Doc/lib/librexec.tex

   1 \section{\module{rexec} ---
   2          Basic restricted execution framework.}
   3 \declaremodule{standard}{rexec}
   4
   5 \modulesynopsis{Basic restricted execution framework.}
   6
   7
   8
   9 This module contains the \class{RExec} class, which supports
  10 \method{r_eval()}, \method{r_execfile()}, \method{r_exec()}, and
  11 \method{r_import()} methods, which are restricted versions of the standard
  12 Python functions \method{eval()}, \method{execfile()} and
  13 the \keyword{exec} and \keyword{import} statements.
  14 Code executed in this restricted environment will
  15 only have access to modules and functions that are deemed safe; you
  16 can subclass \class{RExec} to add or remove capabilities as desired.
  17
  18 \emph{Note:} The \class{RExec} class can prevent code from performing
  19 unsafe operations like reading or writing disk files, or using TCP/IP
  20 sockets.  However, it does not protect against code using extremely
  21 large amounts of memory or CPU time.
  22
  23 \begin{classdesc}{RExec}{\optional{hooks\optional{, verbose}}}
  24 Returns an instance of the \class{RExec} class.
  25
  26 \var{hooks} is an instance of the \class{RHooks} class or a subclass of it.
  27 If it is omitted or \code{None}, the default \class{RHooks} class is
  28 instantiated.
  29 Whenever the \module{RExec} module searches for a module (even a
  30 built-in one) or reads a module's code, it doesn't actually go out to
  31 the file system itself.  Rather, it calls methods of an \class{RHooks}
  32 instance that was passed to or created by its constructor.  (Actually,
  33 the \class{RExec} object doesn't make these calls --- they are made by
  34 a module loader object that's part of the \class{RExec} object.  This
  35 allows another level of flexibility, e.g. using packages.)
  36
  37 By providing an alternate \class{RHooks} object, we can control the
  38 file system accesses made to import a module, without changing the
  39 actual algorithm that controls the order in which those accesses are
  40 made.  For instance, we could substitute an \class{RHooks} object that
  41 passes all filesystem requests to a file server elsewhere, via some
  42 RPC mechanism such as ILU.  Grail's applet loader uses this to support
  43 importing applets from a URL for a directory.
  44
  45 If \var{verbose} is true, additional debugging output may be sent to
  46 standard output.
  47 \end{classdesc}
  48
  49 The \class{RExec} class has the following class attributes, which are
  50 used by the \method{__init__()} method.  Changing them on an existing
  51 instance won't have any effect; instead, create a subclass of
  52 \class{RExec} and assign them new values in the class definition.
  53 Instances of the new class will then use those new values.  All these
  54 attributes are tuples of strings.
  55
  56 \begin{memberdesc}{nok_builtin_names}
  57 Contains the names of built-in functions which will \emph{not} be
  58 available to programs running in the restricted environment.  The
  59 value for \class{RExec} is \code{('open',} \code{'reload',}
  60 \code{'__import__')}.  (This gives the exceptions, because by far the
  61 majority of built-in functions are harmless.  A subclass that wants to
  62 override this variable should probably start with the value from the
  63 base class and concatenate additional forbidden functions --- when new
  64 dangerous built-in functions are added to Python, they will also be
  65 added to this module.)
  66 \end{memberdesc}
  67
  68 \begin{memberdesc}{ok_builtin_modules}
  69 Contains the names of built-in modules which can be safely imported.
  70 The value for \class{RExec} is \code{('audioop',} \code{'array',}
  71 \code{'binascii',} \code{'cmath',} \code{'errno',} \code{'imageop',}
  72 \code{'marshal',} \code{'math',} \code{'md5',} \code{'operator',}
  73 \code{'parser',} \code{'regex',} \code{'rotor',} \code{'select',}
  74 \code{'strop',} \code{'struct',} \code{'time')}.  A similar remark
  75 about overriding this variable applies --- use the value from the base
  76 class as a starting point.
  77 \end{memberdesc}
  78
  79 \begin{memberdesc}{ok_path}
  80 Contains the directories which will be searched when an \keyword{import}
  81 is performed in the restricted environment.
  82 The value for \class{RExec} is the same as \code{sys.path} (at the time
  83 the module is loaded) for unrestricted code.
  84 \end{memberdesc}
  85
  86 \begin{memberdesc}{ok_posix_names}
  87 % Should this be called ok_os_names?
  88 Contains the names of the functions in the \module{os} module which will be
  89 available to programs running in the restricted environment.  The
  90 value for \class{RExec} is \code{('error',} \code{'fstat',}
  91 \code{'listdir',} \code{'lstat',} \code{'readlink',} \code{'stat',}
  92 \code{'times',} \code{'uname',} \code{'getpid',} \code{'getppid',}
  93 \code{'getcwd',} \code{'getuid',} \code{'getgid',} \code{'geteuid',}
  94 \code{'getegid')}.
  95 \end{memberdesc}
  96
  97 \begin{memberdesc}{ok_sys_names}
  98 Contains the names of the functions and variables in the \module{sys}
  99 module which will be available to programs running in the restricted
 100 environment.  The value for \class{RExec} is \code{('ps1',}
 101 \code{'ps2',} \code{'copyright',} \code{'version',} \code{'platform',}
 102 \code{'exit',} \code{'maxint')}.
