1 \section{\module{rexec
} ---
2 Restricted execution framework
}
4 \declaremodule{standard
}{rexec
}
5 \modulesynopsis{Basic restricted execution framework.
}
6 \versionchanged[Disabled module
]{2.3}
8 \begin{notice
}[warning
]
9 The documentation has been left in place to help in reading old code
13 This module contains the
\class{RExec
} class, which supports
14 \method{r_eval()
},
\method{r_execfile()
},
\method{r_exec()
}, and
15 \method{r_import()
} methods, which are restricted versions of the standard
16 Python functions
\method{eval()
},
\method{execfile()
} and
17 the
\keyword{exec
} and
\keyword{import
} statements.
18 Code executed in this restricted environment will
19 only have access to modules and functions that are deemed safe; you
20 can subclass
\class{RExec
} to add or remove capabilities as desired.
22 \begin{notice
}[warning
]
23 While the
\module{rexec
} module is designed to perform as described
24 below, it does have a few known vulnerabilities which could be
25 exploited by carefully written code. Thus it should not be relied
26 upon in situations requiring ``production ready'' security. In such
27 situations, execution via sub-processes or very careful
28 ``cleansing'' of both code and data to be processed may be
29 necessary. Alternatively, help in patching known
\module{rexec
}
30 vulnerabilities would be welcomed.
34 The
\class{RExec
} class can prevent code from performing unsafe
35 operations like reading or writing disk files, or using TCP/IP
36 sockets. However, it does not protect against code using extremely
37 large amounts of memory or processor time.
40 \begin{classdesc
}{RExec
}{\optional{hooks
\optional{, verbose
}}}
41 Returns an instance of the
\class{RExec
} class.
43 \var{hooks
} is an instance of the
\class{RHooks
} class or a subclass of it.
44 If it is omitted or
\code{None
}, the default
\class{RHooks
} class is
46 Whenever the
\module{rexec
} module searches for a module (even a
47 built-in one) or reads a module's code, it doesn't actually go out to
48 the file system itself. Rather, it calls methods of an
\class{RHooks
}
49 instance that was passed to or created by its constructor. (Actually,
50 the
\class{RExec
} object doesn't make these calls --- they are made by
51 a module loader object that's part of the
\class{RExec
} object. This
52 allows another level of flexibility, which can be useful when changing
53 the mechanics of
\keyword{import
} within the restricted environment.)
55 By providing an alternate
\class{RHooks
} object, we can control the
56 file system accesses made to import a module, without changing the
57 actual algorithm that controls the order in which those accesses are
58 made. For instance, we could substitute an
\class{RHooks
} object that
59 passes all filesystem requests to a file server elsewhere, via some
60 RPC mechanism such as ILU. Grail's applet loader uses this to support
61 importing applets from a URL for a directory.
63 If
\var{verbose
} is true, additional debugging output may be sent to
67 It is important to be aware that code running in a restricted
68 environment can still call the
\function{sys.exit()
} function. To
69 disallow restricted code from exiting the interpreter, always protect
70 calls that cause restricted code to run with a
71 \keyword{try
}/
\keyword{except
} statement that catches the
72 \exception{SystemExit
} exception. Removing the
\function{sys.exit()
}
73 function from the restricted environment is not sufficient --- the
74 restricted code could still use
\code{raise SystemExit
}. Removing
75 \exception{SystemExit
} is not a reasonable option; some library code
76 makes use of this and would break were it not available.
80 \seetitle[http://grail.sourceforge.net/
]{Grail Home Page
}{Grail is a
81 Web browser written entirely in Python. It uses the
82 \module{rexec
} module as a foundation for supporting
83 Python applets, and can be used as an example usage of
88 \subsection{RExec Objects
\label{rexec-objects
}}
90 \class{RExec
} instances support the following methods:
92 \begin{methoddesc
}{r_eval
}{code
}
93 \var{code
} must either be a string containing a Python expression, or
94 a compiled code object, which will be evaluated in the restricted
95 environment's
\module{__main__
} module. The value of the expression or
96 code object will be returned.
