this is viewed using the
ls -Z command:
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Temporary Changes: chcon
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
In this example, SELinux provides a user (unconfined_u), a role (object_r), a type
(user_home_t), and a level (s0). This information is used to make access control decisions. On DAC
systems, access is controlled based on Linux user and group IDs. SELinux policy rules are checked
after DAC rules. SELinux policy rules are not used if DAC rules deny access first.
There are multiple commands for managing the SELinux context for files, such as chcon, semanage
fcontext, and restorecon.
5.7.1. Temporary Changes: chcon
The chcon command changes the SELinux context for files. However, changes made with the chcon
command do not survive a file system relabel, or the execution of the /sbin/restorecon command.
SELinux policy controls whether users are able to modify the SELinux context for any given file. When
using chcon, users provide all or part of the SELinux context to change. An incorrect file type is a
common cause of SELinux denying access.
Quick Reference
• Run the chcon -t type file-name command to change the file type, where type is a type,
such as httpd_sys_content_t, and file-name is a file or directory name.
• Run the chcon -R -t type directory-name command to change the type of the directory
and its contents, where type is a type, such as httpd_sys_content_t, and directory-name
is a directory name.
Changing a File's or Directory's Type
The following example demonstrates changing the type, and no other attributes of the SELinux
context:
1. Run the cd command without arguments to change into your home directory.
2. Run the touch file1 command to create a new file. Use the ls -Z file1 command to view
the SELinux context for file1:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
In this example, the SELinux context for file1 includes the SELinux unconfined_u user,
object_r role, user_home_t type, and the s0 level. For a description of each part of the
SELinux context, refer to Chapter 3, SELinux Contexts.
3. Run the chcon -t samba_share_t file1 command to change the type to samba_share_t.
The -t option only changes the type. View the change with ls -Z file1:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:samba_share_t:s0 file1
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4. Use the /sbin/restorecon -v file1 command to restore the SELinux context for the file1
file. Use the -v option to view what changes:
$ /sbin/restorecon -v file1
restorecon reset file1 context unconfined_u:object_r:samba_share_t:s0>system_u:object_r:user_home_t:s0
In this example, the previous type, samba_share_t, is restored to the correct, user_home_t
type. When using targeted policy (the default SELinux policy in Red Hat Enterprise Linux 6), the
/sbin/restorecon command reads the files in the /etc/selinux/targeted/contexts/
files/ directory, to see which SELinux context files should have.
The example in this section works the same for directories, for example, if file1 was a directory.
Changing a Directory and its Contents Types
The following example demonstrates creating a new directory, and changing the directory's file type
(along with its contents) to a type used by the Apache HTTP Server. The configuration in this example
is used if you want Apache HTTP Server to use a different document root (instead of /var/www/
html/):
1. As the Linux root user, run the mkdir /web command to create a new directory, and then the
touch /web/file{1,2,3} command to create 3 empty files (file1, file2, and file3). The
/web/ directory and files in it are labeled with the default_t type:
# ls -dZ /web
drwxr-xr-x root
# ls -lZ /web
-rw-r--r-- root
-rw-r--r-- root
-rw-r--r-- root
root unconfined_u:object_r:default_t:s0 /web
root unconfined_u:object_r:default_t:s0 file1
root unconfined_u:object_r:default_t:s0 file2
root unconfined_u:object_r:default_t:s0 file3
2. As the Linux root user, run the chcon -R -t httpd_sys_content_t /web/ command to
change the type of the /web/ directory (and its contents) to httpd_sys_content_t:
# chcon -R -t httpd_sys_content_t /web/
# ls -dZ /web/
drwxr-xr-x root root unconfined_u:object_r:httpd_sys_content_t:s0
# ls -lZ /web/
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0
/web/
file1
file2
file3
3. As the Linux root user, run the /sbin/restorecon -R -v /web/ command to restore the
default SELinux contexts:
# /sbin/restorecon -R -v /web/
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Persistent Changes: semanage fcontext
restorecon reset /web context unconfined_u:object_r:httpd_sys_content_t:s0>system_u:object_r:default_t:s0
restorecon reset /web/file2 context unconfined_u:object_r:httpd_sys_content_t:s0>system_u:object_r:default_t:s0
restorecon reset /web/file3 context unconfined_u:object_r:httpd_sys_content_t:s0>system_u:object_r:default_t:s0
restorecon reset /web/file1 context unconfined_u:object_r:httpd_sys_content_t:s0>system_u:object_r:default_t:s0
Refer to the chcon(1) manual page for further information about chcon.
Note
Type Enforcement is the main permission control used in SELinux targeted policy. For the
most part, SELinux users and roles can be ignored.
5.7.2. Persistent Changes: semanage fcontext
The /usr/sbin/semanage fcontext command changes the SELinux context for files. When
using targeted policy, changes made with this command are added to the /etc/selinux/
targeted/contexts/files/file_contexts file if the changes are to files that exists in
file_contexts, or are added to file_contexts.local for new files and directories, such as
creating a /web/ directory. setfiles, which is used when a file system is relabeled, and /sbin/
restorecon, which restores the default SELinux contexts, read these files. This means that changes
made by /usr/sbin/semanage fcontext are persistent, even if the file system is relabeled.
SELinux policy controls whether users are able to modify the SELinux context for any given file.
Quick Reference
To make SELinux context changes that survive a file system relabel:
1. Run the /usr/sbin/semanage fcontext -a options file-name|directory-name
command, remembering to use the full path to the file or directory.
2. Run the /sbin/restorecon -v file-name|directory-name command to apply the
context changes.
Changing a File's Type
The following example demonstrates changing a file's type, and no other attributes of the SELinux
context:
1. As the Linux root user, run the touch /etc/file1 command to create a new file. By default,
newly-created files in the /etc/ directory are labeled with the etc_t type:
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0
/etc/file1
2. As the Linux root user, run the /usr/sbin/semanage fcontext -a -t samba_share_t /
etc/file1 command to change the file1 type to samba_share_t. The -a option adds a new
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record, and the -t option defines a type (samba_share_t). Note: running this command does
not directly change the type - file1 is still labeled with the etc_t type:
# /usr/sbin/semanage fcontext -a -t samba_share_t /etc/file1
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0
/etc/file1
The /usr/sbin/semanage fcontext -a -t samba_share_t /etc/file1
command adds the following entry to /etc/selinux/targeted/contexts/files/
file_contexts.local:
/etc/file1
unconfined_u:object_r:samba_share_t:s0
3. As the Linux root user, run the /sbin/restorecon -v /etc/file1 command to change
the type. Since the semanage command added an entry to file.contexts.local for /etc/
file1, the /sbin/restorecon command changes the type to samba_share_t:
# /sbin/restorecon -v /etc/file1
restorecon reset /etc/file1 context unconfined_u:object_r:etc_t:s0>system_u:object_r:samba_share_t:s0
4. As the Linux root user, run the rm -i /etc/file1 command to remove file1.
5. As the Linux root user, run the /usr/sbin/semanage fcontext -d /etc/file1
command to remove the context added for /etc/file1. When the context is removed, running
restorecon changes the type to etc_t, rather than samba_share_t.
Changing a Directory's Type
The following example demonstrates creating a new directory and changing that directory's file type, to
a type used by Apache HTTP Server:
1. As the Linux root user, run the mkdir /web command to create a new directory. This directory is
labeled with the default_t type:
# ls -dZ /web
drwxr-xr-x root root unconfined_u:object_r:default_t:s0 /web
The ls -d option makes ls list information about a directory, rather than its
contents, and the -Z option makes ls display the SELinux context (in this example,
unconfined_u:object_r:default_t:s0).
2. As the Linux root user, run the /usr/sbin/semanage fcontext -a t httpd_sys_content_t /web command to change the /web/ type to
httpd_sys_content_t. The -a option adds a new record, and the -t option defines a type
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(httpd_sys_content_t). Note: running this command does not directly change the type - /
web/ is still labeled with the default_t type:
# /usr/sbin/semanage fcontext -a -t httpd_sys_content_t /web
# ls -dZ /web
drwxr-xr-x root root unconfined_u:object_r:default_t:s0
/web
The /usr/sbin/semanage fcontext -a -t httpd_sys_content_t /web
command adds the following entry to /etc/selinux/targeted/contexts/files/
file_contexts.local:
/web
unconfined_u:object_r:httpd_sys_content_t:s0
3. As the Linux root user, run the /sbin/restorecon -v /web command to change the type.
Since the semanage command added an entry to file.contexts.local for /web, the /
sbin/restorecon command changes the type to httpd_sys_content_t:
# /sbin/restorecon -v /web
restorecon reset /web context unconfined_u:object_r:default_t:s0>system_u:object_r:httpd_sys_content_t:s0
By default, newly-created files and directories inherit the SELinux type of their parent folders.
When using this example, and before removing the SELinux context added for /web/, files and
directories created in the /web/ directory are labeled with the httpd_sys_content_t type.
4. As the Linux root user, run the /usr/sbin/semanage fcontext -d /web command to
remove the context added for /web/.
5. As the Linux root user, run the /sbin/restorecon -v /web command to restore the default
SELinux context.
Changing a Directory and its Contents Types
The following example demonstrates creating a new directory, and changing the directory's file type
(along with its contents) to a type used by Apache HTTP Server. The configuration in this example
is used if you want Apache HTTP Server to use a different document root (instead of /var/www/
html/):
1. As the Linux root user, run the mkdir /web command to create a new directory, and then the
touch /web/file{1,2,3} command to create 3 empty files (file1, file2, and file3). The
/web/ directory and files in it are labeled with the default_t type:
# ls -dZ /web
drwxr-xr-x root
# ls -lZ /web
-rw-r--r-- root
-rw-r--r-- root
-rw-r--r-- root
root unconfined_u:object_r:default_t:s0 /web
root unconfined_u:object_r:default_t:s0 file1
root unconfined_u:object_r:default_t:s0 file2
root unconfined_u:object_r:default_t:s0 file3
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2. As the Linux root user, run the /usr/sbin/semanage fcontext -a -t
httpd_sys_content_t "/web(/.*)?" command to change the type of the /web/ directory
and the files in it, to httpd_sys_content_t. The -a option adds a new record, and the -t
option defines a type (httpd_sys_content_t). The "/web(/.*)?" regular expression causes the
semanage command to apply changes to the /web/ directory, as well as the files in it. Note:
running this command does not directly change the type - /web/ and files in it are still labeled with
the default_t type:
# ls -dZ /web
drwxr-xr-x root
# ls -lZ /web
-rw-r--r-- root
-rw-r--r-- root
-rw-r--r-- root
root unconfined_u:object_r:default_t:s0 /web
root unconfined_u:object_r:default_t:s0 file1
root unconfined_u:object_r:default_t:s0 file2
root unconfined_u:object_r:default_t:s0 file3
The /usr/sbin/semanage fcontext -a -t httpd_sys_content_t "/web(/.*)?"
command adds the following entry to /etc/selinux/targeted/contexts/files/
file_contexts.local:
/web(/.*)?
system_u:object_r:httpd_sys_content_t:s0
3. As the Linux root user, run the /sbin/restorecon -R -v /web command to change the type
of the /web/ directory, as well as all files in it. The -R is for recursive, which means all files and
directories under the /web/ directory are labeled with the httpd_sys_content_t type. Since
the semanage command added an entry to file.contexts.local for /web(/.*)?, the /
sbin/restorecon command changes the types to httpd_sys_content_t:
# /sbin/restorecon -R -v /web
restorecon reset /web context unconfined_u:object_r:default_t:s0>system_u:object_r:httpd_sys_content_t:s0
restorecon reset /web/file2 context unconfined_u:object_r:default_t:s0>system_u:object_r:httpd_sys_content_t:s0
restorecon reset /web/file3 context unconfined_u:object_r:default_t:s0>system_u:object_r:httpd_sys_content_t:s0
restorecon reset /web/file1 context unconfined_u:object_r:default_t:s0>system_u:object_r:httpd_sys_content_t:s0
By default, newly-created files and directories inherit the SELinux type of their parents. In
this example, files and directories created in the /web/ directory will be labeled with the
httpd_sys_content_t type.
