.\" 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.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
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.Dd November 18, 2002
.Dt MAC_BIBA 4
.Os
.Sh NAME
.Nm mac_biba
.Nd "Biba data integrity policy"
.Sh SYNOPSIS
To compile Biba into your kernel, place the following lines in your kernel
configuration file:
.Bd -ragged -offset indent
.Cd "options MAC"
.Cd "options MAC_BIBA"
.Ed
.Pp
Alternately, to load the Biba 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_biba_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
policy module implements the Biba integrity model,
which protects the integrity of system objects and subjects by means of
a strict information flow policy.
In Biba, all system subjects and objects are assigned integrity labels, made
up of hierarchal grades, and non-hierarchal components.
Together, these label elements permit all labels to be placed in a partial
order, with information flow protections based on a dominance operator
describing the order.
The hierarchal grade field is expressed as a value between 0 and 65535,
with higher values reflecting higher integrity.
The non-hierarchal compartment field is expressed as a set of up to 256
components, numbered from 0 to 255.
A complete label consists of both hierarchal and non-hierarchal elements.
.Pp
Three special label values exist:
.Bl -column -offset indent ".Li biba/equal" "lower than all other labels"
.It Sy Label Ta Sy Comparison
.It Li biba/low Ta "lower than all other labels"
.It Li biba/equal Ta "equal to all other labels"
.It Li biba/high Ta "higher than all other labels"
.El
.Pp
The
.Dq Li biba/high
label is assigned to system objects which affect the integrity of the system
as a whole.
The
.Dq Li biba/equal
label
may be used to indicate that a particular subject or object is exempt from
the Biba protections.
These special label values are not specified as containing any compartments,
although in a label comparison,
.Dq Li biba/high
appears to contain all compartments,
.Dq Li biba/equal
the same compartments as the other label to which it is being compared,
and
.Dq Li biba/low
none.
.Pp
In general, Biba access control takes the following model:
.Bl -bullet
.It
A subject at the same integrity level as an object may both read from
and write to the object as though Biba protections were not in place.
.It
A subject at a higher integrity level than an object may write to the object,
but not read the object.
.It
A subject at a lower integrity level than an object may read the object,
but not write to the object.
.It
If the subject and object labels may not be compared in the partial order,
all access is restricted.
.El
.Pp
These rules prevent subjects of lower integrity from influencing the
behavior of higher integrity subjects by preventing the flow of information,
and hence control, from allowing low integrity subjects to modify either
a high integrity object or high integrity subjects acting on those objects.
Biba integrity policies may be appropriate in a number of environments,
both from the perspective of preventing corruption of the operating system,
and corruption of user data if marked as higher integrity than the attacker.
In traditional trusted operating systems, the Biba integrity model is used
to protect the Trusted Code Base (TCB).
.Pp
The Biba integrity model is similar to
.Xr mac_lomac 4 ,
with the exception that LOMAC permits access by a higher integrity subject
to a lower integrity object, but downgrades the integrity level of the subject
to prevent integrity rules from being violated.
Biba is a fixed label policy in that all subject and object label changes are
explicit, whereas LOMAC is a floating label policy.
.Pp
The Biba integrity model is also similar to
.Xr mac_mls 4 ,
with the exception that the dominance operator and access rules are reversed,
preventing the downward flow of information rather than the upward flow of
information.
Multi-Level Security (MLS) protects the confidentiality, rather than the
integrity, of subjects and objects.
.Ss Label Format
Almost all system objects are tagged with an effective, active label element,
reflecting the integrity of the object, or integrity of the data contained
in the object.
In general, objects labels are represented in the following form:
.Pp
.Sm off
.D1 Li biba / Ar grade : compartments
.Sm on
.Pp
For example:
.Bd -literal -offset indent
biba/10:2+3+6
biba/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 Biba 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 biba / Ar effectivegrade : effectivecompartments ( lograde : locompartments -
.D1 Ar higrade : hicompartments )
.Sm on
.Pp
For example:
.Bd -literal -offset indent
biba/10:2+3+6(5:2+3-20:2+3+4+5+6)
biba/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.biba.ptys_equal"
.It Va security.mac.biba.enabled
Enables enforcement of the Biba integrity policy.
(Default: 1).
.It Va security.mac.biba.ptys_equal
Label
.Xr pty 4 Ns s
as
.Dq Li biba/equal
upon creation.
(Default: 0).
.It Va security.mac.biba.revocation_enabled
Revoke access to objects if the label is changed to dominate the subject.
(Default: 0).
.El
.Sh SEE ALSO
.Xr mac 4 ,
.Xr mac_bsdextended 4 ,
.Xr mac_ifoff 4 ,
.Xr mac_lomac 4 ,
.Xr mac_mls 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 Labs,
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.