69 lines
3.2 KiB
ReStructuredText
69 lines
3.2 KiB
ReStructuredText
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.. _restricted:
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********************
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Restricted Execution
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********************
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.. warning::
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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 :mod:`rexec` and :mod:`Bastion` modules.
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*Restricted execution* 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 *supervisor*) 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.
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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 :func:`open` function so that it raises an exception whenever the
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*mode* parameter is ``'w'``. It might also perform a :cfunc:`chroot`\ -like
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operation on the *filename* 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 an built-in :func:`open` function in its environment, with the same
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calling interface. The semantics would be identical too, with :exc:`IOError`\ s
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being raised when the supervisor determined that an unallowable parameter is
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being used.
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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 ``__builtins__`` object
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in its global variables: if this is (the dictionary of) the standard
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:mod:`__builtin__` module, the code is deemed to be unrestricted, else it is
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deemed to be restricted.
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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 :attr:`func_globals` and the class and instance object
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attribute :attr:`__dict__` are unavailable.
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Two modules provide the framework for setting up restricted execution
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environments:
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.. toctree::
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rexec.rst
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bastion.rst
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.. seealso::
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`Grail Home Page <http://grail.sourceforge.net/>`_
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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.
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