<?xml version="1.0" encoding="iso-8859-1"?>
<!--
The FreeBSD Documentation Project
$FreeBSD$
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<chapter id="mac">
<chapterinfo>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Written by </contrib>
</author>
</authorgroup>
</chapterinfo>
<title>Mandatory Access Control</title>
<sect1 id="mac-synopsis">
<title>Synopsis</title>
<indexterm><primary>MAC</primary></indexterm>
<indexterm>
<primary>Mandatory Access Control</primary>
<see>MAC</see>
</indexterm>
<para>&os; 5.X introduced new security extensions from the
TrustedBSD project based on the &posix;.1e draft. Two of the most
significant new security mechanisms are file system Access Control
Lists (<acronym>ACL</acronym>s) and Mandatory Access Control
(<acronym>MAC</acronym>) facilities. Mandatory Access Control allows
new access control modules to be loaded, implementing new security
policies. Some provide protections of a narrow subset of the
system, hardening a particular service. Others provide
comprehensive labeled security across all subjects and objects.
The mandatory part
of the definition comes from the fact that the enforcement of
the controls is done by administrators and the system, and is
not left up to the discretion of users as is done with
discretionary access control (<acronym>DAC</acronym>, the standard
file and System V <acronym>IPC</acronym> permissions on &os;).</para>
<para>This chapter will focus on the
Mandatory Access Control Framework (<acronym>MAC</acronym> Framework), and a set
of pluggable security policy modules enabling various security
mechanisms.</para>
<para>After reading this chapter, you will know:</para>
<itemizedlist>
<listitem>
<para>What <acronym>MAC</acronym> security policy modules are currently
included in &os; and their associated mechanisms.</para>
</listitem>
<listitem>
<para>What <acronym>MAC</acronym> security policy modules implement as
well as the difference between a labeled and non-labeled
policy.</para>
</listitem>
<listitem>
<para>How to efficiently configure a system to use
the <acronym>MAC</acronym> framework.</para>
</listitem>
<listitem>
<para>How to configure the different security policy modules included with the
<acronym>MAC</acronym> framework.</para>
</listitem>
<listitem>
<para>How to implement a more secure environment using the
<acronym>MAC</acronym> framework and the examples
shown.</para>
</listitem>
<listitem>
<para>How to test the <acronym>MAC</acronym> configuration
to ensure the framework has been properly implemented.</para>
</listitem>
</itemizedlist>
<para>Before reading this chapter, you should:</para>
<itemizedlist>
<listitem>
<para>Understand &unix; and &os; basics
(<xref linkend="basics"/>).</para>
</listitem>
<listitem>
<para>Be familiar with
the basics of kernel configuration/compilation
(<xref linkend="kernelconfig"/>).</para>
</listitem>
<listitem>
<para>Have some familiarity with security and how it
pertains to &os; (<xref linkend="security"/>).</para>
</listitem>
</itemizedlist>
<warning>
<para>The improper use of the
information contained herein may cause loss of system access,
aggravation of users, or inability to access the features
provided by X11. More importantly, <acronym>MAC</acronym> should not
be relied upon to completely secure a system. The
<acronym>MAC</acronym> framework only augments
existing security policy; without sound security practices and
regular security checks, the system will never be completely
secure.</para>
<para>It should also be noted that the examples contained
within this chapter are just that, examples. It is not
recommended that these particular settings be rolled out
on a production system. Implementing the various security policy modules takes
a good deal of thought and testing. One who does not fully understand
exactly how everything works may find him or herself going
back through the entire system and reconfiguring many files
or directories.</para>
</warning>
<sect2>
<title>What Will Not Be Covered</title>
<para>This chapter covers a broad range of security issues relating
to the <acronym>MAC</acronym> framework. The
development of new <acronym>MAC</acronym> security policy modules
will not be covered. A number of security policy modules included with the
<acronym>MAC</acronym> framework have specific characteristics
which are provided for both testing and new module
development. These include the &man.mac.test.4;,
&man.mac.stub.4; and &man.mac.none.4;.
For more information on these security policy modules and the various
mechanisms they provide, please review the manual pages.</para>
</sect2>
</sect1>
<sect1 id="mac-inline-glossary">
<title>Key Terms in This Chapter</title>
<para>Before reading this chapter, a few key terms must be
explained. This will hopefully clear up any confusion that
may occur and avoid the abrupt introduction of new terms
and information.</para>
<itemizedlist>
<listitem>
<para><emphasis>compartment</emphasis>: A compartment is a
set of programs and data to be partitioned or separated,
where users are given explicit access to specific components
of a system. Also, a compartment represents a grouping,
such as a work group, department, project, or topic. Using
compartments, it is possible to implement a need-to-know
security policy.</para>
</listitem>
<listitem>
<para><emphasis>high water mark</emphasis>: A high water mark
policy is one which permits the raising of security levels
for the purpose of accessing higher level information. In
most cases, the original level is restored after the process
is complete. Currently, the &os; <acronym>MAC</acronym>
framework does not have a policy for this, but the definition
is included for completeness.</para>
</listitem>
<listitem>
<para><emphasis>integrity</emphasis>: Integrity, as a key
concept, is the level of trust which can be placed on data.
As the integrity of the data is elevated, so does the ability
to trust that data.</para>
</listitem>
<listitem>
<para><emphasis>label</emphasis>: A label is a security
attribute which can be applied to files, directories, or
other items in the system. It could be considered
a confidentiality stamp; when a label is placed on
a file it describes the security properties for that specific
file and will only permit access by files, users, resources,
etc. with a similar security setting. The meaning and
interpretation of label values depends on the policy configuration: while
some policies might treat a label as representing the
integrity or secrecy of an object, other policies might use
labels to hold rules for access.</para>
</listitem>
<listitem>
<para><emphasis>level</emphasis>: The increased or decreased
setting of a security attribute. As the level increases,
its security is considered to elevate as well.</para>
</listitem>
<listitem>
<para><emphasis>low water mark</emphasis>: A low water mark
policy is one which permits lowering of the security levels
for the purpose of accessing information which is less
secure. In most cases, the original security level of the
user is restored after the process is complete. The only
security policy module in &os; to use this is
&man.mac.lomac.4;.</para>
</listitem>
<listitem>
<para><emphasis>multilabel</emphasis>: The
<option>multilabel</option> property is a file system option
which can be set in single user mode using the
&man.tunefs.8; utility, during the boot operation
using the &man.fstab.5; file, or during the creation of
a new file system. This option will permit an administrator
to apply different <acronym>MAC</acronym> labels on different
objects. This option
only applies to security policy modules which support labeling.</para>
</listitem>
<listitem>
<para><emphasis>object</emphasis>: An object or system
object is an entity through which information flows
under the direction of a <emphasis>subject</emphasis>.
This includes directories, files, fields, screens, keyboards,
memory, magnetic storage, printers or any other data
storage/moving device. Basically, an object is a data container or
a system resource; access to an <emphasis>object</emphasis>
effectively means access to the data.</para>
</listitem>
<listitem>
<para><emphasis>policy</emphasis>: A collection of rules
which defines how objectives are to be achieved. A
<emphasis>policy</emphasis> usually documents how certain
items are to be handled. This chapter will
consider the term <emphasis>policy</emphasis> in this
context as a <emphasis>security policy</emphasis>; i.e.
a collection of rules which will control the flow of data
and information and define whom will have access to that
data and information.</para>
</listitem>
<listitem>
<para><emphasis>sensitivity</emphasis>: Usually used when
discussing <acronym>MLS</acronym>. A sensitivity level is
a term used to describe how important or secret the data
should be. As the sensitivity level increases, so does the
importance of the secrecy, or confidentiality of the data.</para>
</listitem>
<listitem>
<para><emphasis>single label</emphasis>: A single label is
when the entire file system uses one label to
enforce access control over the flow of data. When a file
system has this set, which is any time when the
<option>multilabel</option> option is not set, all
files will conform to the same label setting.</para>
</listitem>
<listitem>
<para><emphasis>subject</emphasis>: a subject is any
active entity that causes information to flow between
<emphasis>objects</emphasis>; e.g., a user, user process,
system process, etc. On &os;, this is almost always a thread
acting in a process on behalf of a user.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="mac-initial">
<title>Explanation of MAC</title>
<para>With all of these new terms in mind, consider how the
<acronym>MAC</acronym> framework augments the security of
the system as a whole. The various security policy modules provided by
the <acronym>MAC</acronym> framework could be used to
protect the network and file systems, block users from
accessing certain ports and sockets, and more. Perhaps
the best use of the policy modules is to blend them together, by loading
several security policy modules at a time for a multi-layered
security environment. In a multi-layered security environment,
multiple policy modules are in effect to keep security in check. This
is different to a hardening policy, which typically hardens
elements of a system that is used only for specific purposes.
