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 :c:func:`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 a 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:`~object.__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 69