4. As the Linux root user, run the /usr/sbin/semanage fcontext -d "/web(/.*)?"
command to remove the context added for "/web(/.*)?".
5. As the Linux root user, run the /sbin/restorecon -R -v /web command to restore the
default SELinux contexts.
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The file_t and default_t Types
Deleting an added Context
The following example demonstrates adding and removing an SELinux context:
1. As the Linux root user, run the /usr/sbin/semanage fcontext -a -t
httpd_sys_content_t /test command. The /test/ directory does not have to exist.
This command adds the following context to /etc/selinux/targeted/contexts/files/
file_contexts.local:
/test
system_u:object_r:httpd_sys_content_t:s0
2. To remove the context, as the Linux root user, run the /usr/sbin/semanage fcontext
-d file-name|directory-name command, where file-name|directory-name
is the first part in file_contexts.local. The following is an example of a context in
file_contexts.local:
/test
system_u:object_r:httpd_sys_content_t:s0
With the first part being /test. To prevent the /test/ directory from being labeled with
the httpd_sys_content_t after running /sbin/restorecon, or after a file system
relabel, run the following command as the Linux root user to delete the context from
file_contexts.local:
/usr/sbin/semanage fcontext -d /test
If the context is part of a regular expression, for example, /web(/.*)?, use quotation marks around
the regular expression:
/usr/sbin/semanage fcontext -d "/web(/.*)?"
Refer to the semanage(8) manual page for further information about /usr/sbin/semanage.
Important
When changing the SELinux context with /usr/sbin/semanage fcontext -a,
use the full path to the file or directory to avoid files being mislabeled after a file system
relabel, or after the /sbin/restorecon command is run.
5.8. The file_t and default_t Types
For file systems that support extended attributes, when a file that lacks an SELinux context on disk is
accessed, it is treated as if it had a default context as defined by SELinux policy. In common policies,
this default context uses the file_t type. This should be the only use of this type, so that files without
a context on disk can be distinguished in policy, and generally kept inaccessible to confined domains.
The file_t type should not exist on correctly-labeled file systems, because all files on a system
running SELinux should have an SELinux context, and the file_t type is never used in file-context
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configuration .
Files in /etc/selinux/targeted/contexts/files/ define contexts for files and directories. Files in this directory are read
by restorecon and setfiles to restore files and directories to their default contexts.
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The default_t type is used on files that do not match any other pattern in file-context configuration,
so that such files can be distinguished from files that do not have a context on disk, and generally kept
inaccessible to confined domains. If you create a new top-level directory, such as /mydirectory/,
this directory may be labeled with the default_t type. If services need access to such a directory,
update the file-contexts configuration for this location. Refer to Section 5.7.2, “Persistent Changes:
semanage fcontext” for details on adding a context to the file-context configuration.
5.9. Mounting File Systems
By default, when a file system that supports extended attributes is mounted, the security context for
each file is obtained from the security.selinux extended attribute of the file. Files in file systems that
do not support extended attributes are assigned a single, default security context from the policy
configuration, based on file system type.
Use the mount -o context command to override existing extended attributes, or to specify a
different, default context for file systems that do not support extended attributes. This is useful if
you do not trust a file system to supply the correct attributes, for example, removable media used
in multiple systems. The mount -o context command can also be used to support labeling for
file systems that do not support extended attributes, such as File Allocation Table (FAT) or NFS file
systems. The context specified with the context is not written to disk: the original contexts are
preserved, and are seen when mounting without a context option (if the file system had extended
attributes in the first place).
For further information about file system labeling, refer to James Morris's "Filesystem Labeling in
SELinux" article: http://www.linuxjournal.com/article/7426.
5.9.1. Context Mounts
To mount a file system with the specified context, overriding existing contexts if they exist, or to specify
a different, default context for a file system that does not support extended attributes, as the Linux root
user, use the mount -o context=SELinux_user:role:type:level command when mounting
the desired file system. Context changes are not written to disk. By default, NFS mounts on the client
side are labeled with a default context defined by policy for NFS file systems. In common policies, this
default context uses the nfs_t type. Without additional mount options, this may prevent sharing NFS
file systems via other services, such as the Apache HTTP Server. The following example mounts an
NFS file system so that it can be shared via the Apache HTTP Server:
# mount server:/export /local/mount/point -o\
context="system_u:object_r:httpd_sys_content_t:s0"
Newly-created files and directories on this file system appear to have the SELinux context specified
with -o context; however, since context changes are not written to disk for these situations, the
context specified with the context option is only retained if the context option is used on the next
mount, and if the same context is specified.
Type Enforcement is the main permission control used in SELinux targeted policy. For the most part,
SELinux users and roles can be ignored, so, when overriding the SELinux context with -o context,
use the SELinux system_u user and object_r role, and concentrate on the type. If you are not
using the MLS policy or multi-category security, use the s0 level.
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Note
When a file system is mounted with a context option, context changes (by users and
processes) are prohibited. For example, running chcon on a file system mounted with a
context option results in a Operation not supported error.
5.9.2. Changing the Default Context
As mentioned in Section 5.8, “The file_t and default_t Types”, on file systems that support extended
attributes, when a file that lacks an SELinux context on disk is accessed, it is treated as if it had
a default context as defined by SELinux policy. In common policies, this default context uses the
file_t type. If it is desirable to use a different default context, mount the file system with the
defcontext option.
The following example mounts a newly-created file system (on /dev/sda2) to the newly-created
/test/ directory. It assumes that there are no rules in /etc/selinux/targeted/contexts/
files/ that define a context for the /test/ directory:
# mount /dev/sda2 /test/ -o defcontext="system_u:object_r:samba_share_t:s0"
In this example:
• the defcontext option defines that system_u:object_r:samba_share_t:s0 is "the default
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security context for unlabeled files" .
• when mounted, the root directory (/test/) of the file system is treated as if it is labeled with the
context specified by defcontext (this label is not stored on disk). This affects the labeling for files
created under /test/: new files inherit the samba_share_t type, and these labels are stored on
disk.
• files created under /test/ while the file system was mounted with a defcontext option retain
their labels.
5.9.3. Mounting an NFS File System
By default, NFS mounts on the client side are labeled with a default context defined by policy for
NFS file systems. In common policies, this default context uses the nfs_t type. Depending on policy
configuration, services, such as Apache HTTP Server and MySQL, may not be able to read files
labeled with the nfs_t type. This may prevent file systems labeled with this type from being mounted
and then read or exported by other services.
If you would like to mount an NFS file system and read or export that file system with another service,
use the context option when mounting to override the nfs_t type. Use the following context option
to mount NFS file systems so that they can be shared via the Apache HTTP Server:
mount server:/export /local/mount/point -o\
context="system_u:object_r:httpd_sys_content_t:s0"
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Since context changes are not written to disk for these situations, the context specified with the
context option is only retained if the context option is used on the next mount, and if the same
context is specified.
As an alternative to mounting file systems with context options, Booleans can be turned on to allow
services access to file systems labeled with the nfs_t type. Refer to Section 5.6.3, “Booleans for NFS
and CIFS” for instructions on configuring Booleans to allow services access to the nfs_t type.
5.9.4. Multiple NFS Mounts
When mounting multiple mounts from the same NFS export, attempting to override the SELinux
context of each mount with a different context, results in subsequent mount commands failing. In the
following example, the NFS server has a single export, /export, which has two subdirectories, web/
and database/. The following commands attempt two mounts from a single NFS export, and try to
override the context for each one:
# mount server:/export/web /local/web -o\
context="system_u:object_r:httpd_sys_content_t:s0"
# mount server:/export/database /local/database -o\
context="system_u:object_r:mysqld_db_t:s0"
The second mount command fails, and the following is logged to /var/log/messages:
kernel: SELinux: mount invalid.
type nfs)
Same superblock, different security settings for (dev 0:15,
To mount multiple mounts from a single NFS export, with each mount having a different context, use
the -o nosharecache,context options. The following example mounts multiple mounts from a
single NFS export, with a different context for each mount (allowing a single service access to each
one):
# mount server:/export/web /local/web -o\
nosharecache,context="system_u:object_r:httpd_sys_content_t:s0"
# mount server:/export/database /local/database -o\
nosharecache,context="system_u:object_r:mysqld_db_t:s0"
In this example, server:/export/web is mounted locally to /local/web/, with all files being
labeled with the httpd_sys_content_t type, allowing Apache HTTP Server access. server:/
export/database is mounted locally to /local/database, with all files being labeled with the
mysqld_db_t type, allowing MySQL access. These type changes are not written to disk.
Important
The nosharecache options allows you to mount the same subdirectory of an export
multiple times with different contexts (for example, mounting /export/web multiple
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times). Do not mount the same subdirectory from an export multiple times with different
contexts, as this creates an overlapping mount, where files are accessible under two
different contexts.
5.9.5. Making Context Mounts Persistent
To make context mounts persistent across remounting and reboots, add entries for the file systems in
/etc/fstab or an automounter map, and use the desired context as a mount option. The following
example adds an entry to /etc/fstab for an NFS context mount:
server:/export /local/mount/ nfs context="system_u:object_r:httpd_sys_content_t:s0" 0 0
5.10. Maintaining SELinux Labels
These sections describe what happens to SELinux contexts when copying, moving, and archiving files
and directories. Also, it explains how to preserve contexts when copying and archiving.