The only downside is administrative overhead in cases of
multiple file system labels, setting network access control
user by user, etc.</para>
<para>These downsides are minimal when compared to the lasting
effect of the framework; for instance, the ability to pick and choose
which policies are required for a specific configuration keeps
performance overhead down. The reduction of support for unneeded
policies can increase the overall performance of the system as well as
offer flexibility of choice. A good implementation would
consider the overall security requirements and effectively implement
the various security policy modules offered by the framework.</para>
<para>Thus a system utilizing <acronym>MAC</acronym> features
should at least guarantee that a user will not be permitted
to change security attributes at will; all user utilities,
programs and scripts must work within the constraints of
the access rules provided by the selected security policy modules; and
that total control of the <acronym>MAC</acronym> access
rules are in the hands of the system administrator.</para>
<para>It is the sole duty of the system administrator to
carefully select the correct security policy modules. Some environments
may need to limit access control over the network; in these
cases, the &man.mac.portacl.4;, &man.mac.ifoff.4; and even
&man.mac.biba.4; policy modules might make good starting points. In other
cases, strict confidentiality of file system objects might
be required. Policy modules such as &man.mac.bsdextended.4;
and &man.mac.mls.4; exist for this purpose.</para>
<para>Policy decisions could be made based on network
configuration. Perhaps only certain users should be permitted
access to facilities provided by &man.ssh.1; to access the
network or the Internet. The &man.mac.portacl.4; would be
the policy module of choice for these situations. But what should be
done in the case of file systems? Should all access to certain
directories be severed from other groups or specific
users? Or should we limit user or utility access to specific
files by setting certain objects as classified?</para>
<para>In the file system case, access to objects might be
considered confidential to some users, but not to others.
For an example, a large development team might be broken
off into smaller groups of individuals. Developers in
project A might not be permitted to access objects written
by developers in project B. Yet they might need to access
objects created by developers in project C; that is quite a
situation indeed. Using the different security policy modules provided by
the <acronym>MAC</acronym> framework; users could
be divided into these groups and then given access to the
appropriate areas without fear of information
leakage.</para>
<para>Thus, each security policy module has a unique way of dealing with
the overall security of a system. Module selection should be based
on a well thought out security policy. In many cases, the
overall policy may need to be revised and reimplemented on
the system. Understanding the different security policy modules offered by
the <acronym>MAC</acronym> framework will help administrators
choose the best policies for their situations.</para>
<para>The default &os; kernel does not include the option for
the <acronym>MAC</acronym> framework; thus the following
kernel option must be added before trying any of the examples or
information in this chapter:</para>
<programlisting>options MAC</programlisting>
<para>And the kernel will require a rebuild and a reinstall.</para>
<caution>
<para>While the various manual pages for <acronym>MAC</acronym>
policy modules state that they may be built into the kernel,
it is possible to lock the system out of
the network and more. Implementing <acronym>MAC</acronym>
is much like implementing a firewall, care must be taken
to prevent being completely locked out of the system. The
ability to revert back to a previous configuration should be
considered while the implementation of <acronym>MAC</acronym>
remotely should be done with extreme caution.</para>
</caution>
</sect1>
<sect1 id="mac-understandlabel">
<title>Understanding MAC Labels</title>
<para>A <acronym>MAC</acronym> label is a security attribute
which may be applied to subjects and objects throughout
the system.</para>
<para>When setting a label, the user must be able to comprehend
what it is, exactly, that is being done. The attributes
available on an object depend on the policy module loaded, and that
policy modules interpret their attributes in different
ways. If improperly configured due to lack of comprehension, or
the inability to understand the implications, the result will
be the unexpected and perhaps, undesired, behavior of the
system.</para>
<para>The security label on an object is used as a part of a
security access control decision by a policy. With some
policies, the label by itself contains all information necessary
to make a decision; in other models, the labels may be processed
as part of a larger rule set, etc.</para>
<para>For instance, setting the label of <literal>biba/low</literal>
on a file will represent a label maintained by the Biba security policy module,
with a value of <quote>low</quote>.</para>
<para>A few policy modules which support the labeling feature in
&os; offer three specific predefined labels. These
are the low, high, and equal labels. Although they enforce
access control in a different manner with each policy module, you
can be sure that the low label will be the lowest setting,
the equal label will set the subject or object to be disabled
or unaffected, and the high label will enforce the highest
setting available in the Biba and <acronym>MLS</acronym>
policy modules.</para>
<para>Within single label file system environments, only one label may be
used on objects. This will enforce one set of
access permissions across the entire system and in many
environments may be all that is required. There are a few
cases where multiple labels may be set on objects
or subjects in the file system. For those cases, the
<option>multilabel</option> option may be passed to
&man.tunefs.8;.</para>
<para>In the case of Biba and <acronym>MLS</acronym>, a numeric
label may be set to indicate the precise level of hierarchical
control. This numeric level is used to partition or sort
information into different groups of say, classification only
permitting access to that group or a higher group level.</para>
<para>In most cases the administrator will only be setting up a
single label to use throughout the file system.</para>
<para><emphasis>Hey wait, this is similar to <acronym>DAC</acronym>!
I thought <acronym>MAC</acronym> gave control strictly to the
administrator.</emphasis> That statement still holds true, to some
extent as <username>root</username> is the one in control and who
configures the policies so that users are placed in the
appropriate categories/access levels. Alas, many policy modules can
restrict the <username>root</username> user as well. Basic
control over objects will then be released to the group, but
<username>root</username> may revoke or modify the settings
at any time. This is the hierarchal/clearance model covered
by policies such as Biba and <acronym>MLS</acronym>.</para>
<sect2>
<title>Label Configuration</title>
<para>Virtually all aspects of label policy module configuration
will be performed using the base system utilities. These
commands provide a simple interface for object or subject
configuration or the manipulation and verification of
the configuration.</para>
<para>All configuration may be done by use of the
&man.setfmac.8; and &man.setpmac.8; utilities.
The <command>setfmac</command> command is used to set
<acronym>MAC</acronym> labels on system objects while the
<command>setpmac</command> command is used to set the labels
on system subjects. Observe:</para>
<screen>&prompt.root; <userinput>setfmac biba/high test</userinput></screen>
<para>If no errors occurred with the command above, a prompt
will be returned. The only time these commands are not
quiescent is when an error occurred; similarly to the
&man.chmod.1; and &man.chown.8; commands. In some cases this
error may be a <errorname>Permission denied</errorname> and
is usually obtained when the label is being set or modified
on an object which is restricted.<footnote><para>Other conditions
may produce different failures. For instance, the file may not
be owned by the user attempting to relabel the object, the
object may not exist or may be read only. A mandatory policy
will not allow the process to relabel the file, maybe because
of a property of the file, a property of the process, or a
property of the proposed new label value. For example: a user
running at low integrity tries to change the label of a high
integrity file. Or perhaps a user running at low integrity
tries to change the label of a low integrity file to a high
integrity label.</para></footnote> The system administrator
may use the following commands to overcome this:</para>
<screen>&prompt.root; <userinput>setfmac biba/high test</userinput>
<errorname>Permission denied</errorname>
&prompt.root; <userinput>setpmac biba/low setfmac biba/high test</userinput>
&prompt.root; <userinput>getfmac test</userinput>
test: biba/high</screen>
<para>As we see above, <command>setpmac</command>
can be used to override the policy module's settings by assigning
a different label to the invoked process. The
<command>getpmac</command> utility is usually used with currently
running processes, such as <application>sendmail</application>:
although it takes a process ID in place of
a command the logic is extremely similar. If users
attempt to manipulate a file not in their access, subject to the
rules of the loaded policy modules, the
<errorname>Operation not permitted</errorname> error
will be displayed by the <function>mac_set_link</function>
function.</para>
<sect3>
<title>Common Label Types</title>
<para>For the &man.mac.biba.4;, &man.mac.mls.4; and
&man.mac.lomac.4; policy modules, the ability to assign
simple labels is provided. These take the form of high,
equal and low, what follows is a brief description of
what these labels provide:</para>
<itemizedlist>
<listitem>
<para>The <literal>low</literal> label is considered the
lowest label setting an object or subject may have.
