Configuration Files for Security-Aware Applications

SELinux includes a set of new and modified applications that have some degree of awareness of the mandatory access controls. Some of these applications require their own configuration files that are related to the policy. This section describes these application configuration files.

The application configuration file sources are located in the /etc/selinux/(strict|targeted)/src/policy/appconfig directory. The configuration files used at runtime are installed under the /etc/selinux/(strict|targeted)/contexts directory. The most commonly used configuration files are discussed below, but there are a number of other configuration files included in the example policy and used by additional applications that are not discussed here.

Default_Contexts

Applications that need to set security contexts for user processes use the get_default_context or get_ordered_context_list libselinux functions. Internally, these functions consult the kernel policy to determine the set of legal security contexts for the user that are reachable by the application and then refine and order this set based on the default_contexts configuration file. Any context in a default_contexts configuration that is not within the set of legal contexts for the user that can be reached from the application will be ignored.

Each line of the default_contexts file specifies an entry consisting of a partial context for the application followed by a list of one or more partial contexts for users in the desired prioritization order. A partial context is a context without a user identity value. Partial contexts are used in the list of user contexts since the user identity can be inferred (it is the user who was authenticated or whose crontab file was read). Partial contexts are used for the application as the application may run under different user identities at different times. In the simplest form, an entry identifies the application context and then provides a single user context to use as the default.

In the example default_contexts file, login and ssh sessions default to user_r:user_t or staff_r:staff_t. Users can then use newrole to change to a different role if authorized for another role. System cron jobs default to system_r:system_crond_t, while user cron jobs default to user_r:user_crond_t. A derived domain (user_crond_t) is used so that the policy can grant different permissions to user cron jobs than to user sessions.

An administrator may also create a per-user default_contexts file in the /etc/selinux/(strict|targeted)/contexts/users directory with a filename identical to the username. If such a file exists for the user, then any entries in it are given higher priority than the entries in the system-wide default_contexts file. For example, the root user typically has such a per-user default_contexts file so that he will default to sysadm_r:sysadm_t for local logins.

Default_Type

The default_type file defines the default type (domain) for each role. Each line specifies a role:type pair, and the appropriate type is selected by matching the role field. This file is used by programs like newrole to automatically provide a default domain when the user selects a role. If no entry is specified, then the user must explicitly specify a domain.

Initrc_Context

The initrc_context file defines the security context for running /etc/rc.d scripts via the run_init program. It consists of a single line specifying the proper security context. The run_init program transitions to this security context and then runs the specified script. This ensures that the scripts are executed from the same context as when they are run by init.