5.10.1. Copying Files and Directories
When a file or directory is copied, a new file or directory is created if it does not exist. That new file
or directory's context is based on default-labeling rules, not the original file or directory's context
(unless options were used to preserve the original context). For example, files created in user home
directories are labeled with the user_home_t type:
$ touch file1
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
If such a file is copied to another directory, such as /etc/, the new file is created in accordance
to default-labeling rules for the /etc/ directory. Copying a file (without additional options) may not
preserve the original context:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
# cp file1 /etc/
$ ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0
/etc/file1
When file1 is copied to /etc/, if /etc/file1 does not exist, /etc/file1 is created as a new
file. As shown in the example above, /etc/file1 is labeled with the etc_t type, in accordance to
default-labeling rules.
When a file is copied over an existing file, the existing file's context is preserved, unless the user
specified cp options to preserve the context of the original file, such as --preserve=context.
SELinux policy may prevent contexts from being preserved during copies.
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Copying Without Preserving SELinux Contexts
When copying a file with the cp command, if no options are given, the type is inherited from the
targeted, parent directory:
$ touch file1
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
$ ls -dZ /var/www/html/
drwxr-xr-x root root system_u:object_r:httpd_sys_content_t:s0 /var/www/html/
# cp file1 /var/www/html/
$ ls -Z /var/www/html/file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/file1
In this example, file1 is created in a user's home directory, and is labeled with the user_home_t
type. The /var/www/html/ directory is labeled with the httpd_sys_content_t type, as shown
with the ls -dZ /var/www/html/ command. When file1 is copied to /var/www/html/, it
inherits the httpd_sys_content_t type, as shown with the ls -Z /var/www/html/file1
command.
Preserving SELinux Contexts When Copying
Use the cp --preserve=context command to preserve contexts when copying:
$ touch file1
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
$ ls -dZ /var/www/html/
drwxr-xr-x root root system_u:object_r:httpd_sys_content_t:s0 /var/www/html/
# cp --preserve=context file1 /var/www/html/
$ ls -Z /var/www/html/file1
-rw-r--r-- root root unconfined_u:object_r:user_home_t:s0 /var/www/html/file1
In this example, file1 is created in a user's home directory, and is labeled with the user_home_t
type. The /var/www/html/ directory is labeled with the httpd_sys_content_t type, as shown
with the ls -dZ /var/www/html/ command. Using the --preserve=context option preserves
SELinux contexts during copy operations. As shown with the ls -Z /var/www/html/file1
command, the file1 user_home_t type was preserved when the file was copied to /var/www/
html/.
Copying and Changing the Context
Use the cp -Z command to change the destination copy's context. The following example was
performed in the user's home directory:
$ touch file1
$ cp -Z system_u:object_r:samba_share_t:s0 file1 file2
$ ls -Z file1 file2
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
-rw-rw-r-- user1 group1 system_u:object_r:samba_share_t:s0 file2
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$ rm file1 file2
In this example, the context is defined with the -Z option. Without the -Z option, file2 would be
labeled with the unconfined_u:object_r:user_home_t context.
Copying a File Over an Existing File
When a file is copied over an existing file, the existing file's context is preserved (unless an option is
used to preserve contexts). For example:
# touch /etc/file1
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0
/etc/file1
# touch /tmp/file2
# ls -Z /tmp/file2
-rw-r--r-- root root unconfined_u:object_r:user_tmp_t:s0 /tmp/file2
# cp /tmp/file2 /etc/file1
# ls -Z /etc/file1
-rw-r--r-- root root unconfined_u:object_r:etc_t:s0
/etc/file1
In this example, two files are created: /etc/file1, labeled with the etc_t type, and /tmp/file2,
labeled with the user_tmp_t type. The cp /tmp/file2 /etc/file1 command overwrites file1
with file2. After copying, the ls -Z /etc/file1 command shows file1 labeled with the etc_t
type, not the user_tmp_t type from /tmp/file2 that replaced /etc/file1.
Important
Copy files and directories, rather than moving them. This helps ensure they are labeled
with the correct SELinux contexts. Incorrect SELinux contexts can prevent processes from
accessing such files and directories.
5.10.2. Moving Files and Directories
File and directories keep their current SELinux context when they are moved. In many cases, this is
incorrect for the location they are being moved to. The following example demonstrates moving a file
from a user's home directory to /var/www/html/, which is used by the Apache HTTP Server. Since
the file is moved, it does not inherit the correct SELinux context:
1. Run the cd command without any arguments to change into your home directory. Once in your
home directory, run the touch file1 command to create a file. This file is labeled with the
user_home_t type:
$ ls -Z file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1
2. Run the ls -dZ /var/www/html/ command to view the SELinux context of the /var/www/
html/ directory:
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$ ls -dZ /var/www/html/
drwxr-xr-x root root system_u:object_r:httpd_sys_content_t:s0 /var/www/html/
By default, the /var/www/html/ directory is labeled with the httpd_sys_content_t type.
Files and directories created under the /var/www/html/ directory inherit this type, and as such,
they are labeled with this type.
3. As the Linux root user, run the mv file1 /var/www/html/ command to move file1 to the /
var/www/html/ directory. Since this file is moved, it keeps its current user_home_t type:
# mv file1 /var/www/html/
# ls -Z /var/www/html/file1
-rw-rw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 /var/www/html/file1
By default, the Apache HTTP Server can not read files that are labeled with the user_home_t type.
If all files comprising a web page are labeled with the user_home_t type, or another type that the
Apache HTTP Server can not read, permission is denied when attempting to access them via Firefox
or text-based Web browsers.
Important
Moving files and directories with the mv command may result in the wrong SELinux
context, preventing processes, such as the Apache HTTP Server and Samba, from
accessing such files and directories.
5.10.3. Checking the Default SELinux Context
Use the /usr/sbin/matchpathcon command to check if files and directories have the correct
SELinux context. From the matchpathcon(8) manual page: "matchpathcon queries the system
8
policy and outputs the default security context associated with the file path." . The following example
demonstrates using the /usr/sbin/matchpathcon command to verify that files in /var/www/
html/ directory are labeled correctly:
1. As the Linux root user, run the touch /var/www/html/file{1,2,3} command to create
three files (file1, file2, and file3). These files inherit the httpd_sys_content_t type from
the /var/www/html/ directory:
# touch /var/www/html/file{1,2,3}
# ls -Z /var/www/html/
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file3
The matchpathcon(8) manual page, as shipped with the libselinux-utils package in Red Hat Enterprise Linux, is written by Daniel
Walsh. Any edits or changes in this version were done by Murray McAllister.
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Archiving Files with tar
2. As the Linux root user, run the chcon -t samba_share_t /var/www/html/file1 command
to change the file1 type to samba_share_t. Note: the Apache HTTP Server can not read files
or directories labeled with the samba_share_t type.
3. The /usr/sbin/matchpathcon -V option compares the current SELinux context to the correct,
default context in SELinux policy. Run the /usr/sbin/matchpathcon -V /var/www/html/*
command to check all files in the /var/www/html/ directory:
$ /usr/sbin/matchpathcon -V /var/www/html/*
/var/www/html/file1 has context unconfined_u:object_r:samba_share_t:s0, should be
system_u:object_r:httpd_sys_content_t:s0
/var/www/html/file2 verified.
/var/www/html/file3 verified.
The following output from the /usr/sbin/matchpathcon command explains that file1 is labeled
with the samba_share_t type, but should be labeled with the httpd_sys_content_t type:
/var/www/html/file1 has context unconfined_u:object_r:samba_share_t:s0, should be
system_u:object_r:httpd_sys_content_t:s0
To resolve the label problem and allow the Apache HTTP Server access to file1, as the Linux root
user, run the /sbin/restorecon -v /var/www/html/file1 command:
# /sbin/restorecon -v /var/www/html/file1
restorecon reset /var/www/html/file1 context unconfined_u:object_r:samba_share_t:s0>system_u:object_r:httpd_sys_content_t:s0
5.10.4. Archiving Files with tar
tar does not retain extended attributes by default. Since SELinux contexts are stored in extended
attributes, contexts can be lost when archiving files. Use tar --selinux to create archives that
retain contexts. If a Tar archive contains files without extended attributes, or if you want the extended
attributes to match the system defaults, run the archive through /sbin/restorecon:
$ tar -xvf archive.tar | /sbin/restorecon -f -
Note: depending on the directory, you may need to be the Linux root user to run the /sbin/
restorecon command.
The following example demonstrates creating a Tar archive that retains SELinux contexts:
1. As the Linux root user, run the touch /var/www/html/file{1,2,3} command to create
three files (file1, file2, and file3). These files inherit the httpd_sys_content_t type from
the /var/www/html/ directory:
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# touch /var/www/html/file{1,2,3}
# ls -Z /var/www/html/
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file1
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file2
-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 file3
2. Run the cd /var/www/html/ command to change into the /var/www/html/ directory. Once
in this directory, as the Linux root user, run the tar --selinux -cf test.tar file{1,2,3}
command to create a Tar archive named test.tar.
3. As the Linux root user, run the mkdir /test command to create a new directory, and then, run
the chmod 777 /test/ command to allow all users full-access to the /test/ directory.
4. Run the cp /var/www/html/test.tar /test/ command to copy the test.tar file in to the
/test/ directory.
5. Run the cd /test/ command to change into the /test/ directory. Once in this directory, run the
tar -xvf test.tar command to extract the Tar archive.
6. Run the ls -lZ /test/ command to view the SELinux contexts. The httpd_sys_content_t
type has been retained, rather than being changed to default_t, which would have happened
had the --selinux not been used:
$ ls -lZ /test/
-rw-r--r-- user1
-rw-r--r-- user1
-rw-r--r-- user1
-rw-r--r-- user1
group1
group1
group1
group1
unconfined_u:object_r:httpd_sys_content_t:s0 file1
unconfined_u:object_r:httpd_sys_content_t:s0 file2
unconfined_u:object_r:httpd_sys_content_t:s0 file3
unconfined_u:object_r:default_t:s0 test.tar
7. If the /test/ directory is no longer required, as the Linux root user, run the rm -ri /test/
command to remove it, as well as all files in it.
Refer to the tar(1) manual page for further information about tar, such as the --xattrs option that
retains all extended attributes.
5.10.5. Archiving Files with star
star does not retain extended attributes by default. Since SELinux contexts are stored in extended
attributes, contexts can be lost when archiving files. Use star -xattr -H=exustar to create
archives that retain contexts. The star package is not installed by default. To install star, run the yum
install star command as the Linux root user.
