30.1. "rexec" — Restricted execution framework
**********************************************

Deprecated since version 2.6: The "rexec" module has been removed in
Python 3.

Changed in version 2.3: Disabled module.

Warning: The documentation has been left in place to help in reading
  old code that uses the module.

This module contains the "RExec" class, which supports "r_eval()",
"r_execfile()", "r_exec()", and "r_import()" methods, which are
restricted versions of the standard Python functions "eval()",
"execfile()" and the "exec" and "import" statements. Code executed in
this restricted environment will only have access to modules and
functions that are deemed safe; you can subclass "RExec" to add or
remove capabilities as desired.

Warning: While the "rexec" module is designed to perform as
  described below, it does have a few known vulnerabilities which
  could be exploited by carefully written code.  Thus it should not be
  relied upon in situations requiring “production ready” security.  In
  such situations, execution via sub-processes or very careful
  “cleansing” of both code and data to be processed may be necessary.
  Alternatively, help in patching known "rexec" vulnerabilities would
  be welcomed.

Note: The "RExec" class can prevent code from performing unsafe
  operations like reading or writing disk files, or using TCP/IP
  sockets. However, it does not protect against code using extremely
  large amounts of memory or processor time.

class rexec.RExec([hooks[, verbose]])

   Returns an instance of the "RExec" class.

   *hooks* is an instance of the "RHooks" class or a subclass of it.
   If it is omitted or "None", the default "RHooks" class is
   instantiated. Whenever the "rexec" module searches for a module
   (even a built-in one) or reads a module’s code, it doesn’t actually
   go out to the file system itself. Rather, it calls methods of an
   "RHooks" instance that was passed to or created by its constructor.
   (Actually, the "RExec" object doesn’t make these calls — they are
   made by a module loader object that’s part of the "RExec" object.
   This allows another level of flexibility, which can be useful when
   changing the mechanics of "import" within the restricted
   environment.)

   By providing an alternate "RHooks" object, we can control the file
   system accesses made to import a module, without changing the
   actual algorithm that controls the order in which those accesses
   are made.  For instance, we could substitute an "RHooks" object
   that passes all filesystem requests to a file server elsewhere, via
   some RPC mechanism such as ILU.  Grail’s applet loader uses this to
   support importing applets from a URL for a directory.

   If *verbose* is true, additional debugging output may be sent to
   standard output.

It is important to be aware that code running in a restricted
environment can still call the "sys.exit()" function.  To disallow
restricted code from exiting the interpreter, always protect calls
that cause restricted code to run with a "try"/"except" statement that
catches the "SystemExit" exception.  Removing the "sys.exit()"
function from the restricted environment is not sufficient — the
restricted code could still use "raise SystemExit".  Removing
"SystemExit" is not a reasonable option; some library code makes use
of this and would break were it not available.

See also:

  Grail Home Page
     Grail is a Web browser written entirely in Python.  It uses the
     "rexec" module as a foundation for supporting Python applets, and
     can be used as an example usage of this module.


30.1.1. RExec Objects
=====================

"RExec" instances support the following methods:

RExec.r_eval(code)

   *code* must either be a string containing a Python expression, or a
   compiled code object, which will be evaluated in the restricted
   environment’s "__main__" module.  The value of the expression or
   code object will be returned.

RExec.r_exec(code)

   *code* must either be a string containing one or more lines of
   Python code, or a compiled code object, which will be executed in
   the restricted environment’s "__main__" module.

RExec.r_execfile(filename)

   Execute the Python code contained in the file *filename* in the
   restricted environment’s "__main__" module.

Methods whose names begin with "s_" are similar to the functions
beginning with "r_", but the code will be granted access to restricted
versions of the standard I/O streams "sys.stdin", "sys.stderr", and
"sys.stdout".

RExec.s_eval(code)

   *code* must be a string containing a Python expression, which will
   be evaluated in the restricted environment.

RExec.s_exec(code)

   *code* must be a string containing one or more lines of Python
   code, which will be executed in the restricted environment.

RExec.s_execfile(code)

   Execute the Python code contained in the file *filename* in the
   restricted environment.

"RExec" objects must also support various methods which will be
implicitly called by code executing in the restricted environment.
Overriding these methods in a subclass is used to change the policies
enforced by a restricted environment.

RExec.r_import(modulename[, globals[, locals[, fromlist]]])

   Import the module *modulename*, raising an "ImportError" exception
   if the module is considered unsafe.

RExec.r_open(filename[, mode[, bufsize]])

   Method called when "open()" is called in the restricted
   environment.  The arguments are identical to those of "open()", and
   a file object (or a class instance compatible with file objects)
   should be returned.  "RExec"’s default behaviour is allow opening
   any file for reading, but forbidding any attempt to write a file.
   See the example below for an implementation of a less restrictive
   "r_open()".

