mingw/html/lib/Safe.html

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  13 <big><strong><span class="block">&nbsp;Safe - Compile and execute code in restricted compartments</span></strong></big>
  14 </td></tr>
  15 </table>
  16
  17 <p><a name="__index__"></a></p>
  18 <!-- INDEX BEGIN -->
  19
  20 <ul>
  21
  22         <li><a href="#name">NAME</a></li>
  23         <li><a href="#synopsis">SYNOPSIS</a></li>
  24         <li><a href="#description">DESCRIPTION</a></li>
  25         <li><a href="#warning">WARNING</a></li>
  26         <ul>
  27
  28                 <li><a href="#recent_changes">RECENT CHANGES</a></li>
  29                 <li><a href="#methods_in_class_safe">Methods in class Safe</a></li>
  30                 <li><a href="#some_safety_issues">Some Safety Issues</a></li>
  31                 <li><a href="#author">AUTHOR</a></li>
  32         </ul>
  33
  34 </ul>
  35 <!-- INDEX END -->
  36
  37 <hr />
  38 <p>
  39 </p>
  40 <h1><a name="name">NAME</a></h1>
  41 <p>Safe - Compile and execute code in restricted compartments</p>
  42 <p>
  43 </p>
  44 <hr />
  45 <h1><a name="synopsis">SYNOPSIS</a></h1>
  46 <pre>
  47   use Safe;</pre>
  48 <pre>
  49   $compartment = new Safe;</pre>
  50 <pre>
  51   $compartment-&gt;permit(qw(time sort :browse));</pre>
  52 <pre>
  53   $result = $compartment-&gt;reval($unsafe_code);</pre>
  54 <p>
  55 </p>
  56 <hr />
  57 <h1><a name="description">DESCRIPTION</a></h1>
  58 <p>The Safe extension module allows the creation of compartments
  59 in which perl code can be evaluated. Each compartment has</p>
  60 <dl>
  61 <dt><strong><a name="item_a_new_namespace">a new namespace</a></strong>
  62
  63 <dd>
  64 <p>The ``root'' of the namespace (i.e. ``main::'') is changed to a
  65 different package and code evaluated in the compartment cannot
  66 refer to variables outside this namespace, even with run-time
  67 glob lookups and other tricks.</p>
  68 </dd>
  69 <dd>
  70 <p>Code which is compiled outside the compartment can choose to place
  71 variables into (or <em>share</em> variables with) the compartment's namespace
  72 and only that data will be visible to code evaluated in the
  73 compartment.</p>
  74 </dd>
  75 <dd>
  76 <p>By default, the only variables shared with compartments are the
  77 ``underscore'' variables $_ and @_ (and, technically, the less frequently
  78 used %_, the _ filehandle and so on). This is because otherwise perl
  79 operators which default to $_ will not work and neither will the
  80 assignment of arguments to @_ on subroutine entry.</p>
  81 </dd>
  82 </li>
  83 <dt><strong><a name="item_an_operator_mask">an operator mask</a></strong>
  84
  85 <dd>
  86 <p>Each compartment has an associated ``operator mask''. Recall that
  87 perl code is compiled into an internal format before execution.
  88 Evaluating perl code (e.g. via ``eval'' or ``do 'file''') causes
  89 the code to be compiled into an internal format and then,
  90 provided there was no error in the compilation, executed.