Setting this on objects or subjects will block their
access to objects or subjects marked high.</para>
</listitem>
<listitem>
<para>The <literal>equal</literal> label should only be
placed on objects considered to be exempt from the
policy.</para>
</listitem>
<listitem>
<para>The <literal>high</literal> label grants an object or
subject the highest possible setting.</para>
</listitem>
</itemizedlist>
<para>With respect to each policy module, each of those settings
will instate a different information flow directive. Reading
the proper manual pages will further explain the traits of
these generic label configurations.</para>
<sect4>
<title>Advanced Label Configuration</title>
<para>Numeric grade labels are used for
<literal>comparison:compartment+compartment</literal>; thus
the following:</para>
<programlisting>biba/10:2+3+6(5:2+3-20:2+3+4+5+6)</programlisting>
<para>May be interpreted as:</para>
<para><quote>Biba Policy Label</quote>/<quote>Grade 10</quote>
:<quote>Compartments 2, 3 and 6</quote>:
(<quote>grade 5 ...</quote>)</para>
<para>In this example, the first grade would be considered
the <quote>effective grade</quote> with
<quote>effective compartments</quote>, the second grade
is the low grade and the last one is the high grade.
In most configurations these settings will not be used;
indeed, they offered for more advanced
configurations.</para>
<para>When applied to system objects, they will only have a
current grade/compartments as opposed to system subjects
as they reflect the range of available rights in the system,
and network interfaces, where they are used for access
control.</para>
<para>The grade and compartments in a subject and object pair
are used to construct a relationship referred to as
<quote>dominance</quote>, in which a subject dominates an
object, the object dominates the subject, neither dominates
the other, or both dominate each other. The
<quote>both dominate</quote> case occurs when the two labels
are equal. Due to the information flow nature of Biba, you
have rights to a set of compartments,
<quote>need to know</quote>, that might correspond to
projects, but objects also have a set of compartments.
Users may have to subset their rights using
<command>su</command> or <command>setpmac</command> in order
to access objects in a compartment from which they are not
restricted.</para>
</sect4>
</sect3>
<sect3>
<title>Users and Label Settings</title>
<para>Users themselves are required to have labels so that
their files and processes may properly interact with the
security policy defined on the system. This is
configured through the <filename>login.conf</filename> file
by use of login classes. Every policy module that uses labels
will implement the user class setting.</para>
<para>An example entry containing every policy module setting is displayed
below:</para>
<programlisting>default:\
:copyright=/etc/COPYRIGHT:\
:welcome=/etc/motd:\
:setenv=MAIL=/var/mail/$,BLOCKSIZE=K:\
:path=~/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin:\
:manpath=/usr/share/man /usr/local/man:\
:nologin=/usr/sbin/nologin:\
:cputime=1h30m:\
:datasize=8M:\
:vmemoryuse=100M:\
:stacksize=2M:\
:memorylocked=4M:\
:memoryuse=8M:\
:filesize=8M:\
:coredumpsize=8M:\
:openfiles=24:\
:maxproc=32:\
:priority=0:\
:requirehome:\
:passwordtime=91d:\
:umask=022:\
:ignoretime@:\
:label=partition/13,mls/5,biba/10(5-15),lomac/10[2]:</programlisting>
<para>The <literal>label</literal> option is used to set the
user class default label which will be enforced by
<acronym>MAC</acronym>. Users will never be permitted to
modify this value, thus it can be considered not optional
in the user case. In a real configuration, however, the
administrator will never wish to enable every policy module.
It is recommended that the rest of this chapter be reviewed
before any of this configuration is implemented.</para>
<note>
<para>Users may change their label after the initial login;
however, this change is subject constraints of the policy.
The example above tells the Biba policy that a process's
minimum integrity is 5, its maximum is 15, but the default
effective label is 10. The process will run at 10 until
it chooses to change label, perhaps due to the user using
the setpmac command, which will be constrained by Biba to
the range set at login.</para>
</note>
<para>In all cases, after a change to
<filename>login.conf</filename>, the login class capability
database must be rebuilt using <command>cap_mkdb</command>
and this will be reflected throughout every forthcoming
example or discussion.</para>
<para>It is useful to note that many sites may have a
particularly large number of users requiring several
different user classes. In depth planning is required
as this may get extremely difficult to manage.</para>
</sect3>
<sect3>
<title>Network Interfaces and Label Settings</title>
<para>Labels may also be set on network interfaces to help
control the flow of data across the network. In all cases
they function in the same way the policies function with
respect to objects. Users at high settings in
<literal>biba</literal>, for example, will not be permitted
to access network interfaces with a label of low.</para>
<para>The <option>maclabel</option> may be passed to
<command>ifconfig</command> when setting the
<acronym>MAC</acronym> label on network interfaces. For
example:</para>
<screen>&prompt.root; <userinput>ifconfig bge0 maclabel biba/equal</userinput></screen>
<para>will set the <acronym>MAC</acronym> label of
<literal>biba/equal</literal> on the &man.bge.4; interface.
When using a setting similar to
<literal>biba/high(low-high)</literal> the entire label should
be quoted; otherwise an error will be returned.</para>
<para>Each policy module which supports labeling has a tunable
which may be used to disable the <acronym>MAC</acronym>
label on network interfaces. Setting the label to
<option>equal</option> will have a similar effect. Review
the output from <command>sysctl</command>, the policy manual
pages, or even the information found later in this chapter
for those tunables.</para>
</sect3>
</sect2>
<sect2>
<title>Singlelabel or Multilabel?</title>
<!-- Stopped here with my edits -->
<para>By default the system will use the
<option>singlelabel</option> option. But what does this
mean to the administrator? There are several differences
which, in their own right, offer pros and cons to the
flexibility in the systems security model.</para>
<para>The <option>singlelabel</option> only permits for one
label, for instance <literal>biba/high</literal> to be used
for each subject or object. It provides for lower
administration overhead but decreases the flexibility of
policies which support labeling. Many administrators may
want to use the <option>multilabel</option> option in
their security policy.</para>
<para>The <option>multilabel</option> option will permit each
subject or object to have its own independent
<acronym>MAC</acronym> label in
place of the standard <option>singlelabel</option> option
which will allow only one label throughout the partition.
The <option>multilabel</option> and <option>single</option>
label options are only required for the policies which
implement the labeling feature, including the Biba, Lomac,
<acronym>MLS</acronym> and <acronym>SEBSD</acronym>
policies.</para>
<para>In many cases, the <option>multilabel</option> may not need
to be set at all. Consider the following situation and
security model:</para>
<itemizedlist>
<listitem>
<para>&os; web-server using the <acronym>MAC</acronym>
framework and a mix of the various policies.</para>
</listitem>
<listitem>
<para>This machine only requires one label,
<literal>biba/high</literal>, for everything in the system.
Here the file system would not require the
<option>multilabel</option> option as a single label
will always be in effect.</para>
</listitem>
<listitem>
<para>But, this machine will be a web server and should have
the web server run at <literal>biba/low</literal> to prevent
write up capabilities. The Biba policy and how it works
will be discussed later, so if the previous comment was
difficult to interpret just continue reading and return.
The server could use a separate partition set at
<literal>biba/low</literal> for most if not all of its
runtime state. Much is lacking from this example, for
instance the restrictions on data, configuration and user
settings; however, this is just a quick example to prove the
aforementioned point.</para>
</listitem>
</itemizedlist>
<para>If any of the non-labeling policies are to be used,
then the <option>multilabel</option> option would never
be required. These include the <literal>seeotheruids</literal>,
<literal>portacl</literal> and <literal>partition</literal>
policies.</para>
<para>It should also be noted that using
<option>multilabel</option> with a partition and establishing
a security model based on <option>multilabel</option>
functionality could open the doors for higher administrative
overhead as everything in the file system would have a label.
This includes directories, files, and even device
nodes.</para>
<para>The following command will set <option>multilabel</option>
on the file systems to have multiple labels. This may only be
done in single user mode:</para>
<screen>&prompt.root; <userinput>tunefs -l enable /</userinput></screen>
<para>This is not a requirement for the swap file
system.</para>
<note>
<para>Some users have experienced problems with setting the
<option>multilabel</option> flag on the root partition.
