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<div class="section" id="restricted-execution">
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<span id="restricted"></span><h1>30. Restricted Execution<a class="headerlink" href="#restricted-execution" title="Permalink to this headline">¶</a></h1>
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<div class="admonition warning">
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<p class="first admonition-title">Warning</p>
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<p class="last">In Python 2.3 these modules have been disabled due to various known and not
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readily fixable security holes. The modules are still documented here to help
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in reading old code that uses the <a class="reference internal" href="rexec.html#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal"><span class="pre">rexec</span></code></a> and <a class="reference internal" href="bastion.html#module-Bastion" title="Bastion: Providing restricted access to objects. (deprecated)"><code class="xref py py-mod docutils literal"><span class="pre">Bastion</span></code></a> modules.</p>
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<p><em>Restricted execution</em> is the basic framework in Python that allows for the
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segregation of trusted and untrusted code. The framework is based on the notion
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that trusted Python code (a <em>supervisor</em>) can create a “padded cell’ (or
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environment) with limited permissions, and run the untrusted code within this
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cell. The untrusted code cannot break out of its cell, and can only interact
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with sensitive system resources through interfaces defined and managed by the
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trusted code. The term “restricted execution” is favored over “safe-Python”
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since true safety is hard to define, and is determined by the way the restricted
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environment is created. Note that the restricted environments can be nested,
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with inner cells creating subcells of lesser, but never greater, privilege.</p>
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<p>An interesting aspect of Python’s restricted execution model is that the
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interfaces presented to untrusted code usually have the same names as those
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presented to trusted code. Therefore no special interfaces need to be learned
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to write code designed to run in a restricted environment. And because the
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exact nature of the padded cell is determined by the supervisor, different
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restrictions can be imposed, depending on the application. For example, it
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might be deemed “safe” for untrusted code to read any file within a specified
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directory, but never to write a file. In this case, the supervisor may redefine
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the built-in <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal"><span class="pre">open()</span></code></a> function so that it raises an exception whenever the
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<em>mode</em> parameter is <code class="docutils literal"><span class="pre">'w'</span></code>. It might also perform a <code class="xref c c-func docutils literal"><span class="pre">chroot()</span></code>-like
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operation on the <em>filename</em> parameter, such that root is always relative to some
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safe “sandbox” area of the filesystem. In this case, the untrusted code would
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still see a built-in <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal"><span class="pre">open()</span></code></a> function in its environment, with the same
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calling interface. The semantics would be identical too, with <a class="reference internal" href="exceptions.html#exceptions.IOError" title="exceptions.IOError"><code class="xref py py-exc docutils literal"><span class="pre">IOError</span></code></a>s
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being raised when the supervisor determined that an unallowable parameter is
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<p>The Python run-time determines whether a particular code block is executing in
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restricted execution mode based on the identity of the <code class="docutils literal"><span class="pre">__builtins__</span></code> object
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in its global variables: if this is (the dictionary of) the standard
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<a class="reference internal" href="__builtin__.html#module-__builtin__" title="__builtin__: The module that provides the built-in namespace."><code class="xref py py-mod docutils literal"><span class="pre">__builtin__</span></code></a> module, the code is deemed to be unrestricted, else it is
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deemed to be restricted.</p>
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<p>Python code executing in restricted mode faces a number of limitations that are
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designed to prevent it from escaping from the padded cell. For instance, the
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function object attribute <code class="xref py py-attr docutils literal"><span class="pre">func_globals</span></code> and the class and instance object
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attribute <a class="reference internal" href="stdtypes.html#object.__dict__" title="object.__dict__"><code class="xref py py-attr docutils literal"><span class="pre">__dict__</span></code></a> are unavailable.</p>
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<p>Two modules provide the framework for setting up restricted execution
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<div class="toctree-wrapper compound">
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<li class="toctree-l1"><a class="reference internal" href="rexec.html">30.1. <code class="docutils literal"><span class="pre">rexec</span></code> — Restricted execution framework</a><ul>
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<li class="toctree-l2"><a class="reference internal" href="rexec.html#rexec-objects">30.1.1. RExec Objects</a></li>
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<li class="toctree-l2"><a class="reference internal" href="rexec.html#defining-restricted-environments">30.1.2. Defining restricted environments</a></li>
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<li class="toctree-l2"><a class="reference internal" href="rexec.html#an-example">30.1.3. An example</a></li>
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<li class="toctree-l1"><a class="reference internal" href="bastion.html">30.2. <code class="docutils literal"><span class="pre">Bastion</span></code> — Restricting access to objects</a></li>
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<div class="admonition seealso">
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<p class="first admonition-title">See also</p>
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<dl class="last docutils">
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<dt><a class="reference external" href="http://grail.sourceforge.net/">Grail Home Page</a></dt>
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<dd>Grail, an Internet browser written in Python, uses these modules to support
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Python applets. More information on the use of Python’s restricted execution
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mode in Grail is available on the Web site.</dd>
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