.\" Copyright (c) 2002-2004 Networks Associates Technology, Inc.
.\" All rights reserved.
.\"
.\" This software was developed for the FreeBSD Project by Chris Costello
.\" at Safeport Network Services and 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.
.\"
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.\" modification, are permitted provided that the following conditions
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.Dd July 25, 2015
.Dt MAC_MLS 4
.Os
.Sh NAME
.Nm mac_mls
.Nd "Multi-Level Security confidentiality policy"
.Sh SYNOPSIS
To compile MLS into your kernel, place the following lines in your kernel
configuration file:
.Bd -ragged -offset indent
.Cd "options MAC"
.Cd "options MAC_MLS"
.Ed
.Pp
Alternately, to load the MLS module at boot time, place the following line
in your kernel configuration file:
.Bd -ragged -offset indent
.Cd "options MAC"
.Ed
.Pp
and in
.Xr loader.conf 5 :
.Bd -literal -offset indent
mac_mls_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
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.
.Pp
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.
.Pp
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,
.Dq Li lower
is defined as being dominated by the label to which it is being compared,
and
.Dq Li higher
is defined as dominating the label to which it is being compared,
and
.Dq Li equal
is defined as both labels being able to satisfy the dominance requirements
over one another.
.Pp
Three special label values exist:
.Bl -column -offset indent ".Li mls/equal" "dominated by all other labels"
.It Sy Label Ta Sy Comparison
.It Li mls/low Ta "dominated by all other labels"
.It Li mls/equal Ta "equal to all other labels"
.It Li mls/high Ta "dominates all other labels"
.El
.Pp
The
.Dq Li mls/equal
label may be applied to subjects and objects for which no enforcement of the
MLS security policy is desired.
.Pp
The MLS model enforces the following basic restrictions:
.Bl -bullet
.It
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.
.It
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)
.It
Subjects may not write to objects with a lower classification level than
its own clearance level.
.It
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.
.El
.Pp
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
.Xr ( mac_biba 4 )
in order to protect the Trusted Code Base (TCB).
.Ss 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:
.Pp
.Sm off
.D1 Li mls / Ar grade : compartments
.Sm on
.Pp
For example:
.Bd -literal -offset indent
mls/10:2+3+6
mls/low
.Ed
.Pp
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:
.Pp
.Sm off
.D1 Li mls / Ar effectivegrade : effectivecompartments ( lograde : locompartments No -
.D1 Ar higrade : hicompartments )
.Sm on
.Pp
For example:
.Bd -literal -offset indent
mls/10:2+3+6(5:2+3-20:2+3+4+5+6)
mls/high(low-high)
.Ed
.Pp
Valid ranged labels must meet the following requirement regarding their
elements:
.Pp
.D1 Ar rangehigh No \[>=] Ar effective No \[>=] Ar rangelow
.Pp
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.
.Ss Runtime Configuration
The following
.Xr sysctl 8
MIBs are available for fine-tuning the enforcement of this MAC policy.
.Bl -tag -width ".Va security.mac.mls.ptys_equal"
.It Va security.mac.mls.enabled
Enables the enforcement of the MLS confidentiality policy.
(Default: 1).
.It Va security.mac.mls.ptys_equal
Label
.Xr pty 4 Ns s
as
.Dq Li mls/equal
upon creation.
(Default: 0).
.It Va security.mac.mls.revocation_enabled
Revoke access to objects if the label is changed to a more sensitive
level than the subject.
(Default: 0).
.El
.Sh IMPLEMENTATION NOTES
Currently, the
.Nm
policy relies on superuser status
.Pq Xr 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.
.Sh SEE ALSO
.Xr mac 4 ,
.Xr mac_biba 4 ,
.Xr mac_bsdextended 4 ,
.Xr mac_ddb 4 ,
.Xr mac_ifoff 4 ,
.Xr mac_lomac 4 ,
.Xr mac_none 4 ,
.Xr mac_partition 4 ,
.Xr mac_portacl 4 ,
.Xr mac_seeotheruids 4 ,
.Xr mac_test 4 ,
.Xr maclabel 7 ,
.Xr mac 9
.Sh HISTORY
The
.Nm
policy module first appeared in
.Fx 5.0
and was developed by the
.Tn TrustedBSD
Project.
.Sh AUTHORS
This software was contributed to the
.Fx
Project by Network Associates Laboratories,
the Security Research Division of Network Associates
Inc.\& under DARPA/SPAWAR contract N66001-01-C-8035
.Pq Dq CBOSS ,
as part of the DARPA CHATS research program.
.Sh BUGS
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.