If this is the case, please review the
<xref linkend="mac-troubleshoot"/> of this chapter.</para>
</note>
</sect2>
</sect1>
<sect1 id="mac-planning">
<title>Planning the Security Configuration</title>
<para>Whenever a new technology is implemented, a planning phase is
always a good idea. During the planning stages, an administrator
should in general look at the <quote>big picture</quote>, trying
to keep in view at least the following:</para>
<itemizedlist>
<listitem>
<para>The implementation requirements;</para>
</listitem>
<listitem>
<para>The implementation goals;</para>
</listitem>
</itemizedlist>
<para>For <acronym>MAC</acronym> installations, these include:</para>
<itemizedlist>
<listitem>
<para>How to classify information and resources available on
the target systems.</para>
</listitem>
<listitem>
<para>What sorts of information or resources to restrict
access to along with the type of restrictions that should be
applied.</para>
</listitem>
<listitem>
<para>Which <acronym>MAC</acronym> module or modules will be
required to achieve this goal.</para>
</listitem>
</itemizedlist>
<para>It is always possible to reconfigure and change the
system resources and security settings, it is quite often very inconvenient to
search through the system and fix existing files and user
accounts. Planning helps to ensure a trouble-free and efficient
trusted system implementation. A trial run of the trusted system,
including the configuration, is often vital and definitely
beneficial <emphasis>before</emphasis> a <acronym>MAC</acronym>
implementation is used on production systems. The idea of just
letting loose on a system
with <acronym>MAC</acronym> is like setting up for failure.</para>
<para>Different environments may have explicit needs and
requirements. Establishing an in depth and complete security
profile will decrease the need of changes once the system
goes live. As such, the future sections will cover the
different modules available to administrators; describe their
use and configuration; and in some cases provide insight on
what situations they would be most suitable for. For instance,
a web server might roll out the &man.mac.biba.4; and
&man.mac.bsdextended.4; policies. In other cases, a machine
with very few local users, the &man.mac.partition.4; might
be a good choice.</para>
</sect1>
<sect1 id="mac-modules">
<title>Module Configuration</title>
<para>Every module included with the <acronym>MAC</acronym>
framework may be either compiled into the kernel as noted above
or loaded as a run-time kernel module.
The recommended method is to add the module name to the
<filename>/boot/loader.conf</filename> file so that it will load
during the initial boot operation.</para>
<para>The following sections will discuss the various
<acronym>MAC</acronym> modules and cover their features.
Implementing them into a specific environment will also
be a consideration of this chapter. Some modules support
the use of labeling, which is controlling access by enforcing
a label such as <quote>this is allowed and this is not</quote>.
A label configuration file may control how files may be accessed,
network communication can be exchanged, and more. The previous
section showed how the <option>multilabel</option> flag could
be set on file systems to enable per-file or per-partition
access control.</para>
<para>A single label configuration would enforce only one label
across the system, that is why the <command>tunefs</command>
option is called <option>multilabel</option>.</para>
</sect1>
<sect1 id="mac-seeotheruids">
<title>The MAC seeotheruids Module</title>
<indexterm>
<primary>MAC See Other UIDs Policy</primary>
</indexterm>
<para>Module name: <filename>mac_seeotheruids.ko</filename></para>
<para>Kernel configuration line:
<literal>options MAC_SEEOTHERUIDS</literal></para>
<para>Boot option:
<literal>mac_seeotheruids_load="YES"</literal></para>
<para>The &man.mac.seeotheruids.4; module mimics and extends
the <literal>security.bsd.see_other_uids</literal> and
<literal>security.bsd.see_other_gids</literal>
<command>sysctl</command> tunables. This option does
not require any labels to be set before configuration and
can operate transparently with the other modules.</para>
<para>After loading the module, the following
<command>sysctl</command> tunables may be used to control
the features:</para>
<itemizedlist>
<listitem>
<para><literal>security.mac.seeotheruids.enabled</literal>
will enable the module's features and use the default
settings. These default settings will deny users the
ability to view processes and sockets owned by other
users.</para>
</listitem>
<listitem>
<para>
<literal>security.mac.seeotheruids.specificgid_enabled</literal>
will allow a certain group to be exempt from this policy.
To exempt specific groups from this policy, use the
<literal>security.mac.seeotheruids.specificgid=<replaceable>XXX</replaceable></literal>
<command>sysctl</command> tunable. In the above example,
the <replaceable>XXX</replaceable> should be replaced with the
numeric group ID to be exempted.</para>
</listitem>
<listitem>
<para>
<literal>security.mac.seeotheruids.primarygroup_enabled</literal>
is used to exempt specific primary groups from this policy.
When using this tunable, the
<literal>security.mac.seeotheruids.specificgid_enabled</literal>
may not be set.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="mac-bsdextended">
<title>The MAC bsdextended Module</title>
<indexterm>
<primary>MAC</primary>
<secondary>File System Firewall Policy</secondary>
</indexterm>
<para>Module name: <filename>mac_bsdextended.ko</filename></para>
<para>Kernel configuration line:
<literal>options MAC_BSDEXTENDED</literal></para>
<para>Boot option:
<literal>mac_bsdextended_load="YES"</literal></para>
<para>The &man.mac.bsdextended.4; module enforces the file system
firewall. This module's policy provides an extension to the
standard file system permissions model, permitting an
administrator to create a firewall-like ruleset to protect files,
utilities, and directories in the file system hierarchy. When
access to a file system object is attempted, the list of rules
is iterated until either a matching rule is located or the end
is reached. This behavior may be changed by the use of a
&man.sysctl.8; parameter,
security.mac.bsdextended.firstmatch_enabled. Similar to
other firewall modules in &os;, a file containing access control
rules can be created and read by the system at boot time using
an &man.rc.conf.5; variable.</para>
<para>The rule list may be entered using a utility, &man.ugidfw.8;,
that has a syntax similar to that of &man.ipfw.8;. More tools
can be written by using the functions in the
&man.libugidfw.3; library.</para>
<para>Extreme caution should be taken when working with this
module; incorrect use could block access to certain parts of
the file system.</para>
<sect2>
<title>Examples</title>
<para>After the &man.mac.bsdextended.4; module has
been loaded, the following command may be used to list the
current rule configuration:</para>
<screen>&prompt.root; <userinput>ugidfw list</userinput>
0 slots, 0 rules</screen>
<para>As expected, there are no rules defined. This means that
everything is still completely accessible. To create a rule
which will block all access by users but leave
<username>root</username> unaffected, simply run the
following command:</para>
<screen>&prompt.root; <userinput>ugidfw add subject not uid root new object not uid root mode n</userinput></screen>
<para>This is a very bad idea as it will block all users from
issuing even the most simple commands, such as
<command>ls</command>. A more patriotic list of rules
might be:</para>
<screen>&prompt.root; <userinput>ugidfw set 2 subject uid <replaceable>user1</replaceable> object uid <replaceable>user2</replaceable> mode n</userinput>
&prompt.root; <userinput>ugidfw set 3 subject uid <replaceable>user1</replaceable> object gid <replaceable>user2</replaceable> mode n</userinput></screen>
<para>This will block any and all access, including directory
listings, to <username><replaceable>user2</replaceable></username>'s home
directory from the username <username>user1</username>.</para>
<para>In place of <username>user1</username>, the
<option>not uid <replaceable>user2</replaceable></option> could
be passed. This will enforce the same access restrictions
above for all users in place of just one user.</para>
<note>
<para>The <username>root</username> user will be unaffected
by these changes.</para>
</note>
<para>This should provide a general idea of how the
&man.mac.bsdextended.4; module may be used to help fortify
a file system. For more information, see the
&man.mac.bsdextended.4; and the &man.ugidfw.8; manual
pages.</para>
</sect2>
</sect1>
<sect1 id="mac-ifoff">
<title>The MAC ifoff Module</title>
<indexterm>
<primary>MAC Interface Silencing Policy</primary>
</indexterm>
<para>Module name: <filename>mac_ifoff.ko</filename></para>
<para>Kernel configuration line:
<literal>options MAC_IFOFF</literal></para>
<para>Boot option: <literal>mac_ifoff_load="YES"</literal></para>
<para>The &man.mac.ifoff.4; module exists solely to disable network
interfaces on the fly and keep network interfaces from being
brought up during the initial system boot. It does not require
any labels to be set up on the system, nor does it have a
dependency on other <acronym>MAC</acronym> modules.</para>
<para>Most of the control is done through the
<command>sysctl</command> tunables listed below.</para>
<itemizedlist>
<listitem>
<para><literal>security.mac.ifoff.lo_enabled</literal> will
