||FreeBSD Kernel Interfaces Manual
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:
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
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, “
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:
|dominated by all other labels
|equal to all other labels
|dominates all other labels
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
- 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
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
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
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
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.
MIBs are available for fine-tuning the enforcement of this MAC policy.
- Enables the enforcement of the MLS confidentiality policy. (Default:
mls/equal” upon creation.
- Revoke access to objects if the label is changed to a more sensitive level
than the subject. (Default: 0).
mac_mls policy relies on superuser status
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
mac_mls policy module first appeared in
FreeBSD 5.0 and was developed by the TrustedBSD
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.
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.
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