The following example demonstrates creating a Star archive that retains SELinux contexts:
1. As the Linux root user, run the touch /var/www/html/file{1,2,3} command to create
three files (file1, file2, and file3). These files inherit the httpd_sys_content_t type from
the /var/www/html/ directory:
# touch /var/www/html/file{1,2,3}
# ls -Z /var/www/html/
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-rw-r--r--rw-r--r--rw-r--r--
Other Tools
root root unconfined_u:object_r:httpd_sys_content_t:s0 file1
root root unconfined_u:object_r:httpd_sys_content_t:s0 file2
root root unconfined_u:object_r:httpd_sys_content_t:s0 file3
2. Run the cd /var/www/html/ command to change into the /var/www/html/ directory. Once
in this directory, as the Linux root user, run the star -xattr -H=exustar -c -f=test.star
file{1,2,3} command to create a Star archive named test.star:
# star -xattr -H=exustar -c -f=test.star file{1,2,3}
star: 1 blocks + 0 bytes (total of 10240 bytes = 10.00k).
3. As the Linux root user, run the mkdir /test command to create a new directory, and then, run
the chmod 777 /test/ command to allow all users full-access to the /test/ directory.
4. Run the cp /var/www/html/test.star /test/ command to copy the test.star file in to
the /test/ directory.
5. Run the cd /test/ command to change into the /test/ directory. Once in this directory, run the
star -x -f=test.star command to extract the Star archive:
$ star -x -f=test.star
star: 1 blocks + 0 bytes (total of 10240 bytes = 10.00k).
6. Run the ls -lZ /test/ command to view the SELinux contexts. The httpd_sys_content_t
type has been retained, rather than being changed to default_t, which would have happened
had the --selinux not been used:
$ ls -lZ /test/
-rw-r--r-- user1
-rw-r--r-- user1
-rw-r--r-- user1
-rw-r--r-- user1
group1
group1
group1
group1
unconfined_u:object_r:httpd_sys_content_t:s0 file1
unconfined_u:object_r:httpd_sys_content_t:s0 file2
unconfined_u:object_r:httpd_sys_content_t:s0 file3
unconfined_u:object_r:default_t:s0 test.star
7. If the /test/ directory is no longer required, as the Linux root user, run the rm -ri /test/
command to remove it, as well as all files in it.
8. If star is no longer required, as the Linux root user, run the yum remove star command to
remove the package.
Refer to the star(1) manual page for further information about star.
5.11. Other Tools
This section discusses the tools designed specifically for doing analysis of SELinux policy.
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5.12. Information Gathering Tools
These tools are command line tools, providing formatted output. They are harder to use as part of
command line piping, but they provide gathered and well formatted information quickly.
avcstat
This command provides a short output of the access vector cache statistics since boot. You can watch
the statistics in real time by specifying a time interval in seconds. This provides updated statistics
since the initial output. The statistics file used is /selinux/avc/cache_stats, and you can specify
a different cache file with the -f /path/to/file option.
[root@localhost ~]# avcstat
lookups
hits
misses
47517410
47504630
12780
allocs
12780
reclaims
12176
frees
12275
seinfo
This utility is useful in describing the break-down of a policy, such as the number of classes, types,
Booleans, allow rules, etc. seinfo is a command line utility that uses either the policy.conf file or a
binary policy file as input.
The output of seinfo will vary between binary and source files. For example, the policy source file
uses the { } brackets to group multiple rule elements onto a single line. A similar effect happens with
attributes, where a single attribute expands into one or many types. Because these are expanded
and no longer relevant in the binary policy file, they have a return value of zero in the search results.
However, the number of rules greatly increases as each formerly one line rule using brackets is now a
number of individual lines.
Some items are not present in the binary policy. For example, neverallow rules are only checked
during policy compile, not during runtime, and initial SIDs are not part of the binary policy since they
are required prior to the policy being loaded by the kernel during boot.
[root@localhost ]# seinfo
Statistics for policy file: /etc/selinux/targeted/policy/policy.24
Policy Version & Type: v.24 (binary, mls)
Classes:
Sensitivities:
Types:
Users:
Booleans:
Allow:
Auditallow:
Type_trans:
Type_member:
Role_trans:
Constraints:
Initial SIDs:
Genfscon:
Netifcon:
Permissives:
46
77
1
3001
9
158
262796
44
10760
44
237
62
27
82
0
22
Permissions:
Categories:
Attributes:
Roles:
Cond. Expr.:
Neverallow:
Dontaudit:
Type_change:
Role allow:
Range_trans:
Validatetrans:
Fs_use:
Portcon:
Nodecon:
Polcap:
229
1024
244
13
193
0
156710
38
20
2546
0
22
373
0
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Information Gathering Tools
[root@localhost ]#
The seinfo command can also list the number of types with the domain attribute, giving an estimate
of the number of different confined processes:
# seinfo -adomain -x |
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wc -l
Not all domain types are confined. To look at the number of unconfined domains, use the
unconfined_domain attribute:
# seinfo -aunconfined_domain_type -x | wc -l
52
Permissive domains can be counted with the --permissive option.
# seinfo --permissive -x | wc -l
31
Remove the | wc -l option in the above commands to see the full lists.
sesearch
You can use the sesearch command to search for a particular type in the policy. You can search either
policy source files or the binary file. For example:
[scott@localhost ~]$ sesearch --role_allow -t httpd_sys_content_t \ /etc/selinux/targeted/
policy/policy.24
Found 20 role allow rules:
allow system_r sysadm_r;
allow sysadm_r system_r;
allow sysadm_r staff_r;
allow sysadm_r user_r;
allow system_r git_shell_r;
allow system_r guest_r;
allow logadm_r system_r;
allow system_r logadm_r;
allow system_r nx_server_r;
allow system_r staff_r;
allow staff_r logadm_r;
allow staff_r sysadm_r;
allow staff_r unconfined_r;
allow staff_r webadm_r;
allow unconfined_r system_r;
allow system_r unconfined_r;
allow system_r user_r;
allow webadm_r system_r;
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Chapter 5. Working with SELinux
allow system_r webadm_r;
allow system_r xguest_r;
The sesearch command can provide the number of allow rules:
# sesearch --allow | wc -l
262798
And the number of dontaudit rules:
# sesearch --dontaudit | wc -l
156712
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Confining Users
A number of confined SELinux users are available in Red Hat Enterprise Linux 6. Each Linux user
is mapped to an SELinux user via SELinux policy, allowing Linux users to inherit the restrictions
placed on SELinux users, for example (depending on the user), not being able to: run the X Window
System; use networking; run setuid applications (unless SELinux policy permits it); or run the su and
sudo commands. This helps protect the system from the user. Refer to Section 4.3, “Confined and
Unconfined Users” for further information about confined users.
6.1. Linux and SELinux User Mappings
As the Linux root user, run the semanage login -l command to view the mapping between Linux
users and SELinux users:
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
unconfined_u
unconfined_u
system_u
s0-s0:c0.c1023
s0-s0:c0.c1023
s0-s0:c0.c1023
In Red Hat Enterprise Linux 6, Linux users are mapped to the SELinux __default__ login by default
(which is in turn mapped to the SELinux unconfined_u user). When a Linux user is created with
the useradd command, if no options are specified, they are mapped to the SELinux unconfined_u
user. The following defines the default-mapping:
__default__
unconfined_u
s0-s0:c0.c1023
6.2. Confining New Linux Users: useradd
Linux users mapped to the SELinux unconfined_u user run in the unconfined_t domain. This is
seen by running the id -Z command while logged-in as a Linux user mapped to unconfined_u:
$ id -Z
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
When Linux users run in the unconfined_t domain, SELinux policy rules are applied, but policy
rules exist that allow Linux users running in the unconfined_t domain almost all access. If
unconfined Linux users execute an application that SELinux policy defines can transition from the
unconfined_t domain to its own confined domain, unconfined Linux users are still subject to the
restrictions of that confined domain. The security benefit of this is that, even though a Linux user is
running unconfined, the application remains confined, and therefore, the exploitation of a flaw in the
application can be limited by policy. Note: this does not protect the system from the user. Instead, the
user and the system are being protected from possible damage caused by a flaw in the application.
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When creating Linux users with useradd, use the -Z option to specify which SELinux user they are
mapped to. The following example creates a new Linux user, useruuser, and maps that user to the
SELinux user_u user. Linux users mapped to the SELinux user_u user run in the user_t domain.
In this domain, Linux users are unable to run setuid applications unless SELinux policy permits it (such
as passwd), and can not run su or sudo, preventing them from becoming the Linux root user with
these commands.
1. As the Linux root user, run the /usr/sbin/useradd -Z user_u useruuser command to
create a new Linux user (useruuser) that is mapped to the SELinux user_u user.
2. As the Linux root user, run the semanage login -l command to view the mapping between the
Linux useruuser user and user_u:
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
useruuser
unconfined_u
unconfined_u
system_u
user_u
s0-s0:c0.c1023
s0-s0:c0.c1023
s0-s0:c0.c1023
s0
3. As the Linux root user, run the passwd useruuser command to assign a password to the Linux
useruuser user:
# passwd useruuser
Changing password for user useruuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
4. Log out of your current session, and log in as the Linux useruuser user. When you log in,
pam_selinux maps the Linux user to an SELinux user (in this case, user_u), and sets up the
resulting SELinux context. The Linux user's shell is then launched with this context. Run the id Z command to view the context of a Linux user:
[useruuser@localhost ~]$ id -Z
user_u:user_r:user_t:s0
5. Log out of the Linux useruuser's session, and log back in with your account. If you do not want the
Linux useruuser user, run the /usr/sbin/userdel -r useruuser command as the Linux root
user to remove it, along with its home directory.
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Confining Existing Linux Users: semanage login
6.3. Confining Existing Linux Users: semanage login
If a Linux user is mapped to the SELinux unconfined_u user (the default behavior), and you would
like to change which SELinux user they are mapped to, use the semanage login command.
The following example creates a new Linux user named newuser, then maps that Linux user to the
SELinux user_u user:
1. As the Linux root user, run the /usr/sbin/useradd newuser command to create a new Linux
user (newuser). Since this user uses the default mapping, it does not appear in the /usr/sbin/
semanage login -l output:
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
unconfined_u
unconfined_u
system_u
s0-s0:c0.c1023
s0-s0:c0.c1023
s0-s0:c0.c1023
2. To map the Linux newuser user to the SELinux user_u user, run the following command as the
Linux root user:
/usr/sbin/semanage login -a -s user_u newuser
The -a option adds a new record, and the -s option specifies the SELinux user to map a Linux
user to. The last argument, newuser, is the Linux user you want mapped to the specified SELinux
user.
3. To view the mapping between the Linux newuser user and user_u, run the semanage login l command as the Linux root user:
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
newuser
root
system_u
unconfined_u
user_u
unconfined_u
system_u
s0-s0:c0.c1023
s0
s0-s0:c0.c1023
s0-s0:c0.c1023
4. As the Linux root user, run the passwd newuser command to assign a password to the Linux
newuser user:
# passwd newuser
Changing password for user newuser.
