rexec — Restricted execution framework¶Deprecated since version 2.6:
Deprecated since version 2.6: The rexec module has been removed in Python 3.0.
Changed in version 2.3:
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.
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
rexec
module as a foundation for supporting Python applets, and can be used as an
example usage of this module.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.
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.
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.