  91 Code evaluated in a compartment compiles subject to the
  92 compartment's operator mask. Attempting to evaluate code in a
  93 compartment which contains a masked operator will cause the
  94 compilation to fail with an error. The code will not be executed.</p>
  95 </dd>
  96 <dd>
  97 <p>The default operator mask for a newly created compartment is
  98 the ':default' optag.</p>
  99 </dd>
 100 <dd>
 101 <p>It is important that you read the <code>Opcode(3)</code> module documentation
 102 for more information, especially for detailed definitions of opnames,
 103 optags and opsets.</p>
 104 </dd>
 105 <dd>
 106 <p>Since it is only at the compilation stage that the operator mask
 107 applies, controlled access to potentially unsafe operations can
 108 be achieved by having a handle to a wrapper subroutine (written
 109 outside the compartment) placed into the compartment. For example,</p>
 110 </dd>
 111 <dd>
 112 <pre>
 113     $cpt = new Safe;
 114     sub wrapper {
 115         # vet arguments and perform potentially unsafe operations
 116     }
 117     $cpt-&gt;share('&amp;wrapper');</pre>
 118 </dd>
 119 </li>
 120 </dl>
 121 <p>
 122 </p>
 123 <hr />
 124 <h1><a name="warning">WARNING</a></h1>
 125 <p>The authors make <strong>no warranty</strong>, implied or otherwise, about the
 126 suitability of this software for safety or security purposes.</p>
 127 <p>The authors shall not in any case be liable for special, incidental,
 128 consequential, indirect or other similar damages arising from the use
 129 of this software.</p>
 130 <p>Your mileage will vary. If in any doubt <strong>do not use it</strong>.</p>
 131 <p>
 132 </p>
 133 <h2><a name="recent_changes">RECENT CHANGES</a></h2>
 134 <p>The interface to the Safe module has changed quite dramatically since
 135 version 1 (as supplied with Perl5.002). Study these pages carefully if
 136 you have code written to use Safe version 1 because you will need to
 137 makes changes.</p>
 138 <p>
 139 </p>
 140 <h2><a name="methods_in_class_safe">Methods in class Safe</a></h2>
 141 <p>To create a new compartment, use</p>
 142 <pre>
 143     $cpt = new Safe;</pre>
 144 <p>Optional argument is (NAMESPACE), where NAMESPACE is the root namespace
 145 to use for the compartment (defaults to ``Safe::Root0'', incremented for
 146 each new compartment).</p>
 147 <p>Note that version 1.00 of the Safe module supported a second optional
 148 parameter, MASK.  That functionality has been withdrawn pending deeper
 149 consideration. Use the permit and deny methods described below.</p>
 150 <p>The following methods can then be used on the compartment
 151 object returned by the above constructor. The object argument
 152 is implicit in each case.</p>
 153 <dl>
 154 <dt><strong><a name="item_permit">permit (OP, ...)</a></strong>
 155
 156 <dd>
 157 <p>Permit the listed operators to be used when compiling code in the
 158 compartment (in <em>addition</em> to any operators already permitted).</p>
 159 </dd>
 160 <dd>
 161 <p>You can list opcodes by names, or use a tag name; see
 162 <a href="file://C|\msysgit\mingw\html/lib/auto/Opcode/predefined_opcode_tags.html">Predefined Opcode Tags in the Opcode manpage</a>.</p>
 163 </dd>
 164 </li>
 165 <dt><strong><a name="item_permit_only">permit_only (OP, ...)</a></strong>
 166
 167 <dd>
 168 <p>Permit <em>only</em> the listed operators to be used when compiling code in
 169 the compartment (<em>no</em> other operators are permitted).</p>
 170 </dd>
 171 </li>
 172 <dt><strong><a name="item_deny">deny (OP, ...)</a></strong>
 173
 174 <dd>
 175 <p>Deny the listed operators from being used when compiling code in the
 176 compartment (other operators may still be permitted).</p>
 177 </dd>
 178 </li>
 179 <dt><strong><a name="item_deny_only">deny_only (OP, ...)</a></strong>
 180
 181 <dd>
 182 <p>Deny <em>only</em> the listed operators from being used when compiling code
 183 in the compartment (<em>all</em> other operators will be permitted).</p>
 184 </dd>
 185 </li>
 186 <dt><strong><a name="item_trap">trap (OP, ...)</a></strong>
 187
 188 <dt><strong><a name="item_untrap">untrap (OP, ...)</a></strong>
 189
 190 <dd>
 191 <p>The trap and untrap methods are synonyms for deny and permit
 192 respectfully.</p>
 193 </dd>
 194 </li>
 195 <dt><strong><a name="item_share">share (NAME, ...)</a></strong>
 196
 197 <dd>
 198 <p>This shares the <code>variable(s)</code> in the argument list with the compartment.
