\section{\module{rexec} ---         Restricted execution framework} \declaremodule{standard}{rexec}\modulesynopsis{Basic restricted execution framework.}\versionchanged[Disabled module]{2.3} \begin{notice}[warning]  The documentation has been left in place to help in reading old code  that uses the module.\end{notice} This module contains the \class{RExec} class, which supports\method{r_eval()}, \method{r_execfile()}, \method{r_exec()}, and\method{r_import()} methods, which are restricted versions of the standardPython functions \method{eval()}, \method{execfile()} andthe \keyword{exec} and \keyword{import} statements.Code executed in this restricted environment willonly have access to modules and functions that are deemed safe; youcan subclass \class{RExec} to add or remove capabilities as desired. \begin{notice}[warning]  While the \module{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 \module{rexec}  vulnerabilities would be welcomed.\end{notice} \begin{notice}  The \class{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.\end{notice} \begin{classdesc}{RExec}{\optional{hooks\optional{, verbose}}}Returns an instance of the \class{RExec} class.   \var{hooks} is an instance of the \class{RHooks} class or a subclass of it.If it is omitted or \code{None}, the default \class{RHooks} class isinstantiated.Whenever the \module{rexec} module searches for a module (even abuilt-in one) or reads a module's code, it doesn't actually go out tothe file system itself.  Rather, it calls methods of an \class{RHooks}instance that was passed to or created by its constructor.  (Actually,the \class{RExec} object doesn't make these calls --- they are made bya module loader object that's part of the \class{RExec} object.  Thisallows another level of flexibility, which can be useful when changingthe mechanics of \keyword{import} within the restricted environment.) By providing an alternate \class{RHooks} object, we can control thefile system accesses made to import a module, without changing theactual algorithm that controls the order in which those accesses aremade.  For instance, we could substitute an \class{RHooks} object thatpasses all filesystem requests to a file server elsewhere, via someRPC mechanism such as ILU.  Grail's applet loader uses this to supportimporting applets from a URL for a directory. If \var{verbose} is true, additional debugging output may be sent tostandard output.\end{classdesc} It is important to be aware that code running in a restrictedenvironment can still call the \function{sys.exit()} function.  Todisallow restricted code from exiting the interpreter, always protectcalls that cause restricted code to run with a\keyword{try}/\keyword{except} statement that catches the\exception{SystemExit} exception.  Removing the \function{sys.exit()}function from the restricted environment is not sufficient --- therestricted code could still use \code{raise SystemExit}.  Removing\exception{SystemExit} is not a reasonable option; some library codemakes use of this and would break were it not available.  \begin{seealso}  \seetitle[http://grail.sourceforge.net/]{Grail Home Page}{Grail is a            Web browser written entirely in Python.  It uses the            \module{rexec} module as a foundation for supporting            Python applets, and can be used as an example usage of            this module.}\end{seealso}  \subsection{RExec Objects \label{rexec-objects}} \class{RExec} instances support the following methods: \begin{methoddesc}{r_eval}{code}\var{code} must either be a string containing a Python expression, ora compiled code object, which will be evaluated in the restrictedenvironment's \module{__main__} module.  The value of the expression orcode object will be returned.\end{methoddesc} \begin{methoddesc}{r_exec}{code}\var{code} must either be a string containing one or more lines ofPython code, or a compiled code object, which will be executed in therestricted environment's \module{__main__} module.\end{methoddesc} \begin{methoddesc}{r_execfile}{filename}Execute the Python code contained in the file \var{filename} in therestricted environment's \module{__main__} module.\end{methoddesc} Methods whose names begin with \samp{s_} are similar to the functionsbeginning with \samp{r_}, but the code will be granted access torestricted versions of the standard I/O streams \code{sys.stdin},\code{sys.stderr}, and \code{sys.stdout}. \begin{methoddesc}{s_eval}{code}\var{code} must be a string containing a Python expression, which willbe evaluated in the restricted environment.  \end{methoddesc} \begin{methoddesc}{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{methoddesc} \begin{methoddesc}{s_execfile}{code}Execute the Python code contained in the file \var{filename} in therestricted environment.\end{methoddesc} \class{RExec} objects must also support various methods which will beimplicitly called by code executing in the restricted environment.Overriding these methods in a subclass is used to change the policiesenforced by a restricted environment. \begin{methoddesc}{r_import}{modulename\optional{, globals\optional{,                             locals\optional{, fromlist}}}}Import the module \var{modulename}, raising an \exception{ImportError}exception if the module is considered unsafe.