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.\" $FreeBSD$ 32.\" 33.Dd July 25, 2015 34.Dt MAC_MLS 4 35.Os 36.Sh NAME 37.Nm mac_mls 38.Nd "Multi-Level Security confidentiality policy" 39.Sh SYNOPSIS 40To compile MLS into your kernel, place the following lines in your kernel 41configuration file: 42.Bd -ragged -offset indent 43.Cd "options MAC" 44.Cd "options MAC_MLS" 45.Ed 46.Pp 47Alternately, to load the MLS module at boot time, place the following line 48in your kernel configuration file: 49.Bd -ragged -offset indent 50.Cd "options MAC" 51.Ed 52.Pp 53and in 54.Xr loader.conf 5 : 55.Bd -literal -offset indent 56mac_mls_load="YES" 57.Ed 58.Sh DESCRIPTION 59The 60.Nm 61policy module implements the Multi-Level Security, or MLS model, 62which controls access between subjects and objects based on their 63confidentiality by means of a strict information flow policy. 64Each subject and object in the system has an MLS label associated with it; 65each subject's MLS label contains information on its clearance level, 66and each object's MLS label contains information on its classification. 67.Pp 68In MLS, all system subjects and objects are assigned confidentiality labels, 69made up of a sensitivity level and zero or more compartments. 70Together, these label elements permit all labels to be placed in a partial 71order, with confidentiality protections based on a dominance operator 72describing the order. 73The sensitivity level is expressed as a value between 0 and 7465535, with higher values reflecting higher sensitivity levels. 75The compartment field is expressed as a set of up to 256 components, 76numbered from 1 to 256. 77A complete label consists of both sensitivity and compartment 78elements. 79.Pp 80With normal labels, dominance is defined as a label having a higher 81or equal active sensitivity level, and having at least 82all of the same compartments as the label to which it is being compared. 83With respect to label comparisons, 84.Dq Li lower 85is defined as being dominated by the label to which it is being compared, 86and 87.Dq Li higher 88is defined as dominating the label to which it is being compared, 89and 90.Dq Li equal 91is defined as both labels being able to satisfy the dominance requirements 92over one another. 93.Pp 94Three special label values exist: 95.Bl -column -offset indent ".Li mls/equal" "dominated by all other labels" 96.It Sy Label Ta Sy Comparison 97.It Li mls/low Ta "dominated by all other labels" 98.It Li mls/equal Ta "equal to all other labels" 99.It Li mls/high Ta "dominates all other labels" 100.El 101.Pp 102The 103.Dq Li mls/equal 104label may be applied to subjects and objects for which no enforcement of the 105MLS security policy is desired. 106.Pp 107The MLS model enforces the following basic restrictions: 108.Bl -bullet 109.It 110Subjects may not observe the processes of another subject if its 111clearance level is lower than the clearance level of the object it is 112attempting to observe. 113.It 114Subjects may not read, write, or otherwise observe objects without proper 115clearance (e.g.\& subjects may not observe objects whose classification label 116dominates its own clearance label) 117.It 118Subjects may not write to objects with a lower classification level than 119its own clearance level. 120.It 121A subject may read and write to an object if its clearance level is equal 122to the object's classification level as though MLS protections were not in 123place. 124.El 125.Pp 126These rules prevent subjects of lower clearance from gaining access 127information classified beyond its clearance level in order to protect the 128confidentiality of classified information, subjects of higher clearance 129from writing to objects of lower classification in order to prevent the 130accidental or malicious leaking of information, and subjects of lower 131clearance from observing subjects of higher clearance altogether. 132In traditional trusted operating systems, the MLS confidentiality model is 133used in concert with the Biba integrity model 134.Xr ( mac_biba 4 ) 135in order to protect the Trusted Code Base (TCB). 136.Ss Label Format 137Almost all system objects are tagged with an effective, active label element, 138reflecting the classification of the object, or classification of the data 139contained in the object. 140In general, object labels are represented in the following form: 141.Pp 142.Sm off 143.D1 Li mls / Ar grade : compartments 144.Sm on 145.Pp 146For example: 147.Bd -literal -offset indent 148mls/10:2+3+6 149mls/low 150.Ed 151.Pp 152Subject labels consist of three label elements: an effective (active) label, 153as well as a range of available labels. 154This range is represented using two ordered MLS label elements, and when set 155on a process, permits the process to change its active label to any label of 156greater or equal integrity to the low end of the range, and lesser or equal 157integrity to the high end of the range. 158In general, subject labels are represented in the following form: 159.Pp 160.Sm off 161.D1 Li mls / Ar effectivegrade : effectivecompartments ( lograde : locompartments No - 162.D1 Ar higrade : hicompartments ) 163.Sm on 164.Pp 165For example: 166.Bd -literal -offset indent 167mls/10:2+3+6(5:2+3-20:2+3+4+5+6) 168mls/high(low-high) 169.Ed 170.Pp 171Valid ranged labels must meet the following requirement regarding their 172elements: 173.Pp 174.D1 Ar rangehigh No \[>=] Ar effective No \[>=] Ar rangelow 175.Pp 176One class of objects with ranges currently exists, the network interface. 177In the case of the network interface, the effective label element references 178the default label for packets received over the interface, and the range 179represents the range of acceptable labels of packets to be transmitted over 180the interface. 181.Ss Runtime Configuration 182The following 183.Xr sysctl 8 184MIBs are available for fine-tuning the enforcement of this MAC policy. 185.Bl -tag -width ".Va security.mac.mls.ptys_equal" 186.It Va security.mac.mls.enabled 187Enables the enforcement of the MLS confidentiality policy. 188(Default: 1). 189.It Va security.mac.mls.ptys_equal 190Label 191.Xr pty 4 Ns s 192as 193.Dq Li mls/equal 194upon creation. 195(Default: 0). 196.It Va security.mac.mls.revocation_enabled 197Revoke access to objects if the label is changed to a more sensitive 198level than the subject. 199(Default: 0). 200.El 201.Sh IMPLEMENTATION NOTES 202Currently, the 203.Nm 204policy relies on superuser status 205.Pq Xr suser 9 206in order to change network interface MLS labels. 207This will eventually go away, but it is currently a liability and may 208allow the superuser to bypass MLS protections. 209.Sh SEE ALSO 210.Xr mac 4 , 211.Xr mac_biba 4 , 212.Xr mac_bsdextended 4 , 213.Xr mac_ifoff 4 , 214.Xr mac_lomac 4 , 215.Xr mac_none 4 , 216.Xr mac_partition 4 , 217.Xr mac_portacl 4 , 218.Xr mac_seeotheruids 4 , 219.Xr mac_test 4 , 220.Xr maclabel 7 , 221.Xr mac 9 222.Sh HISTORY 223The 224.Nm 225policy module first appeared in 226.Fx 5.0 227and was developed by the 228.Tn TrustedBSD 229Project. 230.Sh AUTHORS 231This software was contributed to the 232.Fx 233Project by Network Associates Laboratories, 234the Security Research Division of Network Associates 235Inc.\& under DARPA/SPAWAR contract N66001-01-C-8035 236.Pq Dq CBOSS , 237as part of the DARPA CHATS research program. 238.Sh BUGS 239While the MAC Framework design is intended to support the containment of 240the root user, not all attack channels are currently protected by entry 241point checks. 242As such, MAC Framework policies should not be relied on, in isolation, 243to protect against a malicious privileged user. 244