16.11. The MAC Multi-Level Security Module

Module name: mac_mls.ko

Kernel configuration line: options MAC_MLS

Boot option: mac_mls_load="YES"

The mac_mls(4) policy controls access between subjects and objects in the system by enforcing a strict information flow policy.

In MLS environments, a “clearance” level is set in each subject or objects label, along with compartments. Since these clearance or sensibility levels can reach numbers greater than six thousand; it would be a daunting task for any system administrator to thoroughly configure each subject or object. Thankfully, three “instant” labels are already included in this policy.

These labels are mls/low, mls/equal and mls/high. Since these labels are described in depth in the manual page, they will only get a brief description here:

• The mls/low label contains a low configuration which permits it to be dominated by all other objects. Anything labeled with mls/low will have a low clearance level and not be permitted to access information of a higher level. In addition, this label will prevent objects of a higher clearance level from writing or passing information on to them.

• The mls/equal label should be placed on objects considered to be exempt from the policy.

• The mls/high label is the highest level of clearance possible. Objects assigned this label will hold dominance over all other objects in the system; however, they will not permit the leaking of information to objects of a lower class.

MLS provides for:

• A hierarchical security level with a set of non hierarchical categories;

• Fixed rules: no read up, no write down (a subject can have read access to objects on its own level or below, but not above. Similarly, a subject can have write access to objects on its own level or above but not beneath.);

• Secrecy (preventing inappropriate disclosure of data);

• Basis for the design of systems that concurrently handle data at multiple sensitivity levels (without leaking information between secret and confidential).

The following sysctl tunables are available for the configuration of special services and interfaces:

• security.mac.mls.enabled is used to enable/disable the MLS policy.

• security.mac.mls.ptys_equal will label all pty(4) devices as mls/equal during creation.

• security.mac.mls.revocation_enabled is used to revoke access to objects after their label changes to a label of a lower grade.

• security.mac.mls.max_compartments is used to set the maximum number of compartment levels with objects; basically the maximum compartment number allowed on a system.

To manipulate the MLS labels, the setfmac(8) command has been provided. To assign a label to an object, issue the following command:

# setfmac mls/5 test

To get the MLS label for the file test issue the following command:

# getfmac test

This is a summary of the MLS policy's features. Another approach is to create a master policy file in /etc which specifies the MLS policy information and to feed that file into the setfmac command. This method will be explained after all policies are covered.

Observations: an object with lower clearance is unable to observe higher clearance processes. A basic policy would be to enforce mls/high on everything not to be read, even if it needs to be written. Enforce mls/low on everything not to be written, even if it needs to be read. And finally enforce mls/equal on the rest. All users marked insecure should be set at mls/low.