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mac_biba — Biba data integrity policy
To compile Biba into your kernel, place the following lines in your
kernel configuration file:
Alternately, to load the Biba module at boot time, place the following
line in your kernel configuration file:
and in loader.conf(5):
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:
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.
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:
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:
Valid ranged labels must meet the following requirement regarding their
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.
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).
Revoke access to objects if the label is
changed to dominate the subject. (Default:
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)
The mac_biba policy module first appeared in FreeBSD 5.0 and was
developed by the TrustedBSD Project.
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.