Doc/lib/librestricted.tex

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