219 lines
9.3 KiB
TeX
219 lines
9.3 KiB
TeX
\section{Standard Module \sectcode{rexec}}
|
|
\stmodindex{rexec}
|
|
\renewcommand{\indexsubitem}{(in module rexec)}
|
|
|
|
This module contains the \code{RExec} class, which supports
|
|
\code{r_exec()}, \code{r_eval()}, \code{r_execfile()}, and
|
|
\code{r_import()} methods, which are restricted versions of the standard
|
|
Python functions \code{exec()}, \code{eval()}, \code{execfile()}, and
|
|
the \code{import} statement.
|
|
Code executed in this restricted environment will
|
|
only have access to modules and functions that are deemed safe; you
|
|
can subclass \code{RExec} to add or remove capabilities as desired.
|
|
|
|
\emph{Note:} The \code{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 CPU time.
|
|
|
|
\begin{funcdesc}{RExec}{\optional{hooks\optional{\, verbose}}}
|
|
Returns an instance of the \code{RExec} class.
|
|
|
|
\var{hooks} is an instance of the \code{RHooks} class or a subclass of it.
|
|
If it is omitted or \code{None}, the default \code{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, e.g. using packages.)
|
|
|
|
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 \var{verbose} is true, additional debugging output may be sent to
|
|
standard output.
|
|
\end{funcdesc}
|
|
|
|
The RExec class has the following class attributes, which are used by the
|
|
\code{__init__} method. Changing them on an existing instance won't
|
|
have any effect; instead, create a subclass of \code{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.
|
|
|
|
\renewcommand{\indexsubitem}{(RExec object attribute)}
|
|
\begin{datadesc}{nok_builtin_names}
|
|
Contains the names of built-in functions which will \emph{not} be
|
|
available to programs running in the restricted environment. The
|
|
value for \code{RExec} is \code{('open',} \code{'reload',}
|
|
\code{'__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.)
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{ok_builtin_modules}
|
|
Contains the names of built-in modules which can be safely imported.
|
|
The value for \code{RExec} is \code{('audioop',} \code{'array',}
|
|
\code{'binascii',} \code{'cmath',} \code{'errno',} \code{'imageop',}
|
|
\code{'marshal',} \code{'math',} \code{'md5',} \code{'operator',}
|
|
\code{'parser',} \code{'regex',} \code{'rotor',} \code{'select',}
|
|
\code{'strop',} \code{'struct',} \code{'time')}. A similar remark
|
|
about overriding this variable applies --- use the value from the base
|
|
class as a starting point.
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{ok_path}
|
|
Contains the directories which will be searched when an \code{import}
|
|
is performed in the restricted environment.
|
|
The value for \code{RExec} is the same as \code{sys.path} (at the time
|
|
the module is loaded) for unrestricted code.
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{ok_posix_names}
|
|
% Should this be called ok_os_names?
|
|
Contains the names of the functions in the \code{os} module which will be
|
|
available to programs running in the restricted environment. The
|
|
value for \code{RExec} is \code{('error',} \code{'fstat',}
|
|
\code{'listdir',} \code{'lstat',} \code{'readlink',} \code{'stat',}
|
|
\code{'times',} \code{'uname',} \code{'getpid',} \code{'getppid',}
|
|
\code{'getcwd',} \code{'getuid',} \code{'getgid',} \code{'geteuid',}
|
|
\code{'getegid')}.
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{ok_sys_names}
|
|
Contains the names of the functions and variables in the \code{sys}
|
|
module which will be available to programs running in the restricted
|
|
environment. The value for \code{RExec} is \code{('ps1',}
|
|
\code{'ps2',} \code{'copyright',} \code{'version',} \code{'platform',}
|
|
\code{'exit',} \code{'maxint')}.
|
|
\end{datadesc}
|
|
|
|
RExec instances support the following methods:
|
|
\renewcommand{\indexsubitem}{(RExec object method)}
|
|
|
|
\begin{funcdesc}{r_eval}{code}
|
|
\var{code} must either be a string containing a Python expression, or
|
|
a compiled code object, which will be evaluated in the restricted
|
|
environment's \code{__main__} module. The value of the expression or
|
|
code object will be returned.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{r_exec}{code}
|
|
\var{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 \code{__main__} module.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{r_execfile}{filename}
|
|
Execute the Python code contained in the file \var{filename} in the
|
|
restricted environment's \code{__main__} module.
|
|
\end{funcdesc}
|
|
|
|
Methods whose names begin with \code{s_} are similar to the functions
|
|
beginning with \code{r_}, but the code will be granted access to
|
|
restricted versions of the standard I/O streans \code{sys.stdin},
|
|
\code{sys.stderr}, and \code{sys.stdout}.
|
|
|
|
\begin{funcdesc}{s_eval}{code}
|
|
\var{code} must be a string containing a Python expression, which will
|
|
be evaluated in the restricted environment.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{s_exec}{code}
|
|
\var{code} must be a string containing one or more lines of Python code,
|
|
which will be executed in the restricted environment.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{s_execfile}{code}
|
|
Execute the Python code contained in the file \var{filename} in the
|
|
restricted environment.
|
|
\end{funcdesc}
|
|
|
|
\code{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.
|
|
|
|
\begin{funcdesc}{r_import}{modulename\optional{\, globals\, locals\, fromlist}}
|
|
Import the module \var{modulename}, raising an \code{ImportError}
|
|
exception if the module is considered unsafe.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{r_open}{filename\optional{\, mode\optional{\, bufsize}}}
|
|
Method called when \code{open()} is called in the restricted
|
|
environment. The arguments are identical to those of \code{open()},
|
|
and a file object (or a class instance compatible with file objects)
|
|
should be returned. \code{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
|
|
\code{r_open()}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{r_reload}{module}
|
|
Reload the module object \var{module}, re-parsing and re-initializing it.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{r_unload}{module}
|
|
Unload the module object \var{module} (i.e., remove it from the
|
|
restricted environment's \code{sys.modules} dictionary).
|
|
\end{funcdesc}
|
|
|
|
And their equivalents with access to restricted standard I/O streams:
|
|
|
|
\begin{funcdesc}{s_import}{modulename\optional{\, globals, locals, fromlist}}
|
|
Import the module \var{modulename}, raising an \code{ImportError}
|
|
exception if the module is considered unsafe.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{s_reload}{module}
|
|
Reload the module object \var{module}, re-parsing and re-initializing it.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{s_unload}{module}
|
|
Unload the module object \var{module}.
|
|
% XXX what are the semantics of this?
|
|
\end{funcdesc}
|
|
|
|
\subsection{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
|
|
\file{/tmp} to be written, we can subclass the \code{RExec} class:
|
|
|
|
\bcode\begin{verbatim}
|
|
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)
|
|
\end{verbatim}\ecode
|
|
|
|
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 \file{/tmp/foo/../bar}. To fix this, the
|
|
\code{r_open} method would have to simplify the filename to
|
|
\file{/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.
|