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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.
Debian November 18, 2002 MAC_BIBA(4)