Security-Enhanced Linux (SELinux) is a security architecture integrated into the 2.6.x kernel using the Linux Security Modules (LSM).

SELinux Overview

SELinux provides a flexible Mandatory Access Control (MAC) system built into the Linux kernel. Under standard Linux Discretionary Access Control (DAC), an application or process running as a user (UID or SUID) has the user’s permissions to objects such as files, sockets, and other processes. Running a MAC kernel protects the system from malicious or flawed applications that can damage or destroy the system.

SELinux defines the access and transition rights of every user, application, process, and file on the system. SELinux then governs the interactions of these entities using a security policy that specifies how strict or lenient a given Red Hat Enterprise Linux installation should be.

On a day-to-day basis, system users will be largely unaware of SELinux. Only system administrators need to consider how strict a policy to implement for their server environment. The policy can be as strict or as lenient as needed and is very finely detailed. This detail gives the SELinux kernel complete, granular control over the entire system.

The SELinux Decision Making Process

When a subject, (for example, an application), attempts to access an object (for example, a file), the policy enforcement server in the kernel checks an access vector cache (AVC), where subject and object permissions are cached. If a decision cannot be made based on data in the AVC, the request continues to the security server, which looks up the security context of the application and the file in a matrix. Permission is then granted or denied, with an avc: denied message detailed in /var/log/messages if permission is denied. The security context of subjects and objects is applied from the installed policy, which also provides the information to populate the security server’s matrix.

SELinux Decision Process

SELinux Operating Modes

Instead of running in enforcing mode, SELinux can run in permissive mode, where the AVC is checked and denials are logged, but SELinux does not enforce the policy. This can be useful for troubleshooting and for developing or fine-tuning SELinux policy.

The /selinux/ pseudo-file system contains commands that are most commonly used by the kernel subsystem. This type of file system is similar to the /proc/ pseudo-file system. Administrators and users do not normally need to manipulate this component.

There are two ways to configure SELinux under Red Hat Enterprise Linux: using the SELinux Administration Tool (system-config-selinux), or manually editing the
 primary configuration file (/etc/sysconfig/selinux).

The /etc/sysconfig/selinux contains a symbolic link to the actual configuration file, /etc/selinux/config.

SELinux Configuration

SELINUX=enforcing|permissive|disabled — Defines the top-level state of SELinux on a system.

  • enforcing — The SELinux security policy is enforced.
  • permissive — The SELinux system prints warnings but does not enforce policy.This is useful for debugging and troubleshooting purposes. In permissive mode, more denials are logged because subjects can continue with actions that would otherwise be denied in enforcing mode. For example, traversing a directory tree in permissive mode produces avc: denied messages for every directory level read. In enforcing mode, SELinux would have stopped the initial traversal and kept further denial messages from occurring.
  • disabled — SELinux is fully disabled. SELinux hooks are disengaged from the kernel and the pseudo-file system is unregistered.

SELINUXTYPE=targeted|strict — Specifies which policy SELinux should enforce.

  • targeted — Only targeted network daemons are protected.
  • strict — Full SELinux protection, for all daemons. Security contexts are defined for all subjects and objects, and every action is processed by the policy enforcement server.
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By Swatantra Kumar

Swatantra is an Open Source evangelist, a technologist and researcher. Professionally, he does software development, software architecture, server administration and project management. When he's not writing software, he enjoys building web entities and servers, reading about and working with new technologies, and trying to get his friends to make the move to open source software. He's written, co-written and published many articles in international journals, on various domains/topics including Open Source, Networks, Computer Organization, Mobile Technologies, and Business Intelligence. He made a proposal for an information management system at University level during graduation days.

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