mac_biba -- Biba data integrity policy
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"
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 nonhierarchal
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:
+o 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.
+o A subject at a higher integrity level than an object may write to the
object, but not read the object.
+o A subject at a lower integrity level than an object may read the
object, but not write to the object.
+o 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 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 [Toc] [Back]
Almost all system objects are tagged with a single, 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: a single (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/singlegrade:singlecompartments(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 >= single >= rangelow
One class of objects with ranges currently exists, the network interface.
In the case of the network interface, the single 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 [Toc] [Back]
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).
lomac(4), 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.
FreeBSD 5.2.1 November 18, 2002 FreeBSD 5.2.1 [ Back ] |