99 \begin{methoddesc
}{r_exec
}{code
}
100 \var{code
} must either be a string containing one or more lines of
101 Python code, or a compiled code object, which will be executed in the
102 restricted environment's
\module{__main__
} module.
105 \begin{methoddesc
}{r_execfile
}{filename
}
106 Execute the Python code contained in the file
\var{filename
} in the
107 restricted environment's
\module{__main__
} module.
110 Methods whose names begin with
\samp{s_
} are similar to the functions
111 beginning with
\samp{r_
}, but the code will be granted access to
112 restricted versions of the standard I/O streams
\code{sys.stdin
},
113 \code{sys.stderr
}, and
\code{sys.stdout
}.
115 \begin{methoddesc
}{s_eval
}{code
}
116 \var{code
} must be a string containing a Python expression, which will
117 be evaluated in the restricted environment.
120 \begin{methoddesc
}{s_exec
}{code
}
121 \var{code
} must be a string containing one or more lines of Python code,
122 which will be executed in the restricted environment.
125 \begin{methoddesc
}{s_execfile
}{code
}
126 Execute the Python code contained in the file
\var{filename
} in the
127 restricted environment.
130 \class{RExec
} objects must also support various methods which will be
131 implicitly called by code executing in the restricted environment.
132 Overriding these methods in a subclass is used to change the policies
133 enforced by a restricted environment.
135 \begin{methoddesc
}{r_import
}{modulename
\optional{, globals
\optional{,
136 locals
\optional{, fromlist
}}}}
137 Import the module
\var{modulename
}, raising an
\exception{ImportError
}
138 exception if the module is considered unsafe.
141 \begin{methoddesc
}{r_open
}{filename
\optional{, mode
\optional{, bufsize
}}}
142 Method called when
\function{open()
} is called in the restricted
143 environment. The arguments are identical to those of
\function{open()
},
144 and a file object (or a class instance compatible with file objects)
145 should be returned.
\class{RExec
}'s default behaviour is allow opening
146 any file for reading, but forbidding any attempt to write a file. See
147 the example below for an implementation of a less restrictive
151 \begin{methoddesc
}{r_reload
}{module
}
152 Reload the module object
\var{module
}, re-parsing and re-initializing it.
155 \begin{methoddesc
}{r_unload
}{module
}
156 Unload the module object
\var{module
} (remove it from the
157 restricted environment's
\code{sys.modules
} dictionary).
160 And their equivalents with access to restricted standard I/O streams:
162 \begin{methoddesc
}{s_import
}{modulename
\optional{, globals
\optional{,
163 locals
\optional{, fromlist
}}}}
164 Import the module
\var{modulename
}, raising an
\exception{ImportError
}
165 exception if the module is considered unsafe.
168 \begin{methoddesc
}{s_reload
}{module
}
169 Reload the module object
\var{module
}, re-parsing and re-initializing it.
172 \begin{methoddesc
}{s_unload
}{module
}
173 Unload the module object
\var{module
}.
174 % XXX what are the semantics of this?
178 \subsection{Defining restricted environments
\label{rexec-extension
}}
180 The
\class{RExec
} class has the following class attributes, which are
181 used by the
\method{__init__()
} method. Changing them on an existing
182 instance won't have any effect; instead, create a subclass of
183 \class{RExec
} and assign them new values in the class definition.
184 Instances of the new class will then use those new values. All these
185 attributes are tuples of strings.
187 \begin{memberdesc
}{nok_builtin_names
}
188 Contains the names of built-in functions which will
\emph{not
} be
189 available to programs running in the restricted environment. The
190 value for
\class{RExec
} is
\code{('open', 'reload', '__import__')
}.