New UNIX password: Enter a password
Retype new UNIX password: Enter the same password again
passwd: all authentication tokens updated successfully.
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5. Log out of your current session, and log in as the Linux newuser user. Run the id -Z command
to view the newuser's SELinux context:
[newuser@rlocalhost ~]$ id -Z
user_u:user_r:user_t:s0
6. Log out of the Linux newuser's session, and log back in with your account. If you do not want the
Linux newuser user, run the userdel -r newuser command as the Linux root user to remove
it, along with its home directory. Also, the mapping between the Linux newuser user and user_u
is removed:
# /usr/sbin/userdel -r newuser
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
unconfined_u
unconfined_u
system_u
s0-s0:c0.c1023
s0-s0:c0.c1023
s0-s0:c0.c1023
6.4. Changing the Default Mapping
In Red Hat Enterprise Linux 6, Linux users are mapped to the SELinux __default__ login by default
(which is in turn mapped to the SELinux unconfined_u user). If you would like new Linux users, and
Linux users not specifically mapped to an SELinux user to be confined by default, change the default
mapping with the semanage login command.
For example, run the following command as the Linux root user to change the default mapping from
unconfined_u to user_u:
/usr/sbin/semanage login -m -S targeted -s "user_u" -r s0 __default__
Run the semanage login -l command as the Linux root user to verify the __default__ login is
mapped to user_u:
# /usr/sbin/semanage login -l
Login Name
SELinux User
MLS/MCS Range
__default__
root
system_u
user_u
unconfined_u
system_u
s0
s0-s0:c0.c1023
s0-s0:c0.c1023
If a new Linux user is created and an SELinux user is not specified, or if an existing Linux user logs
in and does not match a specific entry from the semanage login -l output, they are mapped to
user_u, as per the __default__ login.
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xguest: Kiosk Mode
To change back to the default behavior, run the following command as the Linux root user to map the
__default__ login to the SELinux unconfined_u user:
/usr/sbin/semanage login -m -S targeted -s "unconfined_u" -r\
s0-s0:c0.c1023 __default__
6.5. xguest: Kiosk Mode
The xguest package provides a kiosk user account. This account is used to secure machines that
people walk up to and use, such as those at libraries, banks, airports, information kiosks, and coffee
shops. The kiosk user account is very limited: essentially, it only allows users to log in and use Firefox
to browse Internet websites. Any changes made while logged in with his account, such as creating
files or changing settings, are lost when you log out.
To set up the kiosk account:
1. As the Linux root user, run yum install xguest command to install the xguest package. Install
dependencies as required.
2. In order to allow the kiosk account to be used by a variety of people, the account is not passwordprotected, and as such, the account can only be protected if SELinux is running in enforcing mode.
Before logging in with this account, use the getenforce command to confirm that SELinux is
running in enforcing mode:
$ /usr/sbin/getenforce
Enforcing
If this is not the case, refer to Section 5.5, “SELinux Modes” for information about changing to
enforcing mode. It is not possible to log in with this account if SELinux is in permissive mode or
disabled.
3. You can only log in to this account via the GNOME Display Manager (GDM). Once the xguest
package is installed, a Guest account is added to the GDM login screen.
6.6. Booleans for Users Executing Applications
Not allowing Linux users to execute applications (which inherit users' permissions) in their home
directories and /tmp/, which they have write access to, helps prevent flawed or malicious applications
from modifying files that users own. In Red Hat Enterprise Linux 6, by default, Linux users in the
guest_t and xguest_t domains can not execute applications in their home directories or /tmp/;
however, by default, Linux users in the user_t and staff_t domains can.
Booleans are available to change this behavior, and are configured with the setsebool command.
The setsebool command must be run as the Linux root user. The setsebool -P command makes
persistent changes. Do not use the -P option if you do not want changes to persist across reboots:
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guest_t
To allow Linux users in the guest_t domain to execute applications in their home directories and /
tmp/:
/usr/sbin/setsebool -P allow_guest_exec_content on
xguest_t
To allow Linux users in the xguest_t domain to execute applications in their home directories and /
tmp/:
/usr/sbin/setsebool -P allow_xguest_exec_content on
user_t
To prevent Linux users in the user_t domain from executing applications in their home directories
and /tmp/:
/usr/sbin/setsebool -P allow_user_exec_content off
staff_t
To prevent Linux users in the staff_t domain from executing applications in their home directories
and /tmp/:
/usr/sbin/setsebool -P allow_staff_exec_content off
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Chapter 7.
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sVirt
sVirt is a technology included in Red Hat Enterprise Linux 6 that integrates SELinux and virtualization.
sVirt applies Mandatory Access Control (MAC) to improve security when using virtual machines. The
main reasons for integrating these technologies are to improve security and harden the system against
bugs in the hypervisor that might be used as an attack vector aimed toward the host or to another
virtual machine.
This chapter describes how sVirt integrates with virtualization technologies in Red Hat Enterprise
Linux 6.
Non-Virtualized Environment
In a non-virtualized environment, hosts are separated from each other physically and each host has
a self-contained environment, consisting of services such as a Web server, or a DNS server. These
services communicate directly to their own user space, host kernel and physical host, offering their
services directly to the network. The following image represents a non-virtualized environment:
Virtualized Environment
In a virtualized environment, several operating systems can be housed (as "guests") within a single
host kernel and physical host. The following image represents a virtualized environment:
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7.1. Security and Virtualization
When services are not virtualized, machines are physically separated. Any exploit is usually contained
to the affected machine, with the obvious exception of network attacks. When services are grouped
together in a virtualized environment, extra vulnerabilities emerge in the system. If there is a security
flaw in the hypervisor that can be exploited by a guest instance, this guest may be able to not only
attack the host, but also other guests running on that host. This is not theoretical; attacks already exist
on hypervisors. These attacks can extend beyond the guest instance and could expose other guests
to attack.
sVirt is an effort to isolate guests and limit their ability to launch further attacks if exploited. This is
demonstrated in the following image, where an attack can not break out of the virtual machine and
extend to another host instance:
SELinux introduces a pluggable security framework for virtualized instances in its implementation
of Mandatory Access Control (MAC). The sVirt framework allows guests and their resources to be
uniquely labelled. Once labelled, rules can be applied which can reject access between different
guests.
7.2. sVirt Labelling
Like other services under the protection of SELinux, sVirt uses process-based mechanisms and
restrictions to provide an extra layer of security over guest instances. Under typical use, you should
not even notice that sVirt is working in the background. This section describes the labelling features of
sVirt.
As shown in the following output, when using sVirt, each Virtual Machine (VM) process is labelled and
runs with a dynamically generated level. Each process is isolated from other VMs with different levels:
# ps -eZ | grep qemu
system_u:system_r:svirt_t:s0:c87,c520 27950 ? 00:00:17 qemu-kvm
system_u:system_r:svirt_t:s0:c639,c757 27989 ? 00:00:06 qemu-system-x86
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The actual disk images are automatically labelled to match the processes, as shown in the following
output:
# ls -lZ /var/lib/libvirt/images/*
system_u:object_r:svirt_image_t:s0:c87,c520
image1
The following table outlines the different labels that can be assigned when using sVirt:
Type
SELinux Context
Description
Virtual Machine Processes
system_u:system_r:svirt_t:MCS1 MCS1 is a randomly
selected MCS field. Currently
approximately 500,000 labels
are supported.
Virtual Machine Image
system_u:object_r:svirt_image_t:MCS1
Only svirt_t processes with the
same MCS fields are able to
read/write these image files
and devices.
Virtual Machine Shared Read/
Write Content
system_u:object_r:svirt_image_t:s0
All svirt_t processes are
allowed to write to the
svirt_image_t:s0 files and
devices.
Virtual Machine Shared Shared
Read Only content
system_u:object_r:svirt_content_t:s0
All svirt_t processes are able
to read files/devices with this
label.
Virtual Machine Image
system_u:object_r:virt_content_t:s0
System default label used
when an image exits. No svirt_t
virtual processes are allowed
to read files/devices with this
label.
Table 7.1. sVirt Labels
It is also possible to perform static labeling when using sVirt. Static labels allow the administrator to
select a specific label, including the MCS/MLS field, for a virtual machine. Administrators who run
statically-labeled virtual machines are responsible for setting the correct label on the image files. The
virtual machine will always be started with that label, and the sVirt system will never modify the label
of a statically-labelled virtual machine's content. This allows the sVirt component to run in an MLS
environment. You can also run multiple virtual machines with different sensitivity levels on a system,
depending on your requirements.
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Troubleshooting
The following chapter describes what happens when SELinux denies access; the top three causes of
problems; where to find information about correct labeling; analyzing SELinux denials; and creating
custom policy modules with audit2allow.
8.1. What Happens when Access is Denied
SELinux decisions, such as allowing or disallowing access, are cached. This cache is known as the
Access Vector Cache (AVC). Denial messages are logged when SELinux denies access. These
denials are also known as "AVC denials", and are logged to a different location, depending on which
daemons are running:
Daemon
auditd on
Log Location
/var/log/audit/audit.log
auditd off; rsyslogd on
/var/log/messages
setroubleshootd, rsyslogd, and
auditd on
/var/log/audit/audit.log. Easier-to-read denial messages
also sent to /var/log/messages
If you are running the X Window System, have the setroubleshoot and setroubleshoot-server
packages installed, and the setroubleshootd and auditd daemons are running, a warning is
displayed when access is denied by SELinux:
Clicking on 'Show' presents a detailed analysis of why SELinux denied access, and a possible solution
for allowing access. If you are not running the X Window System, it is less obvious when access is
denied by SELinux. For example, users browsing your website may receive an error similar to the
following:
Forbidden
You don't have permission to access file name on this server
For these situations, if DAC rules (standard Linux permissions) allow access, check /var/log/
messages and /var/log/audit/audit.log for "SELinux is preventing" and "denied"
errors respectively. This can be done by running the following commands as the Linux root user:
grep "SELinux is preventing" /var/log/messages
grep "denied" /var/log/audit/audit.log
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8.2. Top Three Causes of Problems
The following sections describe the top three causes of problems: labeling problems, configuring
Booleans and ports for services, and evolving SELinux rules.
8.2.1. Labeling Problems
On systems running SELinux, all processes and files are labeled with a label that contains securityrelevant information. This information is called the SELinux context. If these labels are wrong, access
may be denied. If an application is labeled incorrectly, the process it transitions to may not have
the correct label, possibly causing SELinux to deny access, and the process being able to create
mislabeled files.