enable/disable all traffic on the loopback (&man.lo.4;)
interface.</para>
</listitem>
<listitem>
<para><literal>security.mac.ifoff.bpfrecv_enabled</literal> will
enable/disable all traffic on the Berkeley Packet Filter
interface (&man.bpf.4;)</para>
</listitem>
<listitem>
<para><literal>security.mac.ifoff.other_enabled</literal> will
enable/disable traffic on all other interfaces.</para>
</listitem>
</itemizedlist>
<para>One of the most common uses of &man.mac.ifoff.4; is network
monitoring in an environment where network traffic should not
be permitted during the boot sequence. Another suggested use
would be to write a script which uses
<filename role="package">security/aide</filename> to automatically
block network traffic if it finds new or altered files in
protected directories.</para>
</sect1>
<sect1 id="mac-portacl">
<title>The MAC portacl Module</title>
<indexterm>
<primary>MAC Port Access Control List Policy</primary>
</indexterm>
<para>Module name: <filename>mac_portacl.ko</filename></para>
<para>Kernel configuration line:
<literal>MAC_PORTACL</literal></para>
<para>Boot option: <literal>mac_portacl_load="YES"</literal></para>
<para>The &man.mac.portacl.4; module is used to limit binding to
local <acronym>TCP</acronym> and <acronym>UDP</acronym> ports
using a variety of <command>sysctl</command> variables. In
essence &man.mac.portacl.4; makes it possible to allow
non-<username>root</username> users to bind to specified
privileged ports, i.e., ports below 1024.</para>
<para>Once loaded, this module will enable the
<acronym>MAC</acronym> policy on all sockets. The following
tunables are available:</para>
<itemizedlist>
<listitem>
<para><literal>security.mac.portacl.enabled</literal> will
enable/disable the policy completely.</para>
</listitem>
<listitem>
<para><literal>security.mac.portacl.port_high</literal> will set
the highest port number that &man.mac.portacl.4;
will enable protection for.</para>
</listitem>
<listitem>
<para><literal>security.mac.portacl.suser_exempt</literal> will,
when set to a non-zero value, exempt the
<username>root</username> user from this policy.</para>
</listitem>
<listitem>
<para><literal>security.mac.portacl.rules</literal> will
specify the actual mac_portacl policy; see below.</para>
</listitem>
</itemizedlist>
<para>The actual <literal>mac_portacl</literal> policy, as
specified in the <literal>security.mac.portacl.rules</literal>
sysctl, is a text string of the form:
<literal>rule[,rule,...]</literal> with as many rules as
needed. Each rule is of the form:
<literal>idtype:id:protocol:port</literal>. The
<parameter>idtype</parameter> parameter can be
<literal>uid</literal> or <literal>gid</literal> and used to
interpret the <parameter>id</parameter> parameter as either a
user id or group id, respectively. The
<parameter>protocol</parameter> parameter is used to determine if
the rule should apply to <acronym>TCP</acronym> or
<acronym>UDP</acronym> by setting the parameter to
<literal>tcp</literal> or <literal>udp</literal>. The final
<parameter>port</parameter> parameter is the port number to allow
the specified user or group to bind to.</para>
<note>
<para>Since the ruleset is interpreted directly by the kernel
only numeric values can be used for the user ID, group ID, and
port parameters. Names cannot be used for users, groups, or
services.</para>
</note>
<para>By default, on &unix;-like systems, ports below 1024
can only be used by/bound to privileged processes,
i.e., those run as <username>root</username>. For
&man.mac.portacl.4; to allow non-privileged processes to bind
to ports below 1024 this standard &unix; restriction has to be
disabled. This can be accomplished by setting the &man.sysctl.8;
variables <literal>net.inet.ip.portrange.reservedlow</literal> and
<literal>net.inet.ip.portrange.reservedhigh</literal>
to zero.</para>
<para>See the examples below or review the &man.mac.portacl.4;
manual page for further information.</para>
<sect2>
<title>Examples</title>
<para>The following examples should illuminate the above
discussion a little better:</para>
<screen>&prompt.root; <userinput>sysctl security.mac.portacl.port_high=1023</userinput>
&prompt.root; <userinput>sysctl net.inet.ip.portrange.reservedlow=0 net.inet.ip.portrange.reservedhigh=0</userinput></screen>
<para>First we set &man.mac.portacl.4; to cover the standard
privileged ports and disable the normal &unix; bind
restrictions.</para>
<screen>&prompt.root; <userinput>sysctl security.mac.portacl.suser_exempt=1</userinput></screen>
<para>The <username>root</username> user should not be crippled
by this policy, thus set the
<literal>security.mac.portacl.suser_exempt</literal> to a
non-zero value. The &man.mac.portacl.4; module
has now been set up to behave the same way &unix;-like systems
behave by default.</para>
<screen>&prompt.root; <userinput>sysctl security.mac.portacl.rules=uid:80:tcp:80</userinput></screen>
<para>Allow the user with <acronym>UID</acronym> 80 (normally
the <username>www</username> user) to bind to port 80.
This can be used to allow the <username>www</username>
user to run a web server without ever having
<username>root</username> privilege.</para>
<screen>&prompt.root; <userinput>sysctl security.mac.portacl.rules=uid:1001:tcp:110,uid:1001:tcp:995</userinput></screen>
<para>Permit the user with the <acronym>UID</acronym> of
1001 to bind to the <acronym>TCP</acronym> ports 110
(<quote>pop3</quote>) and 995 (<quote>pop3s</quote>).
This will permit this user to start a server that accepts
connections on ports 110 and 995.</para>
</sect2>
</sect1>
<sect1 id="mac-partition">
<title>The MAC partition Module</title>
<indexterm>
<primary>MAC Process Partition Policy</primary>
</indexterm>
<para>Module name: <filename>mac_partition.ko</filename></para>
<para>Kernel configuration line:
<literal>options MAC_PARTITION</literal></para>
<para>Boot option:
<literal>mac_partition_load="YES"</literal></para>
<para>The &man.mac.partition.4; policy will drop processes into
specific <quote>partitions</quote> based on their
<acronym>MAC</acronym> label. Think of it as a special
type of &man.jail.8;, though that is hardly a worthy
comparison.</para>
<para>This is one module that should be added to the
&man.loader.conf.5; file so that it loads
and enables the policy during the boot process.</para>
<para>Most configuration for this policy is done using
the &man.setpmac.8; utility which will be explained below.
The following <command>sysctl</command> tunable is
available for this policy:</para>
<itemizedlist>
<listitem>
<para><literal>security.mac.partition.enabled</literal> will
enable the enforcement of <acronym>MAC</acronym> process
partitions.</para>
</listitem>
</itemizedlist>
<para>When this policy is enabled, users will only be permitted
to see their processes, and any others within their partition,
but will not be permitted to work with
utilities outside the scope of this partition. For instance, a user in the
<literal>insecure</literal> class above will not be permitted
to access the <command>top</command> command as well as many
other commands that must spawn a process.</para>
<para>To set or drop utilities into a partition label, use the
<command>setpmac</command> utility:</para>
<screen>&prompt.root; <userinput>setpmac partition/13 top</userinput></screen>
<para>This will add the <command>top</command> command to the
label set on users in the <literal>insecure</literal> class.
Note that all processes spawned by users
in the <literal>insecure</literal> class will stay in the
<literal>partition/13</literal> label.</para>
<sect2>
<title>Examples</title>
<para>The following command will show you the partition label
and the process list:</para>
<screen>&prompt.root; <userinput>ps Zax</userinput></screen>
<para>This next command will allow the viewing of another
user's process partition label and that user's currently
running processes:</para>
<screen>&prompt.root; <userinput>ps -ZU trhodes</userinput></screen>
<note>
<para>Users can see processes in <username>root</username>'s
label unless the &man.mac.seeotheruids.4; policy is
loaded.</para>
</note>
<para>A really crafty implementation could have all of the
services disabled in <filename>/etc/rc.conf</filename> and
started by a script that starts them with the proper
labeling set.</para>
<note>
<para>The following policies support integer settings
in place of the three default labels offered. These options,
including their limitations, are further explained in
the module manual pages.</para>
</note>
</sect2>
</sect1>
<sect1 id="mac-mls">
<title>The MAC Multi-Level Security Module</title>
<indexterm>
<primary>MAC Multi-Level Security Policy</primary>
</indexterm>
<para>Module name: <filename>mac_mls.ko</filename></para>
<para>Kernel configuration line:
<literal>options MAC_MLS</literal></para>
<para>Boot option: <literal>mac_mls_load="YES"</literal></para>
<para>The &man.mac.mls.4; policy controls access between subjects
and objects in the system by enforcing a strict information
flow policy.</para>
<para>In <acronym>MLS</acronym> environments, a
<quote>clearance</quote> level is set in each subject or objects
label, along with compartments. Since these clearance or
sensibility levels can reach numbers greater than six thousand;
it would be a daunting task for any system administrator to
thoroughly configure each subject or object. Thankfully, three
<quote>instant</quote> labels are already included in this
policy.</para>
<para>These labels are <literal>mls/low</literal>,
<literal>mls/equal</literal> and <literal>mls/high</literal>.
