NAME

     mac_biba — Biba data integrity policy

SYNOPSIS

     To compile Biba into your kernel, place the following lines in your kernel configuration file:

           options MAC
           options MAC_BIBA

     Alternately, to load the Biba module at boot time, place the following line in your kernel configuration
     file:

           options MAC

     and in loader.conf(5):

           mac_biba_load="YES"

DESCRIPTION

     The mac_biba policy module implements the Biba integrity model, which protects the integrity of system
     objects and subjects by means of a strict information flow policy.  In Biba, all system subjects and
     objects are assigned integrity labels, made up of hierarchal grades, and non-hierarchal components.
     Together, these label elements permit all labels to be placed in a partial order, with information flow
     protections based on a dominance operator describing the order.  The hierarchal grade field is expressed as
     a value between 0 and 65535, with higher values reflecting higher integrity.  The non-hierarchal
     compartment field is expressed as a set of up to 256 components, numbered from 0 to 255.  A complete label
     consists of both hierarchal and non-hierarchal elements.

     Three special label values exist:

           Label         Comparison
           biba/low      lower than all other labels
           biba/equal    equal to all other labels
           biba/high     higher than all other labels

     The “biba/high” label is assigned to system objects which affect the integrity of the system as a whole.
     The “biba/equal” label may be used to indicate that a particular subject or object is exempt from the Biba
     protections.  These special label values are not specified as containing any compartments, although in a
     label comparison, “biba/high” appears to contain all compartments, “biba/equal” the same compartments as
     the other label to which it is being compared, and “biba/low” none.

     In general, Biba access control takes the following model:

     •   A subject at the same integrity level as an object may both read from and write to the object as though
         Biba protections were not in place.

     •   A subject at a higher integrity level than an object may write to the object, but not read the object.

     •   A subject at a lower integrity level than an object may read the object, but not write to the object.

     •   If the subject and object labels may not be compared in the partial order, all access is restricted.

     These rules prevent subjects of lower integrity from influencing the behavior of higher integrity subjects
     by preventing the flow of information, and hence control, from allowing low integrity subjects to modify
     either a high integrity object or high integrity subjects acting on those objects.  Biba integrity policies
     may be appropriate in a number of environments, both from the perspective of preventing corruption of the
     operating system, and corruption of user data if marked as higher integrity than the attacker.  In
     traditional trusted operating systems, the Biba integrity model is used to protect the Trusted Code Base
     (TCB).

     The Biba integrity model is similar to mac_lomac(4), with the exception that LOMAC permits access by a
     higher integrity subject to a lower integrity object, but downgrades the integrity level of the subject to
     prevent integrity rules from being violated.  Biba is a fixed label policy in that all subject and object
     label changes are explicit, whereas LOMAC is a floating label policy.

     The Biba integrity model is also similar to mac_mls(4), with the exception that the dominance operator and
     access rules are reversed, preventing the downward flow of information rather than the upward flow of
     information.  Multi-Level Security (MLS) protects the confidentiality, rather than the integrity, of
     subjects and objects.

   Label Format
     Almost all system objects are tagged with an effective, active label element, reflecting the integrity of
     the object, or integrity of the data contained in the object.  In general, objects labels are represented
     in the following form:

           biba/grade:compartments

     For example:

           biba/10:2+3+6
           biba/low

     Subject labels consist of three label elements: an effective (active) label, as well as a range of
     available labels.  This range is represented using two ordered Biba label elements, and when set on a
     process, permits the process to change its active label to any label of greater or equal integrity to the
     low end of the range, and lesser or equal integrity to the high end of the range.  In general, subject
     labels are represented in the following form:

           biba/effectivegrade:effectivecompartments(lograde:locompartments-
           higrade:hicompartments)

     For example:

           biba/10:2+3+6(5:2+3-20:2+3+4+5+6)
           biba/high(low-high)

     Valid ranged labels must meet the following requirement regarding their elements:

           rangehigh ≥ effective ≥ rangelow

     One class of objects with ranges currently exists, the network interface.  In the case of the network
     interface, the effective label element references the default label for packets received over the
     interface, and the range represents the range of acceptable labels of packets to be transmitted over the
     interface.

   Runtime Configuration
     The following sysctl(8) MIBs are available for fine-tuning the enforcement of this MAC policy.

     security.mac.biba.enabled     Enables enforcement of the Biba integrity policy.  (Default: 1).

     security.mac.biba.ptys_equal  Label pty(4)s as “biba/equal” upon creation.  (Default: 0).

     security.mac.biba.revocation_enabled
                                   Revoke access to objects if the label is changed to dominate the subject.
                                   (Default: 0).

SEE ALSO

     mac(4), mac_bsdextended(4), mac_ifoff(4), mac_lomac(4), mac_mls(4), mac_none(4), mac_partition(4),
     mac_portacl(4), mac_seeotheruids(4), mac_test(4), maclabel(7), mac(9)

HISTORY

     The mac_biba policy module first appeared in FreeBSD 5.0 and was developed by the TrustedBSD Project.

AUTHORS

     This software was contributed to the FreeBSD Project by Network Associates Labs, the Security Research
     Division of Network Associates Inc.  under DARPA/SPAWAR contract N66001-01-C-8035 (“CBOSS”), as part of the
     DARPA CHATS research program.