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Chapter 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.
Security-Enhanced Linux (SELinux) refers to files, such as directories and devices, 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, inadvertently giving other users and processes (subjects) access to potentially sensitive information.
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 usually 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 must run with coarse-grained privileges that far exceed their requirements, so that a flaw in any one of these programs might be exploited to obtain further system access. [1]
The following is an example of permissions used on Linux operating systems that do not run Security-Enhanced Linux (SELinux). The permissions in these examples may differ from your system. Use the ls -l command to view file permissions:
$ ls -l file1
-rwxrw-r-- 1 user1 group1 0 2010-01-29 09:17 file1
The first three permission bits, rwx, control the access rights that the Linux user1 user (in this case, the owner) has to file1. The next three permission bits, rw-, control the access rights that the Linux group1 group has to file1. The last three permission bits, r--, control the access rights that 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 information to be protected from legitimate users with limited authorization as well as from authorized users who have unwittingly executed malicious applications.[2]
The following is an example of the labels containing security-relevant information that are applied to 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) for the file1 file. This information is used to make access control decisions. This example also displays the DAC rules, which are shown in the SELinux context via the ls -Z command. SELinux policy rules are checked after DAC rules. SELinux policy rules are not applied if DAC rules deny access first.

Targeted Policy

When SELinux is in enforcing mode, the default policy used in Red Hat Enterprise Linux, is the targeted policy. Processes that are targeted run in a confined domain, and processes that are not targeted run in an unconfined domain. Refer to the Targeted Policy chapter in the Security-Enhanced Linux guide for more information about targeted policy and confined and unconfined processes.

[1] "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.
[2] "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 in the public domain. Refer to the original paper for details.