
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 ``rexec``
  and ``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 ``open()`` function so that it raises an exception whenever
the *mode* parameter is ``'w'``.  It might also perform a
``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
``open()`` function in its environment, with the same calling
interface.  The semantics would be identical too, with ``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 ``__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 ``func_globals`` and the
class and instance object attribute ``__dict__`` are unavailable.

Two modules provide the framework for setting up restricted execution
environments:

* ``rexec`` --- Restricted execution framework
  * RExec Objects
  * Defining restricted environments
  * An example
* ``Bastion`` --- Restricting access to objects
See also:

   Grail Home Page
      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.
