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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.