Since these labels are described in depth in the manual page,
they will only get a brief description here:</para>
<itemizedlist>
<listitem>
<para>The <literal>mls/low</literal> label contains a low
configuration which permits it to be dominated by all other
objects. Anything labeled with <literal>mls/low</literal>
will have a low clearance level and not be permitted to access
information of a higher level. In addition, this label will
prevent objects of a higher clearance level from writing or
passing information on to them.</para>
</listitem>
<listitem>
<para>The <literal>mls/equal</literal> label should be
placed on objects considered to be exempt from the
policy.</para>
</listitem>
<listitem>
<para>The <literal>mls/high</literal> label is the highest level
of clearance possible. Objects assigned this label will
hold dominance over all other objects in the system; however,
they will not permit the leaking of information to objects
of a lower class.</para>
</listitem>
</itemizedlist>
<para><acronym>MLS</acronym> provides for:</para>
<itemizedlist>
<listitem>
<para>A hierarchical security level with a set of non
hierarchical categories;</para>
</listitem>
<listitem>
<para>Fixed rules: no read up, no write down (a subject can
have read access to objects on its own level or below, but
not above. Similarly, a subject can have write access to
objects on its own level or above but not beneath.);</para>
</listitem>
<listitem>
<para>Secrecy (preventing inappropriate disclosure
of data);</para>
</listitem>
<listitem>
<para>Basis for the design of systems that concurrently handle
data at multiple sensitivity levels (without leaking
information between secret and confidential).</para>
</listitem>
</itemizedlist>
<para>The following <command>sysctl</command> tunables are
available for the configuration of special services and
interfaces:</para>
<itemizedlist>
<listitem>
<para><literal>security.mac.mls.enabled</literal> is used to
enable/disable the <acronym>MLS</acronym> policy.</para>
</listitem>
<listitem>
<para><literal>security.mac.mls.ptys_equal</literal> will label
all &man.pty.4; devices as <literal>mls/equal</literal> during
creation.</para>
</listitem>
<listitem>
<para><literal>security.mac.mls.revocation_enabled</literal> is
used to revoke access to objects after their label changes
to a label of a lower grade.</para>
</listitem>
<listitem>
<para><literal>security.mac.mls.max_compartments</literal> is
used to set the maximum number of compartment levels with
objects; basically the maximum compartment number allowed
on a system.</para>
</listitem>
</itemizedlist>
<para>To manipulate the <acronym>MLS</acronym> labels, the
&man.setfmac.8; command has been provided. To assign a label
to an object, issue the following command:</para>
<screen>&prompt.root; <userinput>setfmac mls/5 test</userinput></screen>
<para>To get the <acronym>MLS</acronym> label for the file
<filename>test</filename> issue the following command:</para>
<screen>&prompt.root; <userinput>getfmac test</userinput></screen>
<para>This is a summary of the <acronym>MLS</acronym>
policy's features. Another approach is to create a master policy
file in <filename class="directory">/etc</filename> which
specifies the <acronym>MLS</acronym> policy information and to
feed that file into the <command>setfmac</command> command. This
method will be explained after all policies are covered.</para>
<sect2>
<title>Planning Mandatory Sensitivity</title>
<para>With the Multi-Level Security Policy Module, an
administrator plans for controlling the flow of sensitive
information. By default, with its block read up block write
down nature, the system defaults everything to a low state.
Everything is accessible and an administrator
slowly changes this during the configuration stage; augmenting
the confidentiality of the information.</para>
<para>Beyond the three basic label options above, an administrator
may group users and groups as required to block the information
flow between them. It might be easier to look at the
information in clearance levels familiarized with words, for
instance classifications such as
<literal>Confidential</literal>, <literal>Secret</literal>,
and <literal>Top Secret</literal>. Some administrators might
just create different groups based on project levels.
Regardless of classification method, a well thought out plan
must exist before implementing such a restrictive policy.</para>
<para>Some example situations for this security policy module
could be an e-commerce web server, a file server holding critical
company information, and financial institution environments.
The most unlikely place would be a personal workstation with
only two or three users.</para>
</sect2>
</sect1>
<sect1 id="mac-biba">
<title>The MAC Biba Module</title>
<indexterm>
<primary>MAC Biba Integrity Policy</primary>
</indexterm>
<para>Module name: <filename>mac_biba.ko</filename></para>
<para>Kernel configuration line: <literal>options MAC_BIBA</literal></para>
<para>Boot option: <literal>mac_biba_load="YES"</literal></para>
<para>The &man.mac.biba.4; module loads the <acronym>MAC</acronym>
Biba policy. This policy works much like that of the
<acronym>MLS</acronym> policy with the exception that the rules
for information flow
are slightly reversed. This is said to prevent the downward
flow of sensitive information whereas the <acronym>MLS</acronym>
policy prevents the upward flow of sensitive information; thus,
much of this section can apply to both policies.</para>
<para>In Biba environments, an <quote>integrity</quote> label is
set on each subject or object. These labels are made up of
hierarchal grades, and non-hierarchal components. As an object's
or subject's grade ascends, so does its integrity.</para>
<para>Supported labels are <literal>biba/low</literal>,
<literal>biba/equal</literal>, and <literal>biba/high</literal>;
as explained below:</para>
<itemizedlist>
<listitem>
<para>The <literal>biba/low</literal> label is considered the
lowest integrity an object or subject may have. Setting
this on objects or subjects will block their write access
to objects or subjects marked high. They still have read
access though.</para>
</listitem>
<listitem>
<para>The <literal>biba/equal</literal> label should only be
placed on objects considered to be exempt from the
policy.</para>
</listitem>
<listitem>
<para>The <literal>biba/high</literal> label will permit
writing to objects set at a lower label, but not
permit reading that object. It is recommended that this
label be placed on objects that affect the integrity of
the entire system.</para>
</listitem>
</itemizedlist>
<para>Biba provides for:</para>
<itemizedlist>
<listitem>
<para>Hierarchical integrity level with a set of non
hierarchical integrity categories;</para>
</listitem>
<listitem>
<para>Fixed rules: no write up, no read down (opposite of
<acronym>MLS</acronym>). A subject can have write access
to objects on its own level or below, but not above. Similarly, a
subject can have read access to objects on its own level
or above, but not below;</para>
</listitem>
<listitem>
<para>Integrity (preventing inappropriate modification of
data);</para>
</listitem>
<listitem>
<para>Integrity levels (instead of MLS sensitivity
levels).</para>
</listitem>
</itemizedlist>
<para>The following <command>sysctl</command> tunables can
be used to manipulate the Biba policy.</para>
<itemizedlist>
<listitem>
<para><literal>security.mac.biba.enabled</literal> may be used
to enable/disable enforcement of the Biba policy on the
target machine.</para>
</listitem>
<listitem>
<para><literal>security.mac.biba.ptys_equal</literal> may be
used to disable the Biba policy on &man.pty.4;
devices.</para>
</listitem>
<listitem>
<para><literal>security.mac.biba.revocation_enabled</literal>
will force the revocation of access to objects if the label
is changed to dominate the subject.</para>
</listitem>
</itemizedlist>
<para>To access the Biba policy setting on system objects, use
the <command>setfmac</command> and <command>getfmac</command>
commands:</para>
<screen>&prompt.root; <userinput>setfmac biba/low test</userinput>
&prompt.root; <userinput>getfmac test</userinput>
test: biba/low</screen>
<sect2>
<title>Planning Mandatory Integrity</title>
<para>Integrity, different from sensitivity, guarantees that the
information will never be manipulated by untrusted parties.