 199 This is almost identical to exporting variables using the <a href="file://C|\msysgit\mingw\html/lib/Exporter.html">the Exporter manpage</a>
 200 module.</p>
 201 </dd>
 202 <dd>
 203 <p>Each NAME must be the <strong>name</strong> of a non-lexical variable, typically
 204 with the leading type identifier included. A bareword is treated as a
 205 function name.</p>
 206 </dd>
 207 <dd>
 208 <p>Examples of legal names are '$foo' for a scalar, '@foo' for an
 209 array, '%foo' for a hash, '&amp;foo' or 'foo' for a subroutine and '*foo'
 210 for a glob (i.e.  all symbol table entries associated with ``foo'',
 211 including scalar, array, hash, sub and filehandle).</p>
 212 </dd>
 213 <dd>
 214 <p>Each NAME is assumed to be in the calling package. See share_from
 215 for an alternative method (which share uses).</p>
 216 </dd>
 217 </li>
 218 <dt><strong><a name="item_share_from">share_from (PACKAGE, ARRAYREF)</a></strong>
 219
 220 <dd>
 221 <p>This method is similar to <a href="#item_share"><code>share()</code></a> but allows you to explicitly name the
 222 package that symbols should be shared from. The symbol names (including
 223 type characters) are supplied as an array reference.</p>
 224 </dd>
 225 <dd>
 226 <pre>
 227     $safe-&gt;share_from('main', [ '$foo', '%bar', 'func' ]);</pre>
 228 </dd>
 229 </li>
 230 <dt><strong><a name="item_varglob">varglob (VARNAME)</a></strong>
 231
 232 <dd>
 233 <p>This returns a glob reference for the symbol table entry of VARNAME in
 234 the package of the compartment. VARNAME must be the <strong>name</strong> of a
 235 variable without any leading type marker. For example,</p>
 236 </dd>
 237 <dd>
 238 <pre>
 239     $cpt = new Safe 'Root';
 240     $Root::foo = &quot;Hello world&quot;;
 241     # Equivalent version which doesn't need to know $cpt's package name:
 242     ${$cpt-&gt;varglob('foo')} = &quot;Hello world&quot;;</pre>
 243 </dd>
 244 </li>
 245 <dt><strong><a name="item_reval">reval (STRING)</a></strong>
 246
 247 <dd>
 248 <p>This evaluates STRING as perl code inside the compartment.</p>
 249 </dd>
 250 <dd>
 251 <p>The code can only see the compartment's namespace (as returned by the
 252 <strong>root</strong> method). The compartment's root package appears to be the
 253 <code>main::</code> package to the code inside the compartment.</p>
 254 </dd>
 255 <dd>
 256 <p>Any attempt by the code in STRING to use an operator which is not permitted
 257 by the compartment will cause an error (at run-time of the main program
 258 but at compile-time for the code in STRING).  The error is of the form
 259 ``'%s' trapped by operation mask...''.</p>
 260 </dd>
 261 <dd>
 262 <p>If an operation is trapped in this way, then the code in STRING will
 263 not be executed. If such a trapped operation occurs or any other
 264 compile-time or return error, then $@ is set to the error message, just
 265 as with an eval().</p>
 266 </dd>
 267 <dd>
 268 <p>If there is no error, then the method returns the value of the last
 269 expression evaluated, or a return statement may be used, just as with
 270 subroutines and <strong>eval()</strong>. The context (list or scalar) is determined
 271 by the caller as usual.</p>
 272 </dd>
 273 <dd>
 274 <p>This behaviour differs from the beta distribution of the Safe extension
 275 where earlier versions of perl made it hard to mimic the return
 276 behaviour of the <a href="file://C|\msysgit\mingw\html/pod/perlfunc.html#item_eval"><code>eval()</code></a> command and the context was always scalar.</p>
 277 </dd>
 278 <dd>
 279 <p>Some points to note:</p>
 280 </dd>
 281 <dd>
