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     mac_mls — Multi-Level Security confidentiality policy


     To compile MLS into your kernel, place the following lines in your kernel configuration

           options MAC
           options MAC_MLS

     Alternately, to load the MLS module at boot time, place the following line in your kernel
     configuration file:

           options MAC

     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:

           Label        Comparison
           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).

   Label Format
     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:


     For example:


     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:


     For example:


     Valid ranged labels must meet the following requirement regarding their elements:


     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.     Enables the enforcement of the MLS confidentiality policy.
                                  (Default: 1).  Label pty(4)s as “mls/equal” upon creation.  (Default: 0).
                                  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_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


     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