:mod:`rexec` --- Restricted execution framework

===============================================

.. module:: rexec

   :synopsis: Basic restricted execution framework.

   :deprecated:

.. deprecated:: 2.6

   The :mod:`rexec` module has been removed in Python 3.0.

.. versionchanged:: 2.3

   Disabled module.

.. warning::

   The documentation has been left in place to help in reading old code that uses

   the module.

This module contains the :class:`RExec` class, which supports :meth:`r_eval`,

:meth:`r_execfile`, :meth:`r_exec`, and :meth:`r_import` methods, which are

restricted versions of the standard Python functions :meth:`eval`,

:meth:`execfile` and the :keyword:`exec` and :keyword:`import` statements. Code

executed in this restricted environment will only have access to modules and

functions that are deemed safe; you can subclass :class:`RExec` to add or remove

capabilities as desired.

.. warning::

   While the :mod:`rexec` module is designed to perform as described below, it does

   have a few known vulnerabilities which could be exploited by carefully written

   code.  Thus it should not be relied upon in situations requiring "production

   ready" security.  In such situations, execution via sub-processes or very

   careful "cleansing" of both code and data to be processed may be necessary.

   Alternatively, help in patching known :mod:`rexec` vulnerabilities would be

   welcomed.

.. note::

   The :class:`RExec` class can prevent code from performing unsafe operations like

   reading or writing disk files, or using TCP/IP sockets.  However, it does not

   protect against code using extremely large amounts of memory or processor time.

.. class:: RExec([hooks[, verbose]])

   Returns an instance of the :class:`RExec` class.

   *hooks* is an instance of the :class:`RHooks` class or a subclass of it. If it

   is omitted or ``None``, the default :class:`RHooks` class is instantiated.

   Whenever the :mod:`rexec` module searches for a module (even a built-in one) or

   reads a module's code, it doesn't actually go out to the file system itself.

   Rather, it calls methods of an :class:`RHooks` instance that was passed to or

   created by its constructor.  (Actually, the :class:`RExec` object doesn't make

   these calls --- they are made by a module loader object that's part of the

   :class:`RExec` object.  This allows another level of flexibility, which can be

   useful when changing the mechanics of :keyword:`import` within the restricted

   environment.)

   By providing an alternate :class:`RHooks` object, we can control the file system

   accesses made to import a module, without changing the actual algorithm that

   controls the order in which those accesses are made.  For instance, we could

   substitute an :class:`RHooks` object that passes all filesystem requests to a

   file server elsewhere, via some RPC mechanism such as ILU.  Grail's applet

   loader uses this to support importing applets from a URL for a directory.

   If *verbose* is true, additional debugging output may be sent to standard

   output.

It is important to be aware that code running in a restricted environment can

still call the :func:`sys.exit` function.  To disallow restricted code from

exiting the interpreter, always protect calls that cause restricted code to run

with a :keyword:`try`/:keyword:`except` statement that catches the

:exc:`SystemExit` exception.  Removing the :func:`sys.exit` function from the

restricted environment is not sufficient --- the restricted code could still use

``raise SystemExit``.  Removing :exc:`SystemExit` is not a reasonable option;

some library code makes use of this and would break were it not available.

.. seealso::

   `Grail Home Page <http://grail.sourceforge.net/>`_

      Grail is a Web browser written entirely in Python.  It uses the :mod:`rexec`

      module as a foundation for supporting Python applets, and can be used as an

      example usage of this module.

.. _rexec-objects:

RExec Objects

-------------

:class:`RExec` instances support the following methods:

.. method:: RExec.r_eval(code)

   *code* must either be a string containing a Python expression, or a compiled

   code object, which will be evaluated in the restricted environment's

   :mod:`__main__` module.  The value of the expression or code object will be

   returned.

.. method:: RExec.r_exec(code)

   *code* must either be a string containing one or more lines of Python code, or a

   compiled code object, which will be executed in the restricted environment's

   :mod:`__main__` module.

.. method:: RExec.r_execfile(filename)

   Execute the Python code contained in the file *filename* in the restricted

   environment's :mod:`__main__` module.

Methods whose names begin with ``s_`` are similar to the functions beginning

with ``r_``, but the code will be granted access to restricted versions of the

standard I/O streams ``sys.stdin``, ``sys.stderr``, and ``sys.stdout``.

.. method:: RExec.s_eval(code)

   *code* must be a string containing a Python expression, which will be evaluated

   in the restricted environment.

.. method:: RExec.s_exec(code)

   *code* must be a string containing one or more lines of Python code, which will

   be executed in the restricted environment.

.. method:: RExec.s_execfile(code)

   Execute the Python code contained in the file *filename* in the restricted

   environment.

:class:`RExec` objects must also support various methods which will be

implicitly called by code executing in the restricted environment. Overriding

these methods in a subclass is used to change the policies enforced by a

restricted environment.

.. method:: RExec.r_import(modulename[, globals[, locals[, fromlist]]])

   Import the module *modulename*, raising an :exc:`ImportError` exception if the

   module is considered unsafe.