This includes information passed between subjects, objects,
and both. It ensures that users will only be able to modify
and in some cases even access information they explicitly need
to.</para>
<para>The &man.mac.biba.4; security policy module permits an
administrator to address which files and programs a user or
users may see and invoke while assuring that the programs and
files are free from threats and trusted by the system for that
user, or group of users.</para>
<para>During the initial planning phase, an administrator must be
prepared to partition users into grades, levels, and areas.
Users will be blocked access not only to data but programs
and utilities both before and after they start. The system will
default to a high label once this policy module is enabled, and
it is up to the administrator to configure the different grades
and levels for users. Instead of using clearance levels as
described above, a good planning method could include topics.
For instance, only allow developers modification access to the source code
repository, source code compiler, and other development
utilities. While other users would be grouped into other
categories such as testers, designers, or just ordinary
users and would only be permitted read access.</para>
<para>With its natural security control, a lower integrity subject
is unable to write to a higher integrity subject; a higher
integrity subject cannot observe or read a lower integrity
object. Setting a label at the lowest possible grade could make
it inaccessible to subjects. Some prospective environments for
this security policy module would include a constrained web
server, development and test machine, and source code
repository. A less useful implementation would be a personal
workstation, a machine used as a router, or a network
firewall.</para>
</sect2>
</sect1>
<sect1 id="mac-lomac">
<title>The MAC LOMAC Module</title>
<indexterm>
<primary>MAC LOMAC</primary>
</indexterm>
<para>Module name: <filename>mac_lomac.ko</filename></para>
<para>Kernel configuration line: <literal>options MAC_LOMAC</literal></para>
<para>Boot option: <literal>mac_lomac_load="YES"</literal></para>
<para>Unlike the <acronym>MAC</acronym> Biba policy, the
&man.mac.lomac.4; policy permits access to lower integrity
objects only after decreasing the integrity level to not disrupt
any integrity rules.</para>
<para>The <acronym>MAC</acronym> version of the Low-watermark
integrity policy, not to be confused with the older &man.lomac.4;
implementation, works almost identically to Biba, but with the
exception of using floating labels to support subject
demotion via an auxiliary grade compartment. This secondary
compartment takes the form of <literal>[auxgrade]</literal>.
When assigning a lomac policy with an auxiliary grade, it
should look a little bit like: <literal>lomac/10[2]</literal>
where the number two (2) is the auxiliary grade.</para>
<para>The <acronym>MAC</acronym> LOMAC policy relies on the
ubiquitous labeling of all system objects with integrity labels,
permitting subjects to read from low integrity objects and then
downgrading the label on the subject to prevent future writes to
high integrity objects. This is the
<literal>[auxgrade]</literal> option discussed above, thus the
policy may provide for greater compatibility and require less
initial configuration than Biba.</para>
<sect2>
<title>Examples</title>
<para>Like the Biba and <acronym>MLS</acronym> policies;
the <command>setfmac</command> and <command>setpmac</command>
utilities may be used to place labels on system objects:</para>
<screen>&prompt.root; <userinput>setfmac /usr/home/trhodes lomac/high[low]</userinput>
&prompt.root; <userinput>getfmac /usr/home/trhodes</userinput> lomac/high[low]</screen>
<para>Notice the auxiliary grade here is <literal>low</literal>,
this is a feature provided only by the <acronym>MAC</acronym>
LOMAC policy.</para>
</sect2>
</sect1>
<sect1 id="mac-implementing">
<title>Nagios in a MAC Jail</title>
<indexterm>
<primary>Nagios in a MAC Jail</primary>
</indexterm>
<para>The following demonstration will implement a secure
environment using various <acronym>MAC</acronym> modules
with properly configured policies. This is only a test and
should not be considered the complete answer to everyone's
security woes. Just implementing a policy and ignoring it
never works and could be disastrous in a production
environment.</para>
<para>Before beginning this process, the
<literal>multilabel</literal> option must be set on each file
system as stated at the beginning of this chapter. Not doing
so will result in errors. While at it, ensure that the
<filename role="package">net-mngt/nagios-plugins</filename>,
<filename role="package">net-mngt/nagios</filename>, and
<filename role="package">www/apache22</filename> ports are all
installed, configured, and working correctly.</para>
<sect2>
<title>Create an insecure User Class</title>
<para>Begin the procedure by adding the following user class
to the <filename>/etc/login.conf</filename> file:</para>
<programlisting>insecure:\
:copyright=/etc/COPYRIGHT:\
:welcome=/etc/motd:\
:setenv=MAIL=/var/mail/$,BLOCKSIZE=K:\
:path=~/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin
:manpath=/usr/share/man /usr/local/man:\
:nologin=/usr/sbin/nologin:\
:cputime=1h30m:\
:datasize=8M:\
:vmemoryuse=100M:\
:stacksize=2M:\
:memorylocked=4M:\
:memoryuse=8M:\
:filesize=8M:\
:coredumpsize=8M:\
:openfiles=24:\
:maxproc=32:\
:priority=0:\
:requirehome:\
:passwordtime=91d:\
:umask=022:\
:ignoretime@:\
:label=biba/10(10-10):</programlisting>
<para>And adding the following line to the default user
class:</para>
<programlisting>:label=biba/high:</programlisting>
<para>Once this is completed, the following command must be
issued to rebuild the database:</para>
<screen>&prompt.root; <userinput>cap_mkdb /etc/login.conf</userinput></screen>
</sect2>
<sect2>
<title>Boot Configuration</title>
<para>Do not reboot yet, just add the following lines to
<filename>/boot/loader.conf</filename> so the required
modules will load during system initialization:</para>
<programlisting>mac_biba_load="YES"
mac_seeotheruids_load="YES"</programlisting>
</sect2>
<sect2>
<title>Configure Users</title>
<para>Set the <username>root</username> user to the default
class using:</para>
<screen>&prompt.root; <userinput>pw usermod root -L default</userinput></screen>
<para>All user accounts that are not <username>root</username>
or system users will now require a login class. The login
class is required otherwise users will be refused access
to common commands such as &man.vi.1;.
The following <command>sh</command> script should do the
trick:</para>
<screen>&prompt.root; <userinput>for x in `awk -F: '($3 >= 1001) && ($3 != 65534) { print $1 }' \</userinput>
<userinput>/etc/passwd`; do pw usermod $x -L default; done;</userinput></screen>
<para>Drop the <username>nagios</username> and
<username>www</username> users into the insecure class:</para>
<screen>&prompt.root; <userinput>pw usermod nagios -L insecure</userinput></screen>
<screen>&prompt.root; <userinput>pw usermod www -L insecure</userinput></screen>
</sect2>
<sect2>
<title>Create the Contexts File</title>
<para>A contexts file should now be created; the following example
file should be placed in
<filename>/etc/policy.contexts</filename>.</para>
<programlisting># This is the default BIBA policy for this system.
# System:
/var/run biba/equal
/var/run/* biba/equal
/dev biba/equal
/dev/* biba/equal
/var biba/equal
/var/spool biba/equal
/var/spool/* biba/equal
/var/log biba/equal
/var/log/* biba/equal
/tmp biba/equal
/tmp/* biba/equal
/var/tmp biba/equal
/var/tmp/* biba/equal
/var/spool/mqueue biba/equal
/var/spool/clientmqueue biba/equal
# For Nagios:
/usr/local/etc/nagios
/usr/local/etc/nagios/* biba/10
/var/spool/nagios biba/10
/var/spool/nagios/* biba/10
# For apache
/usr/local/etc/apache biba/10
/usr/local/etc/apache/* biba/10</programlisting>
<para>This policy will enforce security by setting restrictions
on the flow of information. In this specific configuration,
users, <username>root</username> and others, should never be
allowed to access <application>Nagios</application>.
