49.6.3. MLS Policy

SELinux uses the Bell-La Padula BLP model, with Type Enforcement (TE) for integrity. In simple terms, MLS policy ensures that a Subject has an appropriate clearance to access an Object of a particular classification.

For example, under MLS, the system needs to know how to process a request such as: Can a process running with a clearance of { Top Secret / UFO, Rail gun } write to a file classified as { Top Secret / UFO } ?

The MLS model and the policy implemented for it will determine the answer. (Consider, for example, the problem of information leaking out of the Rail gun category into the file).

MLS meets a very narrow (yet critical) set of security requirements based around the way information and personnel are managed in rigidly controlled environments such as the military. MLS is typically difficult to work with and does not map well to general-case scenarios.

Type Enforcement (TE) under SELinux is a more flexible and expressive security scheme, which is in many cases more suitable than MLS.

There are, however, several scenarios where traditional MLS is still required. For example, a file server where the stored data may be of mixed classification and where clients connect at different clearances. This results in a large number of Security Levels and a need for strong isolation all on a single system.

This type of scenario is the reason that SELinux includes MLS as a security model, as an adjunct to TE.