\end{methoddesc} \begin{methoddesc}{r_open}{filename\optional{, mode\optional{, bufsize}}}Method called when \function{open()} is called in the restrictedenvironment.  The arguments are identical to those of \function{open()},and a file object (or a class instance compatible with file objects)should be returned.  \class{RExec}'s default behaviour is allow openingany file for reading, but forbidding any attempt to write a file.  Seethe example below for an implementation of a less restrictive\method{r_open()}.\end{methoddesc} \begin{methoddesc}{r_reload}{module}Reload the module object \var{module}, re-parsing and re-initializing it.  \end{methoddesc} \begin{methoddesc}{r_unload}{module}Unload the module object \var{module} (remove it from therestricted environment's \code{sys.modules} dictionary).\end{methoddesc} And their equivalents with access to restricted standard I/O streams: \begin{methoddesc}{s_import}{modulename\optional{, globals\optional{,                             locals\optional{, fromlist}}}}Import the module \var{modulename}, raising an \exception{ImportError}exception if the module is considered unsafe.\end{methoddesc} \begin{methoddesc}{s_reload}{module}Reload the module object \var{module}, re-parsing and re-initializing it.  \end{methoddesc} \begin{methoddesc}{s_unload}{module}Unload the module object \var{module}.   % XXX what are the semantics of this?  \end{methoddesc}  \subsection{Defining restricted environments \label{rexec-extension}} The \class{RExec} class has the following class attributes, which areused by the \method{__init__()} method.  Changing them on an existinginstance won't have any effect; instead, create a subclass of\class{RExec} and assign them new values in the class definition.Instances of the new class will then use those new values.  All theseattributes are tuples of strings. \begin{memberdesc}{nok_builtin_names}Contains the names of built-in functions which will \emph{not} beavailable to programs running in the restricted environment.  Thevalue for \class{RExec} is \code{('open', 'reload', '__import__')}.(This gives the exceptions, because by far the majority of built-infunctions are harmless.  A subclass that wants to override thisvariable should probably start with the value from the base class andconcatenate additional forbidden functions --- when new dangerousbuilt-in functions are added to Python, they will also be added tothis module.)\end{memberdesc} \begin{memberdesc}{ok_builtin_modules}Contains the names of built-in modules which can be safely imported.The value for \class{RExec} is \code{('audioop', 'array', 'binascii','cmath', 'errno', 'imageop', 'marshal', 'math', 'md5', 'operator','parser', 'regex', 'select', 'sha', '_sre', 'strop','struct', 'time')}.  A similar remark about overriding this variableapplies --- use the value from the base class as a starting point.\end{memberdesc} \begin{memberdesc}{ok_path}Contains the directories which will be searched when an \keyword{import}is performed in the restricted environment.  The value for \class{RExec} is the same as \code{sys.path} (at the timethe module is loaded) for unrestricted code.\end{memberdesc} \begin{memberdesc}{ok_posix_names}% Should this be called ok_os_names?Contains the names of the functions in the \refmodule{os} module which will beavailable to programs running in the restricted environment.  Thevalue for \class{RExec} is \code{('error', 'fstat', 'listdir','lstat', 'readlink', 'stat', 'times', 'uname', 'getpid', 'getppid','getcwd', 'getuid', 'getgid', 'geteuid', 'getegid')}.\end{memberdesc} \begin{memberdesc}{ok_sys_names}Contains the names of the functions and variables in the \refmodule{sys}module which will be available to programs running in the restrictedenvironment.  The value for \class{RExec} is \code{('ps1', 'ps2','copyright', 'version', 'platform', 'exit', 'maxint')}.\end{memberdesc} \begin{memberdesc}{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 \refmodule{imp} module.The meaningful values are \constant{PY_SOURCE}, \constant{PY_COMPILED}, and\constant{C_EXTENSION}.  The value for \class{RExec} is \code{(C_EXTENSION,PY_SOURCE)}.  Adding \constant{PY_COMPILED} in subclasses is not recommended;an attacker could exit the restricted execution mode by putting a forgedbyte-compiled file (\file{.pyc}) anywhere in your file system, for exampleby writing it to \file{/tmp} or uploading it to the \file{/incoming}directory of your public FTP server.\end{memberdesc}  \subsection{An example} Let us say that we want a slightly more relaxed policy than thestandard \class{RExec} class.  For example, if we're willing to allowfiles in \file{/tmp} to be written, we can subclass the \class{RExec}class: \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}%Notice that the above code will occasionally forbid a perfectly validfilename; for example, code in the restricted environment won't beable to open a file called \file{/tmp/foo/../bar}.  To fix this, the\method{r_open()} method would have to simplify the filename to\file{/tmp/bar}, which would require splitting apart the filename andperforming various operations on it.  In cases where security is atstake, it may be preferable to write simple code which is sometimesoverly restrictive, instead of more general code that is also morecomplex and may harbor a subtle security hole.