Linux Advisory Watch – November 4th 2005

Hacks From Pax: SELinux And Access Decisions
Pax Dickinson

Security Contexts

SELinux makes access decisions by checking the security context of the subject (a process, sometimes associated with a user) against the action attempted (e.g. a file read) and the security context of the targeted object (such as a file or network port).

These contexts are divided into three parts: a user identity, a role, and a domain or type. In the current SELinux policy, access is not restricted based on user identities, so we’ll focus on roles and domains in this article.

User Roles

On an SELinux system, unlike a standard Linux system, root has no special privileges inherent to the account. SELinux privileges are denoted by a user’s role. A standard user is assigned a role of user_r, which gives no special privileges. System administrator accounts are assigned a role of staff_r, which permits what is known as a “role transition” to the sysadm_r role. The sysadm_r role is the equivalent of the root account on a non-SELinux system, it has unfettered access to the system.

A staff user transitions to the sysadm_r role by using the newrole command, as shown below.

newrole -r sysadm_r

The user is then prompted for his or her password, successful entry of which will result in transition to the new role. You can view your current role by issuing an id -Z command.

Domains and Types

Domains and types are synonyms, typically the term “domain” is used when referring to processes and the term “type” is used referring to files. Types are the primary method used by SELinux to make authorization decisions. The strict policy defines relatively few users and roles, but contains hundreds of types.

Types are assigned by the security policy based on the path of the file in question, and the policy also transitions processes into an appropriate domain based on the context of the executed file and the domain of the process executing the file.

For example, the Apache webserver executable file has a type of httpd_exec_t. When that file is executed by the init process at bootup, the policy forces the new process to transition into the httpd_t domain. The httpd_t domain has the ability to read web content denoted by the httpd_content_t type, but not to change it or access any other domains not required for proper webserver operation.

You can view the type of a given file by using the -Z option of ls, and you can view the domain a process is running in by using the -Z option of ps. These -Z options are specific to SELinux and will not function on a non-SELinux system.

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