Configuration files and processes that are a part of
<application>Nagios</application> will be completely self
contained or jailed.</para>
<para>This file may now be read into our system by issuing the
following command:</para>
<screen>&prompt.root; <userinput>setfsmac -ef /etc/policy.contexts /</userinput>
&prompt.root; <userinput>setfsmac -ef /etc/policy.contexts /</userinput></screen>
<note>
<para>The above file system layout may be different depending
on environment; however, it must be run on every single file
system.</para>
</note>
<para>The <filename>/etc/mac.conf</filename> file requires
the following modifications in the main section:</para>
<programlisting>default_labels file ?biba
default_labels ifnet ?biba
default_labels process ?biba
default_labels socket ?biba</programlisting>
</sect2>
<sect2>
<title>Enable Networking</title>
<para>Add the following line to
<filename>/boot/loader.conf</filename>:</para>
<programlisting>security.mac.biba.trust_all_interfaces=1</programlisting>
<para>And the following to the network card configuration stored
in <filename>rc.conf</filename>. If the primary Internet
configuration is done via <acronym>DHCP</acronym>, this may
need to be configured manually after every system boot:</para>
<programlisting>maclabel biba/equal</programlisting>
</sect2>
<sect2>
<title>Testing the Configuration</title>
<indexterm>
<primary>MAC Configuration Testing</primary>
</indexterm>
<para>Ensure that the web server and
<application>Nagios</application> will not be started
on system initialization, and reboot. Ensure the
<username>root</username> user cannot access any of the files
in the <application>Nagios</application> configuration
directory. If <username>root</username> can issue an &man.ls.1;
command on <filename>/var/spool/nagios</filename>, then something
is wrong. Otherwise a <quote>permission denied</quote> error
should be returned.</para>
<para>If all seems well, <application>Nagios</application>,
<application>Apache</application>, and
<application>Sendmail</application> can now be started in a way
fitting of the security policy. The following commands will
make this happen:</para>
<screen>&prompt.root; <userinput>cd /etc/mail && make stop && \
setpmac biba/equal make start && setpmac biba/10\(10-10\) apachectl start && \
setpmac biba/10\(10-10\) /usr/local/etc/rc.d/nagios.sh forcestart</userinput></screen>
<para>Double check to ensure that everything is working
properly. If not, check the log files or error messages. Use
the &man.sysctl.8; utility to disable the &man.mac.biba.4;
security policy module enforcement and try starting everything
again, like normal.</para>
<note>
<para>The <username>root</username> user can change the security
enforcement and edit the configuration files without fear.
The following command will permit the degradation of the
security policy to a lower grade for a newly spawned
shell:</para>
<screen>&prompt.root; <userinput>setpmac biba/10 csh</userinput></screen>
<para>To block this from happening, force the user into a range
via &man.login.conf.5;. If &man.setpmac.8; attempts to run
a command outside of the compartment's range, an error will
be returned and the command will not be executed. In this
case, setting root to
<literal>biba/high(high-high)</literal>.</para>
</note>
</sect2>
</sect1>
<sect1 id="mac-userlocked">
<title>User Lock Down</title>
<para>This example considers a relatively small, fewer than fifty
users, storage system. Users would have login capabilities, and
be permitted to not only store data but access resources as
well.</para>
<para>For this scenario, the &man.mac.bsdextended.4; mixed with
&man.mac.seeotheruids.4; could co-exist and block access not
only to system objects, but to hide user processes as
well.</para>
<para>Begin by adding the following line to
<filename>/boot/loader.conf</filename>:</para>
<programlisting>mac_seeotheruids_load="YES"</programlisting>
<para>The &man.mac.bsdextended.4; security policy module may be
activated through the use of the following rc.conf
variable:</para>
<programlisting>ugidfw_enable="YES"</programlisting>
<para>Default rules stored in
<filename>/etc/rc.bsdextended</filename> will be loaded at system
initialization; however, the default entries may need
modification. Since this machine is expected only to service
users, everything may be left commented out except the last
two. These will force the loading of user owned system objects
by default.</para>
<para>Add the required users to this machine and reboot. For
testing purposes, try logging in as a different user across two
consoles. Run the <command>ps aux</command> command to see if
processes of other users are visible. Try to run &man.ls.1; on
another users home directory, it should fail.</para>
<para>Do not try to test with the <username>root</username> user
unless the specific <command>sysctl</command>s have been modified
to block super user access.</para>
<note>
<para>When a new user is added, their &man.mac.bsdextended.4;
rule will not be in the ruleset list. To update the ruleset
quickly, simply unload the security policy module and reload
it again using the &man.kldunload.8; and &man.kldload.8;
utilities.</para>
</note>
</sect1>
<sect1 id="mac-troubleshoot">
<title>Troubleshooting the MAC Framework</title>
<indexterm>
<primary>MAC Troubleshooting</primary>
</indexterm>
<para>During the development stage, a few users reported problems
with normal configuration. Some of these problems
are listed below:</para>
<sect2>
<title>The <option>multilabel</option> option cannot be enabled on
<filename>/</filename></title>
<para>The <option>multilabel</option> flag does not stay
enabled on my root (<filename>/</filename>) partition!</para>
<para>It seems that one out of every fifty users has this
problem, indeed, we had this problem during our initial
configuration. Further observation of this so called
<quote>bug</quote> has lead me to believe that it is a
result of either incorrect documentation or misinterpretation
of the documentation. Regardless of why it happened, the
following steps may be taken to resolve it:</para>
<procedure>
<step>
<para>Edit <filename>/etc/fstab</filename> and set the root
partition at <option>ro</option> for read-only.</para>
</step>
<step>
<para>Reboot into single user mode.</para>
</step>
<step>
<para>Run <command>tunefs</command> <option>-l enable</option>
on <filename>/</filename>.</para>
</step>
<step>
<para>Reboot the system into normal mode.</para>
</step>
<step>
<para>Run <command>mount</command> <option>-urw</option>
<filename>/</filename> and change the <option>ro</option>
back to <option>rw</option> in <filename>/etc/fstab</filename>
and reboot the system again.</para>
</step>
<step>
<para>Double-check the output from the
<command>mount</command> to ensure that
<option>multilabel</option> has been properly set on the
root file system.</para>
</step>
</procedure>
</sect2>
<sect2>
<title>X11 Server Will Not Start After <acronym>MAC</acronym></title>
<para>After establishing a secure environment with
<acronym>MAC</acronym>, I am no longer able to start
X!</para>
<para>This could be caused by the <acronym>MAC</acronym>
<literal>partition</literal> policy or by a mislabeling in
one of the <acronym>MAC</acronym> labeling policies. To
debug, try the following:</para>
<procedure>
<step>
<para>Check the error message; if the user is in the
<literal>insecure</literal> class, the
<literal>partition</literal> policy may be the culprit.
Try setting the user's class back to the
<literal>default</literal> class and rebuild the database
with the <command>cap_mkdb</command> command. If this
does not alleviate the problem, go to step two.</para>
</step>
<step>
<para>Double-check the label policies. Ensure that the
policies are set correctly for the user in question, the
X11 application, and
the <filename class="directory">/dev</filename>
entries.</para>
</step>
<step>
<para>If neither of these resolve the problem, send the
error message and a description of your environment to
the TrustedBSD discussion lists located at the
<ulink url="http://www.TrustedBSD.org">TrustedBSD</ulink>
website or to the &a.questions;
mailing list.</para>
</step>
</procedure>
</sect2>
<sect2>
<title>Error: &man..secure.path.3; cannot stat <filename>.login_conf</filename></title>
<para>When I attempt to switch from the <username>root</username> user
to another user in the system, the error message
<errorname>_secure_path: unable to state .login_conf</errorname> appears.</para>
<para>This message is usually shown when the user has a higher
label setting then that of the user whom they are attempting to
become. For instance a user on the system,
<username>joe</username>, has a default label of
<option>biba/low</option>. The <username>root</username> user,
who has a label of <option>biba/high</option>, cannot view
<username>joe</username>'s home directory. This will happen
regardless if <username>root</username> has used the
<command>su</command> command to become <username>joe</username>,
or not. In this scenario, the Biba integrity model will not
permit <username>root</username> to view objects set at a lower
integrity level.</para>
</sect2>
<sect2>
<title>The <username>root</username> username is broken!</title>
<para>In normal or even single user mode, the
<username>root</username> is not recognized. The
<command>whoami</command> command returns 0 (zero) and
<command>su</command> returns <errorname>who are you?</errorname>.
What could be going on?</para>
<para>This can happen if a labeling policy has been disabled,
either by a &man.sysctl.8; or the policy module was unloaded.
If the policy is being disabled or has been temporarily
disabled, then the login capabilities database needs to be
reconfigured with the <option>label</option> option being
removed. Double check the <filename>login.conf</filename>
file to ensure that all <option>label</option> options have
been removed and rebuild the database with the
<command>cap_mkdb</command> command.</para>
<para>This may also happen if a policy restricts access to the
<filename>master.passwd</filename> file or database. Usually
caused by an administrator altering the file under a label
which conflicts with the general policy being used by the
system. In these cases, the user information would be read
by the system and access would be blocked as the file has
inherited the new label. Disable the policy via a
&man.sysctl.8; and everything should return to normal.</para>
</sect2>
</sect1>
</chapter>