.. method:: RExec.r_open(filename[, mode[, bufsize]])

   Method called when :func:`open` is called in the restricted environment.  The

   arguments are identical to those of :func:`open`, and a file object (or a class

   instance compatible with file objects) should be returned.  :class:`RExec`'s

   default behaviour is allow opening any file for reading, but forbidding any

   attempt to write a file.  See the example below for an implementation of a less

   restrictive :meth:`r_open`.

.. method:: RExec.r_reload(module)

   Reload the module object *module*, re-parsing and re-initializing it.

.. method:: RExec.r_unload(module)

   Unload the module object *module* (remove it from the restricted environment's

   ``sys.modules`` dictionary).

And their equivalents with access to restricted standard I/O streams:

.. method:: RExec.s_import(modulename[, globals[, locals[, fromlist]]])

   Import the module *modulename*, raising an :exc:`ImportError` exception if the

   module is considered unsafe.

.. method:: RExec.s_reload(module)

   Reload the module object *module*, re-parsing and re-initializing it.

.. method:: RExec.s_unload(module)

   Unload the module object *module*.

   .. XXX what are the semantics of this?

.. _rexec-extension:

Defining restricted environments

--------------------------------

The :class:`RExec` class has the following class attributes, which are used by

the :meth:`__init__` method.  Changing them on an existing instance won't have

any effect; instead, create a subclass of :class:`RExec` and assign them new

values in the class definition. Instances of the new class will then use those

new values.  All these attributes are tuples of strings.

.. attribute:: RExec.nok_builtin_names

   Contains the names of built-in functions which will *not* be available to

   programs running in the restricted environment.  The value for :class:`RExec` is

   ``('open', 'reload', '__import__')``. (This gives the exceptions, because by far

   the majority of built-in functions are harmless.  A subclass that wants to

   override this variable should probably start with the value from the base class

   and concatenate additional forbidden functions --- when new dangerous built-in

   functions are added to Python, they will also be added to this module.)

.. attribute:: RExec.ok_builtin_modules

   Contains the names of built-in modules which can be safely imported. The value

   for :class:`RExec` is ``('audioop', 'array', 'binascii', 'cmath', 'errno',

   'imageop', 'marshal', 'math', 'md5', 'operator', 'parser', 'regex', 'select',

   'sha', '_sre', 'strop', 'struct', 'time')``.  A similar remark about overriding

   this variable applies --- use the value from the base class as a starting point.

.. attribute:: RExec.ok_path

   Contains the directories which will be searched when an :keyword:`import` is

   performed in the restricted environment.   The value for :class:`RExec` is the

   same as ``sys.path`` (at the time the module is loaded) for unrestricted code.

.. attribute:: RExec.ok_posix_names

   Contains the names of the functions in the :mod:`os` module which will be

   available to programs running in the restricted environment.  The value for

   :class:`RExec` is ``('error', 'fstat', 'listdir', 'lstat', 'readlink', 'stat',

   'times', 'uname', 'getpid', 'getppid', 'getcwd', 'getuid', 'getgid', 'geteuid',

   'getegid')``.

   .. Should this be called ok_os_names?

.. attribute:: RExec.ok_sys_names

   Contains the names of the functions and variables in the :mod:`sys` module which

   will be available to programs running in the restricted environment.  The value

   for :class:`RExec` is ``('ps1', 'ps2', 'copyright', 'version', 'platform',

   'exit', 'maxint')``.

.. attribute:: RExec.ok_file_types

   Contains the file types from which modules are allowed to be loaded. Each file

   type is an integer constant defined in the :mod:`imp` module. The meaningful

   values are :const:`PY_SOURCE`, :const:`PY_COMPILED`, and :const:`C_EXTENSION`.

   The value for :class:`RExec` is ``(C_EXTENSION, PY_SOURCE)``.  Adding

   :const:`PY_COMPILED` in subclasses is not recommended; an attacker could exit

   the restricted execution mode by putting a forged byte-compiled file

   (:file:`.pyc`) anywhere in your file system, for example by writing it to

   :file:`/tmp` or uploading it to the :file:`/incoming` directory of your public

   FTP server.

An example

----------

Let us say that we want a slightly more relaxed policy than the standard

:class:`RExec` class.  For example, if we're willing to allow files in

:file:`/tmp` to be written, we can subclass the :class:`RExec` class::

   class TmpWriterRExec(rexec.RExec):

       def r_open(self, file, mode='r', buf=-1):

           if mode in ('r', 'rb'):

               pass

           elif mode in ('w', 'wb', 'a', 'ab'):

               # check filename : must begin with /tmp/

               if file[:5]!='/tmp/': 

                   raise IOError, "can't write outside /tmp"

               elif (string.find(file, '/../') >= 0 or

                    file[:3] == '../' or file[-3:] == '/..'):

                   raise IOError, "'..' in filename forbidden"

           else: raise IOError, "Illegal open() mode"

           return open(file, mode, buf)

Notice that the above code will occasionally forbid a perfectly valid filename;

for example, code in the restricted environment won't be able to open a file

called :file:`/tmp/foo/../bar`.  To fix this, the :meth:`r_open` method would

have to simplify the filename to :file:`/tmp/bar`, which would require splitting

apart the filename and performing various operations on it.  In cases where

security is at stake, it may be preferable to write simple code which is

sometimes overly restrictive, instead of more general code that is also more

complex and may harbor a subtle security hole.