1.\" Copyright (c) 2002-2004 Networks Associates Technology, Inc. 2.\" All rights reserved. 3.\" 4.\" This software was developed for the FreeBSD Project by Chris Costello 5.\" at Safeport Network Services and Network Associates Laboratories, the 6.\" Security Research Division of Network Associates, Inc. under 7.\" DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the 8.\" DARPA CHATS research program. 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.Dd November 18, 2002 32.Dt MAC_BIBA 4 33.Os 34.Sh NAME 35.Nm mac_biba 36.Nd "Biba data integrity policy" 37.Sh SYNOPSIS 38To compile Biba into your kernel, place the following lines in your kernel 39configuration file: 40.Bd -ragged -offset indent 41.Cd "options MAC" 42.Cd "options MAC_BIBA" 43.Ed 44.Pp 45Alternately, to load the Biba module at boot time, place the following line 46in your kernel configuration file: 47.Bd -ragged -offset indent 48.Cd "options MAC" 49.Ed 50.Pp 51and in 52.Xr loader.conf 5 : 53.Bd -literal -offset indent 54mac_biba_load="YES" 55.Ed 56.Sh DESCRIPTION 57The 58.Nm 59policy module implements the Biba integrity model, 60which protects the integrity of system objects and subjects by means of 61a strict information flow policy. 62In Biba, all system subjects and objects are assigned integrity labels, made 63up of hierarchal grades, and non-hierarchal components. 64Together, these label elements permit all labels to be placed in a partial 65order, with information flow protections based on a dominance operator 66describing the order. 67The hierarchal grade field is expressed as a value between 0 and 65535, 68with higher values reflecting higher integrity. 69The non-hierarchal compartment field is expressed as a set of up to 256 70components, numbered from 0 to 255. 71A complete label consists of both hierarchal and non-hierarchal elements. 72.Pp 73Three special label values exist: 74.Bl -column -offset indent ".Li biba/equal" "lower than all other labels" 75.It Sy Label Ta Sy Comparison 76.It Li biba/low Ta "lower than all other labels" 77.It Li biba/equal Ta "equal to all other labels" 78.It Li biba/high Ta "higher than all other labels" 79.El 80.Pp 81The 82.Dq Li biba/high 83label is assigned to system objects which affect the integrity of the system 84as a whole. 85The 86.Dq Li biba/equal 87label 88may be used to indicate that a particular subject or object is exempt from 89the Biba protections. 90These special label values are not specified as containing any compartments, 91although in a label comparison, 92.Dq Li biba/high 93appears to contain all compartments, 94.Dq Li biba/equal 95the same compartments as the other label to which it is being compared, 96and 97.Dq Li biba/low 98none. 99.Pp 100In general, Biba access control takes the following model: 101.Bl -bullet 102.It 103A subject at the same integrity level as an object may both read from 104and write to the object as though Biba protections were not in place. 105.It 106A subject at a higher integrity level than an object may write to the object, 107but not read the object. 108.It 109A subject at a lower integrity level than an object may read the object, 110but not write to the object. 111.It 112If the subject and object labels may not be compared in the partial order, 113all access is restricted. 114.El 115.Pp 116These rules prevent subjects of lower integrity from influencing the 117behavior of higher integrity subjects by preventing the flow of information, 118and hence control, from allowing low integrity subjects to modify either 119a high integrity object or high integrity subjects acting on those objects. 120Biba integrity policies may be appropriate in a number of environments, 121both from the perspective of preventing corruption of the operating system, 122and corruption of user data if marked as higher integrity than the attacker. 123In traditional trusted operating systems, the Biba integrity model is used 124to protect the Trusted Code Base (TCB). 125.Pp 126The Biba integrity model is similar to 127.Xr mac_lomac 4 , 128with the exception that LOMAC permits access by a higher integrity subject 129to a lower integrity object, but downgrades the integrity level of the subject 130to prevent integrity rules from being violated. 131Biba is a fixed label policy in that all subject and object label changes are 132explicit, whereas LOMAC is a floating label policy. 133.Pp 134The Biba integrity model is also similar to 135.Xr mac_mls 4 , 136with the exception that the dominance operator and access rules are reversed, 137preventing the downward flow of information rather than the upward flow of 138information. 139Multi-Level Security (MLS) protects the confidentiality, rather than the 140integrity, of subjects and objects. 141.Ss Label Format 142Almost all system objects are tagged with an effective, active label element, 143reflecting the integrity of the object, or integrity of the data contained 144in the object. 145In general, objects labels are represented in the following form: 146.Pp 147.Sm off 148.D1 Li biba / Ar grade : compartments 149.Sm on 150.Pp 151For example: 152.Bd -literal -offset indent 153biba/10:2+3+6 154biba/low 155.Ed 156.Pp 157Subject labels consist of three label elements: an effective (active) label, 158as well as a range of available labels. 159This range is represented using two ordered Biba label elements, and when set 160on a process, permits the process to change its active label to any label of 161greater or equal integrity to the low end of the range, and lesser or equal 162integrity to the high end of the range. 163In general, subject labels are represented in the following form: 164.Pp 165.Sm off 166.D1 Li biba / Ar effectivegrade : effectivecompartments ( lograde : locompartments - 167.D1 Ar higrade : hicompartments ) 168.Sm on 169.Pp 170For example: 171.Bd -literal -offset indent 172biba/10:2+3+6(5:2+3-20:2+3+4+5+6) 173biba/high(low-high) 174.Ed 175.Pp 176Valid ranged labels must meet the following requirement regarding their 177elements: 178.Pp 179.D1 Ar rangehigh No \[>=] Ar effective No \[>=] Ar rangelow 180.Pp 181One class of objects with ranges currently exists, the network interface. 182In the case of the network interface, the effective label element references the 183default label for packets received over the interface, and the range 184represents the range of acceptable labels of packets to be transmitted over 185the interface. 186.Ss Runtime Configuration 187The following 188.Xr sysctl 8 189MIBs are available for fine-tuning the enforcement of this MAC policy. 190.Bl -tag -width ".Va security.mac.biba.ptys_equal" 191.It Va security.mac.biba.enabled 192Enables enforcement of the Biba integrity policy. 193(Default: 1). 194.It Va security.mac.biba.ptys_equal 195Label 196.Xr pty 4 Ns s 197as 198.Dq Li biba/equal 199upon creation. 200(Default: 0). 201.It Va security.mac.biba.revocation_enabled 202Revoke access to objects if the label is changed to dominate the subject. 203(Default: 0). 204.El 205.Sh SEE ALSO 206.Xr mac 4 , 207.Xr mac_bsdextended 4 , 208.Xr mac_ifoff 4 , 209.Xr mac_lomac 4 , 210.Xr mac_mls 4 , 211.Xr mac_none 4 , 212.Xr mac_partition 4 , 213.Xr mac_portacl 4 , 214.Xr mac_seeotheruids 4 , 215.Xr mac_test 4 , 216.Xr maclabel 7 , 217.Xr mac 9 218.Sh HISTORY 219The 220.Nm 221policy module first appeared in 222.Fx 5.0 223and was developed by the 224.Tn TrustedBSD 225Project. 226.Sh AUTHORS 227This software was contributed to the 228.Fx 229Project by Network Associates Labs, 230the Security Research Division of Network Associates 231Inc. 232under DARPA/SPAWAR contract N66001-01-C-8035 233.Pq Dq CBOSS , 234as part of the DARPA CHATS research program. 235