A common cause of labeling problems is when a non-standard directory is used for a service. For
example, instead of using /var/www/html/ for a website, an administrator wants to use /srv/
myweb/. On Red Hat Enterprise Linux 6, the /srv/ directory is labeled with the var_t type. Files
and directories created and /srv/ inherit this type. Also, newly-created top-level directories (such as
/myserver/) may be labeled with the default_t type. SELinux prevents the Apache HTTP Server
(httpd) from accessing both of these types. To allow access, SELinux must know that the files in /
srv/myweb/ are to be accessible to httpd:
# /usr/sbin/semanage fcontext -a -t httpd_sys_content_t \
"/srv/myweb(/.*)?"
This semanage command adds the context for the /srv/myweb/ directory (and all files and
1
directories under it) to the SELinux file-context configuration . The semanage command does not
change the context. As the Linux root user, run the restorecon command to apply the changes:
# /sbin/restorecon -R -v /srv/myweb
Refer to Section 5.7.2, “Persistent Changes: semanage fcontext” for further information about adding
contexts to the file-context configuration.
8.2.1.1. What is the Correct Context?
The matchpathcon command checks the context of a file path and compares it to the default label
for that path. The following example demonstrates using matchpathcon on a directory that contains
incorrectly labeled files:
$ /usr/sbin/matchpathcon -V /var/www/html/*
/var/www/html/index.html has context unconfined_u:object_r:user_home_t:s0, should be
system_u:object_r:httpd_sys_content_t:s0
/var/www/html/page1.html has context unconfined_u:object_r:user_home_t:s0, should be
system_u:object_r:httpd_sys_content_t:s0
Files in /etc/selinux/targeted/contexts/files/ define contexts for files and directories. Files in this directory are read
by restorecon and setfiles to restore files and directories to their default contexts.
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In this example, the index.html and page1.html files are labeled with the user_home_t type.
This type is used for files in user home directories. Using the mv command to move files from your
home directory may result in files being labeled with the user_home_t type. This type should not
exist outside of home directories. Use the restorecon command to restore such files to their correct
type:
# /sbin/restorecon -v /var/www/html/index.html
restorecon reset /var/www/html/index.html context unconfined_u:object_r:user_home_t:s0>system_u:object_r:httpd_sys_content_t:s0
To restore the context for all files under a directory, use the -R option:
# /sbin/restorecon -R -v /var/www/html/
restorecon reset /var/www/html/page1.html context unconfined_u:object_r:samba_share_t:s0>system_u:object_r:httpd_sys_content_t:s0
restorecon reset /var/www/html/index.html context unconfined_u:object_r:samba_share_t:s0>system_u:object_r:httpd_sys_content_t:s0
Refer to Section 5.10.3, “Checking the Default SELinux Context” for a more detailed example of
matchpathcon.
8.2.2. How are Confined Services Running?
Services can be run in a variety of ways. To cater for this, you must tell SELinux how you are running
services. This can be achieved via Booleans that allow parts of SELinux policy to be changed at
runtime, without any knowledge of SELinux policy writing. This allows changes, such as allowing
services access to NFS file systems, without reloading or recompiling SELinux policy. Also, running
services on non-default port numbers requires policy configuration to be updated via the semanage
command.
For example, to allow the Apache HTTP Server to communicate with MySQL, turn the
httpd_can_network_connect_db Boolean on:
# /usr/sbin/setsebool -P httpd_can_network_connect_db on
If access is denied for a particular service, use the getsebool and grep commands to see if
any Booleans are available to allow access. For example, use the getsebool -a | grep ftp
command to search for FTP related Booleans:
$ /usr/sbin/getsebool -a | grep ftp
allow_ftpd_anon_write --> off
allow_ftpd_full_access --> off
allow_ftpd_use_cifs --> off
allow_ftpd_use_nfs --> off
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ftp_home_dir --> off
httpd_enable_ftp_server --> off
tftp_anon_write --> off
For a list of Booleans and whether they are on or off, run the /usr/sbin/getsebool -a command.
For a list of Booleans, an explanation of what each one is, and whether they are on or off, run the
/usr/sbin/semanage boolean -l command as the Linux root user. Refer to Section 5.6,
“Booleans” for information about listing and configuring Booleans.
Port Numbers
Depending on policy configuration, services may only be allowed to run on certain port numbers.
Attempting to change the port a service runs on without changing policy may result in the service
failing to start. For example, run the semanage port -l | grep http command as the Linux root
user to list http related ports:
# /usr/sbin/semanage port -l | grep http
http_cache_port_t
tcp
3128, 8080, 8118
http_cache_port_t
udp
3130
http_port_t
tcp
80, 443, 488, 8008, 8009, 8443
pegasus_http_port_t
tcp
5988
pegasus_https_port_t
tcp
5989
The http_port_t port type defines the ports Apache HTTP Server can listen on, which in this case,
are TCP ports 80, 443, 488, 8008, 8009, and 8443. If an administrator configures httpd.conf so
that httpd listens on port 9876 (Listen 9876), but policy is not updated to reflect this, the service
httpd start command fails:
# /sbin/service httpd start
Starting httpd: (13)Permission denied: make_sock: could not bind to address [::]:9876
(13)Permission denied: make_sock: could not bind to address 0.0.0.0:9876
no listening sockets available, shutting down
Unable to open logs
[FAILED]
An SELinux denial similar to the following is logged to /var/log/audit/audit.log:
type=AVC msg=audit(1225948455.061:294): avc: denied { name_bind } for pid=4997 comm="httpd"
src=9876 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=system_u:object_r:port_t:s0
tclass=tcp_socket
To allow httpd to listen on a port that is not listed for the http_port_t port type, run the semanage
2
port command to add a port to policy configuration :
The semanage port -a command adds an entry to the /etc/selinux/targeted/modules/active/ports.local file.
Note: by default, this file can only be viewed by the Linux root user.
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# /usr/sbin/semanage port -a -t http_port_t -p tcp 9876
The -a option adds a new record; the -t option defines a type; and the -p option defines a protocol.
The last argument is the port number to add.
8.2.3. Evolving Rules and Broken Applications
Applications may be broken, causing SELinux to deny access. Also, SELinux rules are evolving
- SELinux may not have seen an application running in a certain way, possibly causing it to deny
access, even though the application is working as expected. For example, if a new version of
PostgreSQL is released, it may perform actions the current policy has not seen before, causing access
to be denied, even though access should be allowed.
For these situations, after access is denied, use audit2allow to create a custom policy module
to allow access. Refer to Section 8.3.8, “Allowing Access: audit2allow” for information about using
audit2allow.
8.3. Fixing Problems
The following sections help troubleshoot issues. They go over: checking Linux permissions, which
are checked before SELinux rules; possible causes of SELinux denying access, but no denials
being logged; manual pages for services, which contain information about labeling and Booleans;
permissive domains, for allowing one process to run permissive, rather than the whole system; how
to search for and view denial messages; analyzing denials; and creating custom policy modules with
audit2allow.
8.3.1. Linux Permissions
When access is denied, check standard Linux permissions. As mentioned in Chapter 2, Introduction,
most operating systems use a Discretionary Access Control (DAC) system to control access, allowing
users to control the permissions of files that they own. SELinux policy rules are checked after DAC
rules. SELinux policy rules are not used if DAC rules deny access first.
If access is denied and no SELinux denials are logged, use the ls -l command to view the standard
Linux permissions:
$ ls -l /var/www/html/index.html
-rw-r----- 1 root root 0 2009-05-07 11:06 index.html
In this example, index.html is owned by the root user and group. The root user has read and write
permissions (-rw), and members of the root group have read permissions (-r-). Everyone else has
no access (---). By default, such permissions do not allow httpd to read this file. To resolve this
issue, use the chown command to change the owner and group. This command must be run as the
Linux root user:
# chown apache:apache /var/www/html/index.html
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This assumes the default configuration, in which httpd runs as the Linux apache user. If you run
httpd with a different user, replace apache:apache with that user.
3
Refer to the Fedora Documentation Project "Permissions" draft for information about managing Linux
permissions.
8.3.2. Possible Causes of Silent Denials
In certain situations, AVC denials may not be logged when SELinux denies access. Applications and
system library functions often probe for more access than required to perform their tasks. To maintain
least privilege without filling audit logs with AVC denials for harmless application probing, the policy
can silence AVC denials without allowing a permission by using dontaudit rules. These rules are
common in standard policy. The downside of dontaudit is that, although SELinux denies access,
denial messages are not logged, making troubleshooting hard.
To temporarily disable dontaudit rules, allowing all denials to be logged, run the following command
as the Linux root user:
/usr/sbin/semodule -DB
The -D option disables dontaudit rules; the -B option rebuilds policy. After running semodule -DB,
try exercising the application that was encountering permission problems, and see if SELinux denials
— relevant to the application — are now being logged. Take care in deciding which denials should be
allowed, as some should be ignored and handled via dontaudit rules. If in doubt, or in search of
4
guidance, contact other SELinux users and developers on an SELinux list, such as fedora-selinux-list .
To rebuild policy and enable dontaudit rules, run the following command as the Linux root user:
/usr/sbin/semodule -B
This restores the policy to its original state. For a full list of dontaudit rules, run the sesearch -dontaudit command. Narrow down searches using the -s domain option and the grep command.
For example:
$ sesearch --dontaudit -s smbd_t | grep squid
WARNING: This policy contained disabled aliases; they have been removed.
dontaudit smbd_t squid_port_t : tcp_socket name_bind ;
dontaudit smbd_t squid_port_t : udp_socket name_bind ;
Refer to Section 8.3.6, “Raw Audit Messages” and Section 8.3.7, “sealert Messages” for information
about analyzing denials.
8.3.3. Manual Pages for Services
Manual pages for services contain valuable information, such as what file type to use for a given
situation, and Booleans to change the access a service has (such as httpd accessing NFS file
systems). This information may be in the standard manual page, or a manual page with selinux
prepended or appended.
3
4
http://fedoraproject.org/wiki/Docs/Drafts/AdministrationGuide/Permissions
http://www.redhat.com/mailman/listinfo/fedora-selinux-list
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For example, the httpd_selinux(8) manual page has information about what file type to use for a given
situation, as well as Booleans to allow scripts, sharing files, accessing directories inside user home
directories, and so on. Other manual pages with SELinux information for services include:
• Samba: the samba_selinux(8) manual page describes that files and directories to be exported via
Samba must be labeled with the samba_share_t type, as well as Booleans to allow files labeled
with types other than samba_share_t to be exported via Samba.
• NFS: the nfs_selinux(8) manual page describes that, by default, file systems can not be exported
via NFS, and that to allow file systems to be exported, Booleans such as nfs_export_all_ro or
nfs_export_all_rw must be turned on.
• Berkeley Internet Name Domain (BIND): the named(8) manual page describes what file type to
use for a given situation (see the Red Hat SELinux BIND Security Profile section). The
named_selinux(8) manual page describes that, by default, named can not write to master zone files,
and to allow such access, the named_write_master_zones Boolean must be turned on.
The information in manual pages helps you configure the correct file types and Booleans, helping to
prevent SELinux from denying access.
