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mac_mls — Multi-Level Security confidentiality policy
To compile MLS into your kernel, place the following lines in your kernel
Alternately, to load the MLS module at boot time, place the following
line in your kernel configuration file:
and in loader.conf(5):
The mac_mls policy module implements the Multi-Level Security, or MLS
model, which controls access between subjects and objects based on their
confidentiality by means of a strict information flow policy. Each
subject and object in the system has an MLS label associated with it;
each subject's MLS label contains information on its clearance level, and
each object's MLS label contains information on its classification.
In MLS, all system subjects and objects are assigned confidentiality
labels, made up of a sensitivity level and zero or more compartments.
Together, these label elements permit all labels to be placed in a
partial order, with confidentiality protections based on a dominance
operator describing the order. The sensitivity level is expressed as a
value between 0 and 65535, with higher values reflecting higher
sensitivity levels. The compartment field is expressed as a set of up to
256 components, numbered from 1 to 256. A complete label consists of
both sensitivity and compartment elements.
With normal labels, dominance is defined as a label having a higher or
equal active sensitivity level, and having at least all of the same
compartments as the label to which it is being compared. With respect to
label comparisons, “lower” is defined as being dominated by the label to
which it is being compared, and “higher” is defined as dominating the
label to which it is being compared, and “equal” is defined as both
labels being able to satisfy the dominance requirements over one another.
Three special label values exist:
mls/low dominated by all other labels
mls/equal equal to all other labels
mls/high dominates all other labels
The “mls/equal” label may be applied to subjects and objects for which no
enforcement of the MLS security policy is desired.
The MLS model enforces the following basic restrictions:
· Subjects may not observe the processes of another subject if its
clearance level is lower than the clearance level of the object it is
attempting to observe.
· Subjects may not read, write, or otherwise observe objects without
proper clearance (e.g. subjects may not observe objects whose
classification label dominates its own clearance label)
· Subjects may not write to objects with a lower classification level
than its own clearance level.
· A subject may read and write to an object if its clearance level is
equal to the object's classification level as though MLS protections
were not in place.
These rules prevent subjects of lower clearance from gaining access
information classified beyond its clearance level in order to protect the
confidentiality of classified information, subjects of higher clearance
from writing to objects of lower classification in order to prevent the
accidental or malicious leaking of information, and subjects of lower
clearance from observing subjects of higher clearance altogether. In
traditional trusted operating systems, the MLS confidentiality model is
used in concert with the Biba integrity model (mac_biba(4)) in order to
protect the Trusted Code Base (TCB).
Almost all system objects are tagged with an effective, active label
element, reflecting the classification of the object, or classification
of the data contained in the object. In general, object 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 MLS 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.mls.enabled Enables the enforcement of the MLS
confidentiality policy. (Default: 1).
security.mac.mls.ptys_equal Label pty(4)s as “mls/equal” upon creation.
Revoke access to objects if the label is
changed to a more sensitive level than the
subject. (Default: 0).
Currently, the mac_mls policy relies on superuser status (suser(9)) in
order to change network interface MLS labels. This will eventually go
away, but it is currently a liability and may allow the superuser to
bypass MLS protections.
mac(4), mac_biba(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_mls 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 Laboratories, 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.
See mac(9) concerning appropriateness for production use. The TrustedBSD
MAC Framework is considered experimental in FreeBSD.
While the MAC Framework design is intended to support the containment of
the root user, not all attack channels are currently protected by entry
point checks. As such, MAC Framework policies should not be relied on,
in isolation, to protect against a malicious privileged user.