191 (This gives the exceptions, because by far the majority of built-in
192 functions are harmless. A subclass that wants to override this
193 variable should probably start with the value from the base class and
194 concatenate additional forbidden functions --- when new dangerous
195 built-in functions are added to Python, they will also be added to
199 \begin{memberdesc
}{ok_builtin_modules
}
200 Contains the names of built-in modules which can be safely imported.
201 The value for
\class{RExec
} is
\code{('audioop', 'array', 'binascii',
202 'cmath', 'errno', 'imageop', 'marshal', 'math', 'md5', 'operator',
203 'parser', 'regex', 'select', 'sha', '_sre', 'strop',
204 'struct', 'time')
}. A similar remark about overriding this variable
205 applies --- use the value from the base class as a starting point.
208 \begin{memberdesc
}{ok_path
}
209 Contains the directories which will be searched when an
\keyword{import
}
210 is performed in the restricted environment.
211 The value for
\class{RExec
} is the same as
\code{sys.path
} (at the time
212 the module is loaded) for unrestricted code.
215 \begin{memberdesc
}{ok_posix_names
}
216 % Should this be called ok_os_names?
217 Contains the names of the functions in the
\refmodule{os
} module which will be
218 available to programs running in the restricted environment. The
219 value for
\class{RExec
} is
\code{('error', 'fstat', 'listdir',
220 'lstat', 'readlink', 'stat', 'times', 'uname', 'getpid', 'getppid',
221 'getcwd', 'getuid', 'getgid', 'geteuid', 'getegid')
}.
224 \begin{memberdesc
}{ok_sys_names
}
225 Contains the names of the functions and variables in the
\refmodule{sys
}
226 module which will be available to programs running in the restricted
227 environment. The value for
\class{RExec
} is
\code{('ps1', 'ps2',
228 'copyright', 'version', 'platform', 'exit', 'maxint')
}.
231 \begin{memberdesc
}{ok_file_types
}
232 Contains the file types from which modules are allowed to be loaded.
233 Each file type is an integer constant defined in the
\refmodule{imp
} module.
234 The meaningful values are
\constant{PY_SOURCE
},
\constant{PY_COMPILED
}, and
235 \constant{C_EXTENSION
}. The value for
\class{RExec
} is
\code{(C_EXTENSION,
236 PY_SOURCE)
}. Adding
\constant{PY_COMPILED
} in subclasses is not recommended;
237 an attacker could exit the restricted execution mode by putting a forged
238 byte-compiled file (
\file{.pyc
}) anywhere in your file system, for example
239 by writing it to
\file{/tmp
} or uploading it to the
\file{/incoming
}
240 directory of your public FTP server.
244 \subsection{An example
}
246 Let us say that we want a slightly more relaxed policy than the
247 standard
\class{RExec
} class. For example, if we're willing to allow
248 files in
\file{/tmp
} to be written, we can subclass the
\class{RExec
}
252 class TmpWriterRExec(rexec.RExec):
253 def r_open(self, file, mode='r', buf=-
1):
254 if mode in ('r', 'rb'):
256 elif mode in ('w', 'wb', 'a', 'ab'):
257 # check filename : must begin with /tmp/
258 if file
[:
5]!='/tmp/':
259 raise IOError, "can't write outside /tmp"
260 elif (string.find(file, '/../') >=
0 or
261 file
[:
3] == '../' or file
[-
3:
] == '/..'):
262 raise IOError, "'..' in filename forbidden"
263 else: raise IOError, "Illegal open() mode"
264 return open(file, mode, buf)
267 Notice that the above code will occasionally forbid a perfectly valid
268 filename; for example, code in the restricted environment won't be
269 able to open a file called
\file{/tmp/foo/../bar
}. To fix this, the
270 \method{r_open()
} method would have to simplify the filename to
271 \file{/tmp/bar
}, which would require splitting apart the filename and
272 performing various operations on it. In cases where security is at
273 stake, it may be preferable to write simple code which is sometimes
274 overly restrictive, instead of more general code that is also more
275 complex and may harbor a subtle security hole.