8.3.4. Permissive Domains
When SELinux is running in permissive mode, SELinux does not deny access, but denials are logged
for actions that would have been denied if running in enforcing mode. Previously, it was not possible to
make a single domain permissive (remember: processes run in domains). In certain situations, this led
to making the whole system permissive to troubleshoot issues.
Red Hat Enterprise Linux includes permissive domains, where an administrator can configure a single
process (domain) to run permissive, rather than making the whole system permissive. SELinux checks
are still performed for permissive domains; however, the kernel allows access and reports an AVC
denial for situations where SELinux would have denied access.
In Red Hat Enterprise Linux 4, 5 and 6, domain_disable_trans Booleans are available to
prevent an application from transitioning to a confined domain, and therefore, the process runs in an
unconfined domain, such as initrc_t. Turning such Booleans on can cause major problems. For
example, if the httpd_disable_trans Boolean is turned on:
• httpd runs in the unconfined initrc_t domain. Files created by processes running in the
initrc_t domain may not have the same labeling rules applied as files created by a process
running in the httpd_t domain, potentially allowing processes to create mislabeled files. This
causes access problems later on.
• confined domains that are allowed to communicate with httpd_t can not communicate with
initrc_t, possibly causing additional failures.
Permissive domains can be used for:
• making a single process (domain) run permissive to troubleshoot an issue, rather than putting the
entire system at risk by making the entire system permissive.
• creating policies for new applications. Previously, it was recommended that a minimal policy be
created, and then the entire machine put into permissive mode, so that the application could run, but
SELinux denials still logged. audit2allow could then be used to help write the policy. This put the
whole system at risk. With permissive domains, only the domain in the new policy can be marked
permissive, without putting the whole system at risk.
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8.3.4.1. Making a Domain Permissive
To make a domain permissive, run the semanage permissive -a domain command, where
domain is the domain you want to make permissive. For example, run the following command as
the Linux root user to make the httpd_t domain (the domain the Apache HTTP Server runs in)
permissive:
/usr/sbin/semanage permissive -a httpd_t
To view a list of domains you have made permissive, run the semodule -l | grep permissive
command as the Linux root user. For example:
# /usr/sbin/semodule -l | grep permissive
permissive_httpd_t
1.0
If you no longer want a domain to be permissive, run the semanage permissive -d domain
command as the Linux root user. For example:
/usr/sbin/semanage permissive -d httpd_t
8.3.4.2. Denials for Permissive Domains
The SYSCALL message is different for permissive domains. The following is an example AVC denial
(and the associated system call) from the Apache HTTP Server:
type=AVC msg=audit(1226882736.442:86): avc: denied { getattr } for pid=2427 comm="httpd"
path="/var/www/html/file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0
tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1226882736.442:86): arch=40000003 syscall=196 success=no exit=-13
a0=b9a1e198 a1=bfc2921c a2=54dff4 a3=2008171 items=0 ppid=2425 pid=2427 auid=502 uid=48
gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4 comm="httpd" exe="/
usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
By default, the httpd_t domain is not permissive, and as such, the action is denied, and the
SYSCALL message contains success=no. The following is an example AVC denial for the same
situation, except the semanage permissive -a httpd_t command has been run to make the
httpd_t domain permissive:
type=AVC msg=audit(1226882925.714:136): avc: denied { read } for pid=2512
comm="httpd" name="file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0
tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1226882925.714:136): arch=40000003 syscall=5 success=yes exit=11
a0=b962a1e8 a1=8000 a2=0 a3=8000 items=0 ppid=2511 pid=2512 auid=502 uid=48 gid=48 euid=48
suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4 comm="httpd" exe="/usr/sbin/httpd"
subj=unconfined_u:system_r:httpd_t:s0 key=(null)
In this case, although an AVC denial was logged, access was not denied, as shown by success=yes
in the SYSCALL message.
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Refer to Dan Walsh's "Permissive Domains" blog entry for further information about permissive
domains.
8.3.5. Searching For and Viewing Denials
This section assumes the setroubleshoot, setroubleshoot-server, dbus and audit packages are
installed, and that the auditd, rsyslogd, and setroubleshootd daemons are running. Refer
to Section 5.2, “Which Log File is Used” for information about starting these daemons. A number of
tools are available for searching for and viewing SELinux denials, such as ausearch, aureport, and
sealert.
ausearch
The audit package provides ausearch. From the ausearch(8) manual page: "ausearch is a tool that
6
can query the audit daemon logs based for events based on different search criteria" . The ausearch
tool accesses /var/log/audit/audit.log, and as such, must be run as the Linux root user:
Searching For
all denials
Command
/sbin/ausearch -m avc
denials for that today
/sbin/ausearch -m avc -ts today
denials from the last 10 minutes
/sbin/ausearch -m avc -ts recent
To search for SELinux denials for a particular service, use the -c comm-name option, where comm7
name "is the executable’s name" , for example, httpd for the Apache HTTP Server, and smbd for
Samba:
/sbin/ausearch -m avc -c httpd
/sbin/ausearch -m avc -c smbd
Refer to the ausearch(8) manual page for further ausearch options.
aureport
The audit package provides aureport. From the aureport(8) manual page: "aureport is a tool
8
that produces summary reports of the audit system logs" . The aureport tool accesses /var/
log/audit/audit.log, and as such, must be run as the Linux root user. To view a list of SELinux
denials and how often each one occurred, run the aureport -a command. The following is example
output that includes two denials:
# /sbin/aureport -a
AVC Report
========================================================
# date time comm subj syscall class permission obj event
========================================================
1. 05/01/2009 21:41:39 httpd unconfined_u:system_r:httpd_t:s0 195 file getattr
system_u:object_r:samba_share_t:s0 denied 2
2. 05/03/2009 22:00:25 vsftpd unconfined_u:system_r:ftpd_t:s0 5 file read
unconfined_u:object_r:cifs_t:s0 denied 4
5
http://danwalsh.livejournal.com/24537.html
From the ausearch(8) manual page, as shipped with the audit package in Red Hat Enterprise Linux 6.
From the ausearch(8) manual page, as shipped with the audit package in Red Hat Enterprise Linux 6.
From the aureport(8) manual page, as shipped with the audit package in Red Hat Enterprise Linux 6.
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Refer to the aureport(8) manual page for further aureport options.
sealert
The setroubleshoot-server package provides sealert, which reads denial messages translated by
setroubleshoot-server. Denials are assigned IDs, as seen in /var/log/messages. The following is
an example denial from messages:
setroubleshoot: SELinux is preventing httpd (httpd_t) "getattr" to /var/www/html/
file1 (samba_share_t). For complete SELinux messages. run sealert -l 84e0b04dd0ad-4347-8317-22e74f6cd020
In this example, the denial ID is 84e0b04d-d0ad-4347-8317-22e74f6cd020. The -l option takes
an ID as an argument. Running the sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020
command presents a detailed analysis of why SELinux denied access, and a possible solution for
allowing access.
If you are running the X Window System, have the setroubleshoot and setroubleshoot-server
packages installed, and the setroubleshootd, dbus and auditd daemons are running, a warning
is displayed when access is denied by SELinux. Clicking on 'Show' launches the sealert GUI, and
displays denials in HTML output:
• Run the sealert -b command to launch the sealert GUI.
• Run the sealert -l \* command to view a detailed analysis of all denials.
• As the Linux root user, run the sealert -a /var/log/audit/audit.log -H > audit.html
command to create a HTML version of the sealert analysis, as seen with the sealert GUI.
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Raw Audit Messages
Refer to the sealert(8) manual page for further sealert options.
8.3.6. Raw Audit Messages
Raw audit messages are logged to /var/log/audit/audit.log. The following is an example
AVC denial (and the associated system call) that occurred when the Apache HTTP Server (running
in the httpd_t domain) attempted to access the /var/www/html/file1 file (labeled with the
samba_share_t type):
type=AVC msg=audit(1226874073.147:96): avc: denied { getattr } for pid=2465 comm="httpd"
path="/var/www/html/file1" dev=dm-0 ino=284133 scontext=unconfined_u:system_r:httpd_t:s0
tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=file
type=SYSCALL msg=audit(1226874073.147:96): arch=40000003 syscall=196 success=no exit=-13
a0=b98df198 a1=bfec85dc a2=54dff4 a3=2008171 items=0 ppid=2463 pid=2465 auid=502 uid=48
gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=6 comm="httpd" exe="/
usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
{ getattr }
The item in braces indicates the permission that was denied. getattr indicates the source
process was trying to read the target file's status information. This occurs before reading files.
This action is denied due to the file being accessed having the wrong label. Commonly seen
permissions include getattr, read, and write.
comm="httpd"
The executable that launched the process. The full path of the executable is found in the exe=
section of the system call (SYSCALL) message, which in this case, is exe="/usr/sbin/httpd".
path="/var/www/html/file1"
The path to the object (target) the process attempted to access.
scontext="unconfined_u:system_r:httpd_t:s0"
The SELinux context of the process that attempted the denied action. In this case, it is the
SELinux context of the Apache HTTP Server, which is running in the httpd_t domain.
tcontext="unconfined_u:object_r:samba_share_t:s0"
The SELinux context of the object (target) the process attempted to access. In this case, it is
the SELinux context of file1. Note: the samba_share_t type is not accessible to processes
running in the httpd_t domain.
In certain situations, the tcontext may match the scontext, for example, when a process
attempts to execute a system service that will change characteristics of that running process, such
as the user ID. Also, the tcontext may match the scontext when a process tries to use more
resources (such as memory) than normal limits allow, resulting in a security check to see if that
process is allowed to break those limits.
From the system call (SYSCALL) message, two items are of interest:
• success=no: indicates whether the denial (AVC) was enforced or not. success=no indicates
the system call was not successful (SELinux denied access). success=yes indicates the system
call was successful - this can be seen for permissive domains or unconfined domains, such as
initrc_t and kernel_t.
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• exe="/usr/sbin/httpd": the full path to the executable that launched the process, which in this
case, is exe="/usr/sbin/httpd".
An incorrect file type is a common cause for SELinux denying access. To start troubleshooting,
compare the source context (scontext) with the target context (tcontext). Should the process
(scontext) be accessing such an object (tcontext)? For example, the Apache HTTP Server
(httpd_t) should only be accessing types specified in the httpd_selinux(8) manual page, such as
httpd_sys_content_t, public_content_t, and so on, unless configured otherwise.
