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<!-- Copyright (c) 2016 Tresys Technology, LLC.  All rights reserved. -->
<title>Domain Transition Analysis</title>
<h1>Domain Transition Analysis</h1>


<p>A key feature of Type Enforcement (TE) security is the ability to
define domain types with which programs run, use that domain type to
control access to objects (which are also typed), and strictly control
the ability of a process to change its domain type.  This last ability
is known as domain transition.</p>

<p>Apol supports analysis of an SELinux policy to understand the domain
transitions it allows.  As with all access in SELinux, the ability to
transition from one domain to another is controlled by 'allow' rules
in the policy.  Below, we describe how apol performs a domain
transition analysis.</p>


<h2>Types Used in Domain Transitions</h2>

<p>When discussing domain transition access, there are three different
types we must consider:</p>
<ul>
<li>SOURCE TYPE: This is the domain type associated with a process
that is trying to change (transition) its domain type to another
type.</li>
<li>TARGET TYPE: This is the domain type to which the source type is
trying to transition.</li>
<li>FILE TYPE (ENTRYPOINT TYPE): This is a type associated with an
executable file object that allows the target type to be entered
as part of an exec() system call.</li>
</ul>


<h2>Domain Transition Criteria</h2>

<p>In SELinux, there are two ways for a domain transition to occur,
on <code>exec()</code>, and on <code>setcon()</code>.
The common case is for a domain transition to occur on <code>exec()</code>.</p>


<h3>Transitions on Exec()</h3>
<p>In SELinux, four types of access (and hence at four rules)
must be allowed by the policy for a domain transition to occur.  These
access types form the criteria used by apol to determine allowed
transitions.</p>

<p>The criteria for an allowed domain transition are as
follows.  In the examples below, assume <code>user_t</code> is the source type,
<code>passwd_t</code> is the target type, and <code>passwd_exec_t</code> is the file entry
point type.</p>

<ol>
<li>A rule must exist that allows the SOURCE domain type <code>transition</code>
access for <code>process</code> object class for the TARGET domain type. For
example, the rule:<pre>

    allow user_t passwd_t : process transition;

</pre>meets this criterion by allowing the source type (<code>user_t</code>) <code>process:
transition</code> permission to the target type (<code>passwd_t</code>).</li>

<li>A rule must exist that allows the SOURCE domain type <code>execute</code>
access to the FILE ENTRYPOINT type. For example, the rule:<pre>

    allow user_t passwd_exec_t : file { read getattr execute };

</pre>meets the criterion by allowing the source type (<code>user_t</code>) <code>execute</code>
access to the file entrypoint type (<code>passwd_exec_t</code>).</li>

<li>A rule must exist that allows the TARGET domain type <code>entrypoint</code>
access to the FILE ENTRYPOINT type for file objects.  For
example, the rule:<pre>

    allow passwd_t passwd_exec_t : file entrypoint;

</pre>meets this criterion by allowing the target type (<code>passwd_t</code>) <code>file:
entrypoint</code> access to the file entrypoint type (<code>passwd_exec_t</code>).</li>

<li>There must be a way for the transition to be triggered.  Typically
this is accomplished in the policy with a TYPE TRANSITION statement.
For example, the statement:<pre>

    type_transition user_t password_exec_t : process passwd_t;

</pre>meets this criterion by specifying that when <code>user_t</code> executes
a program with the <code>passwd_exec_t</code> type, the default type of the
new process is <code>passwd_t</code>.  This is the most common specifier because
it does not require the programs to be SELinux-aware.  Alternatively,
the program can be made SELinux-aware and the program itself may
specify the type of the new process.  For example, the statement:<pre>

    allow user_t self : process setexec;

</pre>allows the source type (<code>user_t</code>) to specify the type of new processes
when executing programs.  In both the type transition and setexec
cases, the types that the source domain may transition to are
limited by the previous three criterion.</li>
</ol>

<p>In the analysis results, apol will list all the types that meet the above four criteria.</p>


<h3>Transitions on Setcon()</h3>
<p>SELinux also supports domain transitions that are requested by SELinux-aware
programs (also known as a dynamic domain transition),
using the <code>setcon()</code> libselinux function.  Two types of access
must be allowed by the policy for a dynamic domain
transition to occur.  These access types form the criteria used by apol
to determine allowed transitions.</p>

<p>The criteria for an allowed dynamic domain transition are as
follows.  In the examples below, assume <code>souce_t</code> is the source type
and, <code>target_t</code> is the target type.</p>

<ol>
<li>A rule must exist that allows the SOURCE domain type <code>dyntransition</code>
access for <code>process</code> object class for the TARGET domain type. For
example, the rule:<pre>

    allow source_t target_t : process transition;

</pre>meets this criterion by allowing the source type (<code>source_t</code>) <code>process:
dyntransition</code> permission to the target type (<code>target_t</code>).</li>

<li>A rule must exist that allows the SOURCE domain type <code>setcurrent</code>
access so it can set its current SELinux context.  For example, the rule:<pre>

    allow source_t source_t : process setcurrent;

</pre>meets the criterion by allowing the source type (<code>source_t</code>) <code>setcurrent</code>
access on itself.</li>
</ol>

<p>In the analysis results, apol will list all the types that meet the above two criteria.</p>


<h2>Reverse Transitions</h2>

<p>Apol supports both forward and reverse domain transition analysis.  A
forward analysis determines all the TARGET types to which the selected
SOURCE types may transition (find child domains/processes).
A reverse analysis is the opposite; select a TARGET type and determine all the SOURCE
types that may transition to the target type (find parent domains/processes).</p>