Security-Enhanced Linux

SELinux

Security-Enhanced Linux (SELinux) is a mandatory access control (MAC) security mechanism implemented in the kernel. SELinux was first introduced in CentOS 4 and significantly enhanced in CentOS 5 and 6. These enhancements mean that content varies as to how to approach SELinux over time to solve problems. 

Access can be constrained on such variables as which users and applications can access which resources. These resources may take the form of files. Standard Linux access controls, such as file modes (-rwxr-xr-x) are modifiable by the user and the applications which the user runs. Conversely, SELinux access controls are determined by a policy loaded on the system which may not be changed by careless users or misbehaving applications. SELinux also adds finer granularity to access controls. Instead of only being able to specify who can read, write or execute a file, for example, SELinux lets you specify who can unlink, append only, move a file and so on. SELinux allows you to specify access to many resources other than files as well, such as network resources and interprocess communication (IPC).

SELinux follows the model of least-privilege more closely. By default under a strict enforcing setting, everything is denied and then a series of exceptions policies are written that give each element of the system (a service, program or user) only the access required to function. If a service, program or user subsequently tries to access or modify a file or resource not necessary for it to function, then access is denied and the action is can be logged.

SELinux has three basic modes of operation, of which Enforcing is set as the installation default mode. There is, however, an additional qualifier of targeted or mls which control how pervasive SELinux rules are applied, with targeted being the less stringent level.

Enforcing:   The default mode which will enable and enforce the SELinux security policy on the system,   

                  denying access and logging actions

Permissive: In Permissive mode, SELinux is enabled but will not enforce the security policy, only warn and   

                   log actions. Permissive mode is useful for troubleshooting SELinux issues
Disabled:    SELinux is turned off

The SELinux mode can be viewed and changed by using the SELinux Management GUI tool available on the Administration menu or from the command line by running 'system-config-selinux' (the SELinux Management GUI tool is part of the policycoreutils-gui package and is not installed by default).

SELinux can potentially control which activities a system allows each user, process and daemon, with very precise specifications.

However, it is mostly used to confine daemons like database engines or web servers that have more clearly-defined data access and activity rights. This limits potential harm from a confined daemon that becomes compromised. Ordinary user-processes often run in the unconfined domain, not restricted by SELinux but still restricted by the classic Linux access rights.

More on the NSA web site.