1 

     2 .. _restricted:

     3 

     4 ********************

     5 Restricted Execution

     6 ********************

     7 

     8 .. warning::

     9 

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

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

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

    13 

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

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

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

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

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

    19 with sensitive system resources through interfaces defined and managed by the

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

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

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

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

    24 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    39 being raised when the supervisor determined that an unallowable parameter is

    40 being used.

    41 

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

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

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

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

    46 deemed to be restricted.

    47 

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

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

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

    51 attribute :attr:`__dict__` are unavailable.

    52 

    53 Two modules provide the framework for setting up restricted execution

    54 environments:

    55 

    56 

    57 .. toctree::

    58 

    59    rexec.rst

    60    bastion.rst

    61 

    62 .. seealso::

    63 

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

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

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

    67       mode in Grail is available on the Web site.

    68