.. _restricted:

********************

Restricted Execution

********************

.. warning::

   In Python 2.3 these modules have been disabled due to various known and not

   readily fixable security holes.  The modules are still documented here to help

   in reading old code that uses the :mod:`rexec` and :mod:`Bastion` modules.

*Restricted execution* is the basic framework in Python that allows for the

segregation of trusted and untrusted code.  The framework is based on the notion

that trusted Python code (a *supervisor*) can create a "padded cell' (or

environment) with limited permissions, and run the untrusted code within this

cell.  The untrusted code cannot break out of its cell, and can only interact

with sensitive system resources through interfaces defined and managed by the

trusted code.  The term "restricted execution" is favored over "safe-Python"

since true safety is hard to define, and is determined by the way the restricted

environment is created.  Note that the restricted environments can be nested,

with inner cells creating subcells of lesser, but never greater, privilege.

An interesting aspect of Python's restricted execution model is that the

interfaces presented to untrusted code usually have the same names as those

presented to trusted code.  Therefore no special interfaces need to be learned

to write code designed to run in a restricted environment.  And because the

exact nature of the padded cell is determined by the supervisor, different

restrictions can be imposed, depending on the application.  For example, it

might be deemed "safe" for untrusted code to read any file within a specified

directory, but never to write a file.  In this case, the supervisor may redefine

the built-in :func:`open` function so that it raises an exception whenever the

*mode* parameter is ``'w'``.  It might also perform a :cfunc:`chroot`\ -like

operation on the *filename* parameter, such that root is always relative to some

safe "sandbox" area of the filesystem.  In this case, the untrusted code would

still see an built-in :func:`open` function in its environment, with the same

calling interface.  The semantics would be identical too, with :exc:`IOError`\ s

being raised when the supervisor determined that an unallowable parameter is

being used.

The Python run-time determines whether a particular code block is executing in

restricted execution mode based on the identity of the ``__builtins__`` object

in its global variables: if this is (the dictionary of) the standard

:mod:`__builtin__` module, the code is deemed to be unrestricted, else it is

deemed to be restricted.

Python code executing in restricted mode faces a number of limitations that are

designed to prevent it from escaping from the padded cell. For instance, the

function object attribute :attr:`func_globals` and the class and instance object

attribute :attr:`__dict__` are unavailable.

Two modules provide the framework for setting up restricted execution

environments:

.. toctree::

   rexec.rst

   bastion.rst

.. seealso::

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

      Grail, an Internet browser written in Python, uses these modules to support

      Python applets.  More information on the use of Python's restricted execution

      mode in Grail is available on the Web site.