Doc/lib/librestricted.tex

   1 \chapter{Restricted Execution \label{restricted}}
   2
   3 In general, Python programs have complete access to the underlying
   4 operating system throug the various functions and classes, For
   5 example, a Python program can open any file for reading and writing by
   6 using the \function{open()} built-in function (provided the underlying
   7 OS gives you permission!).  This is exactly what you want for most
   8 applications.
   9
  10 There exists a class of applications for which this ``openness'' is
  11 inappropriate.  Take Grail: a web browser that accepts ``applets,''
  12 snippets of Python code, from anywhere on the Internet for execution
  13 on the local system.  This can be used to improve the user interface
  14 of forms, for instance.  Since the originator of the code is unknown,
  15 it is obvious that it cannot be trusted with the full resources of the
  16 local machine.
  17
  18 \emph{Restricted execution} is the basic framework in Python that allows
  19 for the segregation of trusted and untrusted code.  It is based on the
  20 notion that trusted Python code (a \emph{supervisor}) can create a
  21 ``padded cell' (or environment) with limited permissions, and run the
  22 untrusted code within this cell.  The untrusted code cannot break out
  23 of its cell, and can only interact with sensitive system resources
  24 through interfaces defined and managed by the trusted code.  The term
  25 ``restricted execution'' is favored over ``safe-Python''
  26 since true safety is hard to define, and is determined by the way the
  27 restricted environment is created.  Note that the restricted
  28 environments can be nested, with inner cells creating subcells of
  29 lesser, but never greater, privilege.
  30
  31 An interesting aspect of Python's restricted execution model is that
  32 the interfaces presented to untrusted code usually have the same names
  33 as those presented to trusted code.  Therefore no special interfaces
  34 need to be learned to write code designed to run in a restricted
  35 environment.  And because the exact nature of the padded cell is
  36 determined by the supervisor, different restrictions can be imposed,
  37 depending on the application.  For example, it might be deemed
  38 ``safe'' for untrusted code to read any file within a specified
  39 directory, but never to write a file.  In this case, the supervisor
  40 may redefine the built-in \function{open()} function so that it raises
  41 an exception whenever the \var{mode} parameter is \code{'w'}.  It
  42 might also perform a \cfunction{chroot()}-like operation on the
  43 \var{filename} parameter, such that root is always relative to some
  44 safe ``sandbox'' area of the filesystem.  In this case, the untrusted
  45 code would still see an built-in \function{open()} function in its
  46 environment, with the same calling interface.  The semantics would be
  47 identical too, with \exception{IOError}s being raised when the
  48 supervisor determined that an unallowable parameter is being used.
  49
  50 The Python run-time determines whether a particular code block is
  51 executing in restricted execution mode based on the identity of the
  52 \code{__builtins__} object in its global variables: if this is (the
  53 dictionary of) the standard \refmodule[builtin]{__builtin__} module,
  54 the code is deemed to be unrestricted, else it is deemed to be
  55 restricted.
  56
  57 Python code executing in restricted mode faces a number of limitations
  58 that are designed to prevent it from escaping from the padded cell.
  59 For instance, the function object attribute \member{func_globals} and
  60 the class and instance object attribute \member{__dict__} are
  61 unavailable.
  62
  63 Two modules provide the framework for setting up restricted execution
  64 environments:
  65
  66 \localmoduletable
  67
  68 \begin{seealso}
  69   \seetext{Andrew Kuchling, ``Restricted Execution HOWTO.''  Available
  70            online at \url{http://www.python.org/doc/howto/rexec/}.}
  71
  72   \seetext{Grail, an Internet browser written in Python, is available
  73            at \url{http://grail.cnri.reston.va.us/grail/}.  More
  74            information on the use of Python's restricted execution
  75            mode in Grail is available on the Web site.}
  76 \end{seealso}