 103 \end{memberdesc}
 104
 105
 106 \class{RExec} instances support the following methods:
 107
 108 \begin{methoddesc}{r_eval}{code}
 109 \var{code} must either be a string containing a Python expression, or
 110 a compiled code object, which will be evaluated in the restricted
 111 environment's \module{__main__} module.  The value of the expression or
 112 code object will be returned.
 113 \end{methoddesc}
 114
 115 \begin{methoddesc}{r_exec}{code}
 116 \var{code} must either be a string containing one or more lines of
 117 Python code, or a compiled code object, which will be executed in the
 118 restricted environment's \module{__main__} module.
 119 \end{methoddesc}
 120
 121 \begin{methoddesc}{r_execfile}{filename}
 122 Execute the Python code contained in the file \var{filename} in the
 123 restricted environment's \module{__main__} module.
 124 \end{methoddesc}
 125
 126 Methods whose names begin with \samp{s_} are similar to the functions
 127 beginning with \samp{r_}, but the code will be granted access to
 128 restricted versions of the standard I/O streams \code{sys.stdin},
 129 \code{sys.stderr}, and \code{sys.stdout}.
 130
 131 \begin{methoddesc}{s_eval}{code}
 132 \var{code} must be a string containing a Python expression, which will
 133 be evaluated in the restricted environment.
 134 \end{methoddesc}
 135
 136 \begin{methoddesc}{s_exec}{code}
 137 \var{code} must be a string containing one or more lines of Python code,
 138 which will be executed in the restricted environment.
 139 \end{methoddesc}
 140
 141 \begin{methoddesc}{s_execfile}{code}
 142 Execute the Python code contained in the file \var{filename} in the
 143 restricted environment.
 144 \end{methoddesc}
 145
 146 \class{RExec} objects must also support various methods which will be
 147 implicitly called by code executing in the restricted environment.
 148 Overriding these methods in a subclass is used to change the policies
 149 enforced by a restricted environment.
 150
 151 \begin{methoddesc}{r_import}{modulename\optional{, globals\optional{,
 152                              locals\optional{, fromlist}}}}
 153 Import the module \var{modulename}, raising an \exception{ImportError}
 154 exception if the module is considered unsafe.
 155 \end{methoddesc}
 156
 157 \begin{methoddesc}{r_open}{filename\optional{, mode\optional{, bufsize}}}
 158 Method called when \function{open()} is called in the restricted
 159 environment.  The arguments are identical to those of \function{open()},
 160 and a file object (or a class instance compatible with file objects)
 161 should be returned.  \class{RExec}'s default behaviour is allow opening
 162 any file for reading, but forbidding any attempt to write a file.  See
 163 the example below for an implementation of a less restrictive
 164 \method{r_open()}.
 165 \end{methoddesc}
 166
 167 \begin{methoddesc}{r_reload}{module}
 168 Reload the module object \var{module}, re-parsing and re-initializing it.
 169 \end{methoddesc}
 170
 171 \begin{methoddesc}{r_unload}{module}
 172 Unload the module object \var{module} (i.e., remove it from the
 173 restricted environment's \code{sys.modules} dictionary).
 174 \end{methoddesc}
 175
 176 And their equivalents with access to restricted standard I/O streams:
 177
 178 \begin{methoddesc}{s_import}{modulename\optional{, globals\optional{,
 179                              locals\optional{, fromlist}}}}
 180 Import the module \var{modulename}, raising an \exception{ImportError}
 181 exception if the module is considered unsafe.
 182 \end{methoddesc}
 183
 184 \begin{methoddesc}{s_reload}{module}
 185 Reload the module object \var{module}, re-parsing and re-initializing it.
 186 \end{methoddesc}
 187
 188 \begin{methoddesc}{s_unload}{module}
 189 Unload the module object \var{module}.
 190 % XXX what are the semantics of this?
 191 \end{methoddesc}
 192
 193 \subsection{An example}
 194
 195 Let us say that we want a slightly more relaxed policy than the
 196 standard \class{RExec} class.  For example, if we're willing to allow
 197 files in \file{/tmp} to be written, we can subclass the \class{RExec}
 198 class:
 199
 200 \begin{verbatim}
 201 class TmpWriterRExec(rexec.RExec):
 202     def r_open(self, file, mode='r', buf=-1):
 203         if mode in ('r', 'rb'):
 204             pass
 205         elif mode in ('w', 'wb', 'a', 'ab'):
 206             # check filename : must begin with /tmp/
 207             if file[:5]!='/tmp/':
 208                 raise IOError, "can't write outside /tmp"
 209             elif (string.find(file, '/../') >= 0 or
 210                  file[:3] == '../' or file[-3:] == '/..'):
 211                 raise IOError, "'..' in filename forbidden"
 212         else: raise IOError, "Illegal open() mode"
 213         return open(file, mode, buf)
 214 \end{verbatim}
 215 %
 216 Notice that the above code will occasionally forbid a perfectly valid
 217 filename; for example, code in the restricted environment won't be
 218 able to open a file called \file{/tmp/foo/../bar}.  To fix this, the
 219 \method{r_open()} method would have to simplify the filename to
 220 \file{/tmp/bar}, which would require splitting apart the filename and
 221 performing various operations on it.  In cases where security is at
 222 stake, it may be preferable to write simple code which is sometimes
 223 overly restrictive, instead of more general code that is also more
 224 complex and may harbor a subtle security hole.