15.12 The MAC Biba Module

Module name: mac_biba.ko

Kernel configuration line: options MAC_BIBA

Boot option: mac_biba_load="YES"

The mac_biba(4) module loads the MAC Biba policy. This policy works much like that of the MLS policy with the exception that the rules for information flow are slightly reversed. This is said to prevent the downward flow of sensitive information whereas the MLS policy prevents the upward flow of sensitive information; thus, much of this section can apply to both policies.

In Biba environments, an “integrity” label is set on each subject or object. These labels are made up of hierarchal grades, and non-hierarchal components. As an object's or subject's grade ascends, so does its integrity.

Supported labels are biba/low, biba/equal, and biba/high; as explained below:

• The biba/low label is considered the lowest integrity an object or subject may have. Setting this on objects or subjects will block their write access to objects or subjects marked high. They still have read access though.

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

• The biba/high label will permit writing to objects set at a lower label, but not permit reading that object. It is recommended that this label be placed on objects that affect the integrity of the entire system.

Biba provides for:

• Hierarchical integrity level with a set of non hierarchical integrity categories;

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

• Integrity (preventing inappropriate modification of data);

• Integrity levels (instead of MLS sensitivity levels).

The following sysctl tunables can be used to manipulate the Biba policy.

• security.mac.biba.enabled may be used to enable/disable enforcement of the Biba policy on the target machine.

• security.mac.biba.ptys_equal may be used to disable the Biba policy on pty(4) devices.

• security.mac.biba.revocation_enabled will force the revocation of access to objects if the label is changed to dominate the subject.

To access the Biba policy setting on system objects, use the setfmac and getfmac commands:

# setfmac biba/low test
# getfmac test
test: biba/low

Observations: a lower integrity subject is unable to write to a higher integrity subject; a higher integrity subject cannot observe or read a lower integrity object.