Doc/lib/librexec.tex

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