Files, such as directories and devices, are called objects. Processes, such as a user running a command or the Mozilla® Firefox® application, are called 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. On Linux® operating systems, users can make their home directories world-readable, giving users and processes (subjects) access to potentially sensitive information.
DAC mechanisms are 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 can be exploited to obtain complete 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 Aug 18 10:08 file1
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. This includes all processes and users.
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.[2]
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. 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.
SELinux provides:
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 SELinux policy rules determine how processes access files and other processes, if a service is compromised, the attacker only has access to the normal functions of that service, and to files that the service has been configured to have access to. For example, if the Apache HTTP Server is compromised, an attacker is unable 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.
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, continue to follow good security practices, such as keeping software up-to-date, using hard-to-guess passwords, firewalls, and so on.
[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 and the document as it was first released. Any edits and changes were done by Murray McAllister.
[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, 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.