238 lines
8.2 KiB
Groff
238 lines
8.2 KiB
Groff
.\" Copyright (c) 2002 Networks Associates Technology, Inc.
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.\" All rights reserved.
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.\"
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.\" This software was developed for the FreeBSD Project by Chris Costello
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.\" at Safeport Network Services and Network Associates Laboratories, the
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.\" Security Research Division of Network Associates, Inc. under
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.\" DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
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.\" DARPA CHATS research program.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd November 18, 2002
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.Os
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.Dt MAC_BIBA 4
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.Sh NAME
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.Nm mac_biba
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.Nd "Biba data integrity policy"
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.Sh SYNOPSIS
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To compile Biba into your kernel, place the following lines in your kernel
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configuration file:
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.Bd -ragged -offset indent
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.Cd "options MAC"
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.Cd "options MAC_BIBA"
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.Ed
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.Pp
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Alternately, to load the Biba module at boot time, place the following line
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in your kernel configuration file:
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.Bd -ragged -offset indent
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.Cd "options MAC"
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.Ed
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.Pp
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and in
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.Xr loader.conf 5 :
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.Bd -literal -offset indent
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mac_biba_load="YES"
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.Ed
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.Sh DESCRIPTION
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The
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.Nm
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policy module implements the Biba integrity model,
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which protects the integrity of system objects and subjects by means of
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a strict information flow policy.
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In Biba, all system subjects and objects are assigned integrity labels, made
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up of hierarchal grades, and non-hierarchal components.
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Together, these label elements permit all labels to be placed in a partial
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order, with information flow protections based on a dominance operator
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describing the order.
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The hierarchal grade field is expressed as a value between 0 and 65535,
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with higher values reflecting higher integrity.
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The non-hierarchal compartment field is expressed as a set of up to 256
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components, numbered from 0 to 255.
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A complete label consists of both hierarchal and non-hierarchal elements.
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.Pp
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Three special label values exist:
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.Bl -column -offset indent ".Li biba/equal" "lower than all other labels"
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.It Sy Label Ta Sy Comparison
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.It Li biba/low Ta "lower than all other labels"
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.It Li biba/equal Ta "equal to all other labels"
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.It Li biba/high Ta "higher than all other labels"
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.El
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.Pp
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The
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.Dq Li biba/high
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label is assigned to system objects which affect the integrity of the system
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as a whole.
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The
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.Dq Li biba/equal
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label
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may be used to indicate that a particular subject or object is exempt from
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the Biba protections.
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These special label values are not specified as containing any compartments,
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although in a label comparison,
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.Dq Li biba/high
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appears to contain all compartments,
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.Dq Li biba/equal
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the same compartments as the other label to which it is being compared,
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and
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.Dq Li biba/low
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none.
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.Pp
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In general, Biba access control takes the following model:
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.Bl -bullet
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.It
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A subject at the same integrity level as an object may both read from
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and write to the object as though Biba protections were not in place.
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.It
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A subject at a higher integrity level than an object may write to the object,
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but not read the object.
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.It
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A subject at a lower integrity level than an object may read the object,
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but not write to the object.
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.It
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If the subject and object labels may not be compared in the partial order,
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all access is restricted.
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.El
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.Pp
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These rules prevent subjects of lower integrity from influencing the
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behavior of higher integrity subjects by preventing the flow of information,
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and hence control, from allowing low integrity subjects to modify either
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a high integrity object or high integrity subjects acting on those objects.
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Biba integrity policies may be appropriate in a number of environments,
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both from the perspective of preventing corruption of the operating system,
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and corruption of user data if marked as higher integrity than the attacker.
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In traditional trusted operating systems, the Biba integrity model is used
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to protect the Trusted Code Base (TCB).
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.Pp
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The Biba integrity model is similar to
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.Xr lomac 4 ,
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with the exception that LOMAC permits access by a higher integrity subject
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to a lower integrity object, but downgrades the integrity level of the subject
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to prevent integrity rules from being violated.
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Biba is a fixed label policy in that all subject and object label changes are
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explicit, whereas LOMAC is a floating label policy.
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.Pp
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The Biba integrity model is also similar to
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.Xr mac_mls 4 ,
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with the exception that the dominance operator and access rules are reversed,
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preventing the downward flow of information rather than the upward flow of
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information.
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Multi-Level Security (MLS) protects the confidentiality, rather than the
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integrity, of subjects and objects.
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.Ss Label Format
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Almost all system objects are tagged with a single, active label element,
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reflecting the integrity of the object, or integrity of the data contained
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in the object.
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In general, objects labels are represented in the following form:
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.Pp
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.Sm off
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.D1 Li biba / Ar grade : compartments
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.Sm on
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.Pp
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For example:
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.Pp
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.Bd -literal -offset indent
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biba/10:2+3+6
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biba/low
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.Ed
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.Pp
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Subject labels consist of three label elements: a single (active) label,
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as well as a range of available labels.
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This range is represented using two ordered Biba label elements, and when set
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on a process, permits the process to change its active label to any label of
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greater or equal integrity to the low end of the range, and lesser or equal
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integrity to the high end of the range.
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In general, subject labels are represented in the following form:
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.Pp
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.Sm off
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.D1 Li biba / Ar singlegrade : singlecompartments ( lograde : locompartments -
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.D1 Ar higrade : hicompartments )
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.Sm on
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.Pp
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For example:
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.Bd -literal -offset indent
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biba/10:2+3+6(5:2+3-20:2+3+4+5+6)
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biba/high(low-high)
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.Ed
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.Pp
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Valid ranged labels must meet the following requirement regarding their
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elements:
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.Pp
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.D1 Ar rangehigh No \[>=] Ar single No \[>=] Ar rangelow
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.Pp
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One class of objects with ranges currently exists, the network interface.
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In the case of the network interface, the single label element references the
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default label for packets received over the interface, and the range
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represents the range of acceptable labels of packets to be transmitted over
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the interface.
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.Ss Runtime Configuration
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The following
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.Xr sysctl 8
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MIBs are available for fine-tuning the enforcement of this MAC policy.
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.Bl -tag -width ".Va security.mac.biba.ptys_equal"
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.It Va security.mac.biba.enabled
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Enables enforcement of the Biba integrity policy.
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(Default: 1).
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.It Va security.mac.biba.ptys_equal
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Label
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.Xr pty 4 Ns s
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as
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.Dq Li biba/equal
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upon creation.
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(Default: 0).
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.It Va security.mac.biba.revocation_enabled
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Revoke access to objects if the label is changed to dominate the subject.
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(Default: 0).
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.El
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.Sh SEE ALSO
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.Xr lomac 4 ,
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.Xr mac 4 ,
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.Xr mac_bsdextended 4 ,
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.Xr mac_ifoff 4 ,
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.Xr mac_lomac 4 ,
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.Xr mac_mls 4 ,
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.Xr mac_none 4 ,
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.Xr mac_partition 4 ,
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.Xr mac_portacl 4 ,
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.Xr mac_seeotheruids 4 ,
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.Xr mac_test 4 ,
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.Xr maclabel 7 ,
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.Xr mac 9
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.Sh HISTORY
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The
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.Nm
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policy module first appeared in
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.Fx 5.0
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and was developed by the
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.Tn TrustedBSD
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Project.
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.Sh AUTHORS
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This software was contributed to the
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.Fx
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Project by Network Associates Labs,
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the Security Research Division of Network Associates
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Inc. under DARPA/SPAWAR contract N66001-01-C-8035
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.Pq Dq CBOSS ,
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as part of the DARPA CHATS research program.
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