RExec.r_reload(module)

   Reload the module object *module*, re-parsing and re-initializing
   it.

RExec.r_unload(module)

   Unload the module object *module* (remove it from the restricted
   environment’s "sys.modules" dictionary).

And their equivalents with access to restricted standard I/O streams:

RExec.s_import(modulename[, globals[, locals[, fromlist]]])

   Import the module *modulename*, raising an "ImportError" exception
   if the module is considered unsafe.

RExec.s_reload(module)

   Reload the module object *module*, re-parsing and re-initializing
   it.

RExec.s_unload(module)

   Unload the module object *module*.


30.1.2. Defining restricted environments
========================================

The "RExec" class has the following class attributes, which are used
by the "__init__()" method.  Changing them on an existing instance
won’t have any effect; instead, create a subclass of "RExec" and
assign them new values in the class definition. Instances of the new
class will then use those new values.  All these attributes are tuples
of strings.

RExec.nok_builtin_names

   Contains the names of built-in functions which will *not* be
   available to programs running in the restricted environment.  The
   value for "RExec" is "('open', 'reload', '__import__')". (This
   gives the exceptions, because by far the majority of built-in
   functions are harmless.  A subclass that wants to override this
   variable should probably start with the value from the base class
   and concatenate additional forbidden functions — when new dangerous
   built-in functions are added to Python, they will also be added to
   this module.)

RExec.ok_builtin_modules

   Contains the names of built-in modules which can be safely
   imported. The value for "RExec" is "('audioop', 'array',
   'binascii', 'cmath', 'errno', 'imageop', 'marshal', 'math', 'md5',
   'operator', 'parser', 'regex', 'select', 'sha', '_sre', 'strop',
   'struct', 'time')".  A similar remark about overriding this
   variable applies — use the value from the base class as a starting
   point.

RExec.ok_path

   Contains the directories which will be searched when an "import" is
   performed in the restricted environment.   The value for "RExec" is
   the same as "sys.path" (at the time the module is loaded) for
   unrestricted code.

RExec.ok_posix_names

   Contains the names of the functions in the "os" module which will
   be available to programs running in the restricted environment.
   The value for "RExec" is "('error', 'fstat', 'listdir', 'lstat',
   'readlink', 'stat', 'times', 'uname', 'getpid', 'getppid',
   'getcwd', 'getuid', 'getgid', 'geteuid', 'getegid')".

RExec.ok_sys_names

   Contains the names of the functions and variables in the "sys"
   module which will be available to programs running in the
   restricted environment.  The value for "RExec" is "('ps1', 'ps2',
   'copyright', 'version', 'platform', 'exit', 'maxint')".

RExec.ok_file_types

   Contains the file types from which modules are allowed to be
   loaded. Each file type is an integer constant defined in the "imp"
   module. The meaningful values are "PY_SOURCE", "PY_COMPILED", and
   "C_EXTENSION". The value for "RExec" is "(C_EXTENSION, PY_SOURCE)".
   Adding "PY_COMPILED" in subclasses is not recommended; an attacker
   could exit the restricted execution mode by putting a forged byte-
   compiled file (".pyc") anywhere in your file system, for example by
   writing it to "/tmp" or uploading it to the "/incoming" directory
   of your public FTP server.


30.1.3. An example
==================

Let us say that we want a slightly more relaxed policy than the
standard "RExec" class.  For example, if we’re willing to allow files
in "/tmp" to be written, we can subclass the "RExec" class:

   class TmpWriterRExec(rexec.RExec):
       def r_open(self, file, mode='r', buf=-1):
           if mode in ('r', 'rb'):
               pass
           elif mode in ('w', 'wb', 'a', 'ab'):
               # check filename: must begin with /tmp/
               if file[:5]!='/tmp/':
                   raise IOError("can't write outside /tmp")
               elif (string.find(file, '/../') >= 0 or
                    file[:3] == '../' or file[-3:] == '/..'):
                   raise IOError("'..' in filename forbidden")
           else: raise IOError("Illegal open() mode")
           return open(file, mode, buf)

Notice that the above code will occasionally forbid a perfectly valid
filename; for example, code in the restricted environment won’t be
able to open a file called "/tmp/foo/../bar".  To fix this, the
"r_open()" method would have to simplify the filename to "/tmp/bar",
which would require splitting apart the filename and performing
various operations on it.  In cases where security is at stake, it may
be preferable to write simple code which is sometimes overly
restrictive, instead of more general code that is also more complex
and may harbor a subtle security hole.
