79 lines
4.0 KiB
TeX
79 lines
4.0 KiB
TeX
\chapter{Restricted Execution \label{restricted}}
|
|
|
|
In general, Python programs have complete access to the underlying
|
|
operating system throug the various functions and classes, For
|
|
example, a Python program can open any file for reading and writing by
|
|
using the \function{open()} built-in function (provided the underlying
|
|
operating system gives you permission!). This is exactly what you want
|
|
for most applications.
|
|
|
|
There exists a class of applications for which this ``openness'' is
|
|
inappropriate. Take Grail: a Web browser that accepts ``applets,''
|
|
snippets of Python code, from anywhere on the Internet for execution
|
|
on the local system. This can be used to improve the user interface
|
|
of forms, for instance. Since the originator of the code is unknown,
|
|
it is obvious that it cannot be trusted with the full resources of the
|
|
local machine.
|
|
|
|
\emph{Restricted execution} is the basic framework in Python that allows
|
|
for the segregation of trusted and untrusted code. It is based on the
|
|
notion that trusted Python code (a \emph{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 \function{open()} function so that it raises
|
|
an exception whenever the \var{mode} parameter is \code{'w'}. It
|
|
might also perform a \cfunction{chroot()}-like operation on the
|
|
\var{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 \function{open()} function in its
|
|
environment, with the same calling interface. The semantics would be
|
|
identical too, with \exception{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
|
|
\code{__builtins__} object in its global variables: if this is (the
|
|
dictionary of) the standard \refmodule[builtin]{__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 \member{func_globals} and
|
|
the class and instance object attribute \member{__dict__} are
|
|
unavailable.
|
|
|
|
Two modules provide the framework for setting up restricted execution
|
|
environments:
|
|
|
|
\localmoduletable
|
|
|
|
\begin{seealso}
|
|
\seetitle[http://www.python.org/doc/howto/rexec/]
|
|
{Restricted Execution HOWTO}
|
|
{Andrew Kuchling's tutorial on the use of the restricted
|
|
execution facilities in Python.}
|
|
\seetitle[http://grail.sourceforge.net/]{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.}
|
|
\end{seealso}
|