80 lines
3.7 KiB
TeX
80 lines
3.7 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 \code{open()} built-in function (provided the underlying OS
|
|
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
|
|
\code{open()} function so that it raises an exception whenever the
|
|
\var{mode} parameter is \code{'w'}. It might also perform a
|
|
\code{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 \code{open()} function in its environment, with the same
|
|
calling interface. The semantics would be identical too, with
|
|
\code{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 \code{__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 \code{func_globals} and the
|
|
class and instance object attribute \code{__dict__} are unavailable.
|
|
|
|
Two modules provide the framework for setting up restricted execution
|
|
environments:
|
|
|
|
\begin{description}
|
|
|
|
\item[rexec]
|
|
--- Basic restricted execution framework.
|
|
|
|
\item[Bastion]
|
|
--- Providing restricted access to objects.
|
|
|
|
\end{description}
|
|
|
|
\begin{seealso}
|
|
\seetext{Andrew Kuchling, ``Restricted Execution HOWTO.'' Available
|
|
online at \url{http://www.python.org/doc/howto/rexec/}.}
|
|
\end{seealso}
|