8.3.7. sealert Messages
Denials are assigned IDs, as seen in /var/log/messages. The following is an example AVC denial
(logged to messages) that occurred when the Apache HTTP Server (running in the httpd_t domain)
attempted to access the /var/www/html/file1 file (labeled with the samba_share_t type):
hostname setroubleshoot: SELinux is preventing httpd (httpd_t) "getattr" to /var/www/
html/file1 (samba_share_t). For complete SELinux messages. run sealert -l 84e0b04dd0ad-4347-8317-22e74f6cd020
As suggested, run the sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020 command to
view the complete message. This command only works on the local machine, and presents the same
information as the sealert GUI:
$ sealert -l 84e0b04d-d0ad-4347-8317-22e74f6cd020
Summary:
SELinux is preventing httpd (httpd_t) "getattr" to /var/www/html/file1
(samba_share_t).
Detailed Description:
SELinux denied access to /var/www/html/file1 requested by httpd.
/var/www/html/file1 has a context used for sharing by different program. If you
would like to share /var/www/html/file1 from httpd also, you need to change its
file context to public_content_t. If you did not intend to this access, this
could signal a intrusion attempt.
Allowing Access:
You can alter the file context by executing chcon -t public_content_t
'/var/www/html/file1'
Fix Command:
chcon -t public_content_t '/var/www/html/file1'
Additional Information:
Source
Target
Target
Source
Source
Port
70
Context
Context
Objects
Path
unconfined_u:system_r:httpd_t:s0
unconfined_u:object_r:samba_share_t:s0
/var/www/html/file1 [ file ]
httpd
/usr/sbin/httpd
<Unknown>
Draft
sealert Messages
Host
hostname
Source RPM Packages
httpd-2.2.10-2
Target RPM Packages
Policy RPM
selinux-policy-3.5.13-11.fc12
Selinux Enabled
True
Policy Type
targeted
MLS Enabled
True
Enforcing Mode
Enforcing
Plugin Name
public_content
Host Name
hostname
Platform
Linux hostname 2.6.27.4-68.fc12.i686 #1 SMP Thu Oct
30 00:49:42 EDT 2008 i686 i686
Alert Count
4
First Seen
Wed Nov 5 18:53:05 2008
Last Seen
Wed Nov 5 01:22:58 2008
Local ID
84e0b04d-d0ad-4347-8317-22e74f6cd020
Line Numbers
Raw Audit Messages
node=hostname type=AVC msg=audit(1225812178.788:101): avc: denied { getattr }
for pid=2441 comm="httpd" path="/var/www/html/file1" dev=dm-0 ino=284916
scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0
tclass=file
node=hostname type=SYSCALL msg=audit(1225812178.788:101): arch=40000003 syscall=196 success=no
exit=-13 a0=b8e97188 a1=bf87aaac a2=54dff4 a3=2008171 items=0 ppid=2439 pid=2441 auid=502
uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=3 comm="httpd"
exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)
Summary
A brief summary of the denied action. This is the same as the denial in /var/log/messages. In
this example, the httpd process was denied access to a file (file1), which is labeled with the
samba_share_t type.
Detailed Description
A more verbose description. In this example, file1 is labeled with the samba_share_t type.
This type is used for files and directories that you want to export via Samba. The description
suggests changing the type to a type that can be accessed by the Apache HTTP Server and
Samba, if such access is desired.
Allowing Access
A suggestion for how to allow access. This may be relabeling files, turning a Boolean on, or
making a local policy module. In this case, the suggestion is to label the file with a type accessible
to both the Apache HTTP Server and Samba.
Fix Command
A suggested command to allow access and resolve the denial. In this example, it gives the
command to change the file1 type to public_content_t, which is accessible to the Apache
HTTP Server and Samba.
Additional Information
Information that is useful in bug reports, such as the policy package name and version (selinuxpolicy-3.5.13-11.fc12), but may not help towards solving why the denial occurred.
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Raw Audit Messages
The raw audit messages from /var/log/audit/audit.log that are associated with the denial.
Refer to Section 8.3.6, “Raw Audit Messages” for information about each item in the AVC denial.
8.3.8. Allowing Access: audit2allow
Do not use the example in this section in production. It is used only to demonstrate the use of
audit2allow.
From the audit2allow(1) manual page: "audit2allow - generate SELinux policy allow rules from
9
logs of denied operations" . After analyzing denials as per Section 8.3.7, “sealert Messages”, and if no
label changes or Booleans allowed access, use audit2allow to create a local policy module. After
access is denied by SELinux, running the audit2allow command presents Type Enforcement rules
that allow the previously denied access.
The following example demonstrates using audit2allow to create a policy module:
1. A denial and the associated system call are logged to /var/log/audit/audit.log:
type=AVC msg=audit(1226270358.848:238): avc: denied { write }
for pid=13349 comm="certwatch" name="cache" dev=dm-0 ino=218171
scontext=system_u:system_r:certwatch_t:s0 tcontext=system_u:object_r:var_t:s0 tclass=dir
type=SYSCALL msg=audit(1226270358.848:238): arch=40000003 syscall=39 success=no exit=-13
a0=39a2bf a1=3ff a2=3a0354 a3=94703c8 items=0 ppid=13344 pid=13349 auid=4294967295
uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=4294967295
comm="certwatch" exe="/usr/bin/certwatch" subj=system_u:system_r:certwatch_t:s0
key=(null)
In this example, certwatch (comm="certwatch") was denied write access ({ write }) to a
directory labeled with the var_t type (tcontext=system_u:object_r:var_t:s0). Analyze
the denial as per Section 8.3.7, “sealert Messages”. If no label changes or Booleans allowed
access, use audit2allow to create a local policy module.
2. With a denial logged, such as the certwatch denial in step 1, run the audit2allow -w -a
command to produce a human-readable description of why access was denied. The -a option
causes all audit logs to be read. The -w option produces the human-readable description. The
audit2allow tool accesses /var/log/audit/audit.log, and as such, must be run as the
Linux root user:
# audit2allow -w -a
type=AVC msg=audit(1226270358.848:238): avc: denied { write }
for pid=13349 comm="certwatch" name="cache" dev=dm-0 ino=218171
scontext=system_u:system_r:certwatch_t:s0 tcontext=system_u:object_r:var_t:s0 tclass=dir
Was caused by:
Missing type enforcement (TE) allow rule.
You can use audit2allow to generate a loadable module to allow this access.
From the audit2allow(1) manual page, as shipped with the policycoreutils package in Red Hat Enterprise Linux 6.
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Allowing Access: audit2allow
As shown, access was denied due to a missing Type Enforcement rule.
3. Run the audit2allow -a command to view the Type Enforcement rule that allows the denied
access:
# audit2allow -a
#============= certwatch_t ==============
allow certwatch_t var_t:dir write;
Important
Missing Type Enforcement rules are usually caused by bugs in SELinux policy,
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and should be reported in Red Hat Bugzilla . For Red Hat Enterprise Linux,
create bugs against the Red Hat Enterprise Linux product, and select the
selinux-policy component. Include the output of the audit2allow -w -a and
audit2allow -a commands in such bug reports.
4. To use the rule displayed by audit2allow -a, run the audit2allow -a -M mycertwatch
command as the Linux root user to create custom module. The -M option creates a Type
Enforcement file (.te) with the name specified with -M, in your current working directory:
# audit2allow -a -M mycertwatch
******************** IMPORTANT ***********************
To make this policy package active, execute:
semodule -i mycertwatch.pp
# ls
mycertwatch.pp
mycertwatch.te
Also, audit2allow compiles the Type Enforcement rule into a policy package (.pp). To install
the module, run the /usr/sbin/semodule -i mycertwatch.pp command as the Linux root
user.
Important
Modules created with audit2allow may allow more access than required. It is
recommended that policy created with audit2allow be posted to an SELinux list,
11
such as fedora-selinux-list , for review. If you believe their is a bug in policy, create a
12
bug in Red Hat Bugzilla .
If you have multiple denials from multiple processes, but only want to create a custom policy
for a single process, use the grep command to narrow down the input for audit2allow. The
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following example demonstrates using grep to only send denials related to certwatch through
audit2allow:
# grep certwatch /var/log/audit/audit.log | audit2allow -M mycertwatch2
******************** IMPORTANT ***********************
To make this policy package active, execute:
# /usr/sbin/semodule -i mycertwatch2.pp
13
Refer to Dan Walsh's "Using audit2allow to build policy modules. Revisited."
information about using audit2allow to build policy modules.
13
http://danwalsh.livejournal.com/24750.html
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blog entry for further
Chapter 9.
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Further Information
9.1. Contributors
1
• Geert Warrink (translation - Dutch)
2
• Domingo Becker (translation - Spanish)
3
• Daniel Cabrera (translation - Spanish)
9.2. Other Resources
The National Security Agency (NSA)
4
From the NSA Contributors to SELinux page:
Researchers in NSA's National Information Assurance Research Laboratory (NIARL) designed and
implemented flexible mandatory access controls in the major subsystems of the Linux kernel and
implemented the new operating system components provided by the Flask architecture, namely the
security server and the access vector cache. The NSA researchers reworked the LSM-based SELinux
for inclusion in Linux 2.6. NSA has also led the development of similar controls for the X Window
System (XACE/XSELinux) and for Xen (XSM/Flask).
• Main SELinux website: http://www.nsa.gov/research/selinux/index.shtml.
• SELinux documentation: http://www.nsa.gov/research/selinux/docs.shtml.
• SELinux background: http://www.nsa.gov/research/selinux/background.shtml.
Tresys Technology
5
Tresys Technology are the upstream for:
6
• SELinux userland libraries and tools .
7
• SELinux Reference Policy .
SELinux News
• News: http://selinuxnews.org/wp/.
• Planet SELinux (blogs): http://selinuxnews.org/planet/.
SELinux Project Wiki
• Main page: http://selinuxproject.org/page/Main_Page.
• User resources, including links to documentation, mailing lists, websites, and tools: http://
selinuxproject.org/page/User_Resources.
4
5
http://www.nsa.gov/research/selinux/contrib.shtml
http://www.tresys.com/
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Chapter 9. Further Information
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Red Hat Enterprise Linux
8
9
• The Red Hat Enterprise Linux Deployment Guide contains an SELinux References section, that
has links to SELinux tutorials, general information, and the technology behind SELinux.
10
• The Red Hat Enterprise Linux 4 SELinux Guide .
Fedora
• Main page: http://fedoraproject.org/wiki/SELinux.
• Troubleshooting: http://fedoraproject.org/wiki/SELinux/Troubleshooting.
• Fedora Core 5 SELinux FAQ: http://docs.fedoraproject.org/selinux-faq-fc5/.
• SELinux Managing Confined Services Guide: http://docs.fedoraproject.org/
The UnOfficial SELinux FAQ
http://www.crypt.gen.nz/selinux/faq.html
IRC
11
On Freenode :
• #selinux
• #fedora-selinux
• #security
11
http://freenode.net/
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Appendix A. Revision History
Revision 1.9
Wed Mar 3 2010
Scott Radvan sradvan@redhat.com
Revision for Red Hat Enterprise Linux 6
77
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