Product SiteDocumentation Site

Chapter 9. SELinux

9.1. Introduction
9.1.1. Benefits of running SELinux
9.1.2. Examples
9.1.3. SELinux Architecture
9.1.4. SELinux on Other Operating Systems
9.2. SELinux Contexts
9.2.1. Domain Transitions
9.2.2. SELinux Contexts for Processes
9.2.3. SELinux Contexts for Users
9.3. Targeted Policy
9.3.1. Confined Processes
9.3.2. Unconfined Processes
9.3.3. Confined and Unconfined Users
9.4. Working with SELinux
9.4.1. SELinux Packages
9.4.2. Which Log File is Used
9.4.3. Main Configuration File
9.4.4. Enabling and Disabling SELinux
9.4.5. SELinux Modes
9.4.6. Booleans
9.4.7. SELinux Contexts - Labeling Files
9.4.8. The file_t and default_t Types
9.4.9. Mounting File Systems
9.4.10. Maintaining SELinux Labels
9.5. Confining Users
9.5.1. Linux and SELinux User Mappings
9.5.2. Confining New Linux Users: useradd
9.5.3. Confining Existing Linux Users: semanage login
9.5.4. Changing the Default Mapping
9.5.5. xguest: Kiosk Mode
9.5.6. Booleans for Users Executing Applications
9.6. Troubleshooting
9.6.1. What Happens when Access is Denied
9.6.2. Top Three Causes of Problems
9.6.3. Fixing Problems
9.7. Further Information
9.7.1. Contributors
9.7.2. Other Resources

9.1. 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 has complete discretion over their files, making it impossible 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 no protection is provided against malicious software. Many system services and privileged programs must run with coarse-grained privileges that far exceed their requirements, so that a flaw in any one of these programs could be exploited to obtain further system access.[14]
The following is an example of permissions used on Linux operating systems that do not run Security-Enhanced Linux (SELinux). The permissions and output in these examples may differ from your system. Use the ls -l command to view file permissions:
$ ls -l file1
-rw-rw-r--. 1 user1 group1 0 May 11 10:46 file1
The first three permission bits, rw, 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 Fedora. 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 information to be protected from legitimate users with limited authorization as well as from authorized users who have unwittingly executed malicious applications.[15]
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
-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. 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.

9.1.1. 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.
  • Confined services. SELinux ships with the ability to confine services and daemons so that they are more predictable and are only allowed access that is required for their normal operation.
  • SELinux can be used to enforce data confidentiality and integrity, as well as protecting processes from untrusted inputs.
SELinux is not:
  • 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-to-date, using hard-to-guess passwords, firewalls, and so on.

9.1.2. Examples

The following examples demonstrate how SELinux increases security:
  • The default action is deny. If a specific 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®.
  • 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 Berkeley Internet Name Daemon (BIND) as a DNS server in Fedora, even if an administrator forgets to limit which servers can perform a zone transfer, the default SELinux policy prevents zone files [16] from being updated via zone transfers, by the BIND named daemon itself, and by other processes.
  • Refer to the Red Hat® Magazine article, Risk report: Three years of Red Hat Enterprise Linux 4[17], for exploits that were restricted due to the default SELinux targeted policy in Red Hat® Enterprise Linux® 4.
  • Refer to the LinuxWorld.com article, A seatbelt for server software: SELinux blocks real-world exploits[18], for background information about SELinux, and information about various exploits that SELinux has prevented.
  • Refer to James Morris's SELinux mitigates remote root vulnerability in OpenPegasus blog post for information about an exploit in OpenPegasus that was mitigated by SELinux as shipped with Red Hat Enterprise Linux 4 and 5.
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.

9.1.3. 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 decreases 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.

9.1.4. SELinux on Other Operating Systems

Refer to the following for information about running SELinux on operating systems:


[14] "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 for details and the document as it was first released. Any edits and changes were done by Murray McAllister.
[15] "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 for details and the document as it was first released. Any edits and changes were done by Murray McAllister.
[16] Text files that include information, such as hostname to IP address mappings, that are used by DNS servers.
[17] Cox, Mark. "Risk report: Three years of Red Hat Enterprise Linux 4". Published 26 February 2008. Accessed 27 August 2009: http://www.redhatmagazine.com/2008/02/26/risk-report-three-years-of-red-hat-enterprise-linux-4/.
[18] Marti, Don. "A seatbelt for server software: SELinux blocks real-world exploits". Published 24 February 2008. Accessed 27 August 2009: http://www.linuxworld.com/news/2008/022408-selinux.html?page=1.