 282 <p>If the entereval op is permitted then the code can use eval ``...'' to
 283 'hide' code which might use denied ops. This is not a major problem
 284 since when the code tries to execute the eval it will fail because the
 285 opmask is still in effect. However this technique would allow clever,
 286 and possibly harmful, code to 'probe' the boundaries of what is
 287 possible.</p>
 288 </dd>
 289 <dd>
 290 <p>Any string eval which is executed by code executing in a compartment,
 291 or by code called from code executing in a compartment, will be eval'd
 292 in the namespace of the compartment. This is potentially a serious
 293 problem.</p>
 294 </dd>
 295 <dd>
 296 <p>Consider a function <code>foo()</code> in package pkg compiled outside a compartment
 297 but shared with it. Assume the compartment has a root package called
 298 'Root'. If <code>foo()</code> contains an eval statement like eval '$foo = 1' then,
 299 normally, $pkg::foo will be set to 1.  If <code>foo()</code> is called from the
 300 compartment (by whatever means) then instead of setting $pkg::foo, the
 301 eval will actually set $Root::pkg::foo.</p>
 302 </dd>
 303 <dd>
 304 <p>This can easily be demonstrated by using a module, such as the Socket
 305 module, which uses eval ``...'' as part of an AUTOLOAD function. You can
 306 'use' the module outside the compartment and share an (autoloaded)
 307 function with the compartment. If an autoload is triggered by code in
 308 the compartment, or by any code anywhere that is called by any means
 309 from the compartment, then the eval in the Socket module's AUTOLOAD
 310 function happens in the namespace of the compartment. Any variables
 311 created or used by the eval'd code are now under the control of
 312 the code in the compartment.</p>
 313 </dd>
 314 <dd>
 315 <p>A similar effect applies to <em>all</em> runtime symbol lookups in code
 316 called from a compartment but not compiled within it.</p>
 317 </dd>
 318 </li>
 319 <dt><strong><a name="item_rdo">rdo (FILENAME)</a></strong>
 320
 321 <dd>
 322 <p>This evaluates the contents of file FILENAME inside the compartment.
 323 See above documentation on the <strong>reval</strong> method for further details.</p>
 324 </dd>
 325 </li>
 326 <dt><strong><a name="item_root">root (NAMESPACE)</a></strong>
 327
 328 <dd>
 329 <p>This method returns the name of the package that is the root of the
 330 compartment's namespace.</p>
 331 </dd>
 332 <dd>
 333 <p>Note that this behaviour differs from version 1.00 of the Safe module
 334 where the root module could be used to change the namespace. That
 335 functionality has been withdrawn pending deeper consideration.</p>
 336 </dd>
 337 </li>
 338 <dt><strong><a name="item_mask">mask (MASK)</a></strong>
 339
 340 <dd>
 341 <p>This is a get-or-set method for the compartment's operator mask.</p>
 342 </dd>
 343 <dd>
 344 <p>With no MASK argument present, it returns the current operator mask of
 345 the compartment.</p>
 346 </dd>
 347 <dd>
 348 <p>With the MASK argument present, it sets the operator mask for the
 349 compartment (equivalent to calling the deny_only method).</p>
 350 </dd>
 351 </li>
 352 </dl>
 353 <p>
 354 </p>
 355 <h2><a name="some_safety_issues">Some Safety Issues</a></h2>
 356 <p>This section is currently just an outline of some of the things code in
 357 a compartment might do (intentionally or unintentionally) which can
 358 have an effect outside the compartment.</p>
 359 <dl>
 360 <dt><strong><a name="item_memory">Memory</a></strong>
 361
 362 <dd>
 363 <p>Consuming all (or nearly all) available memory.</p>
 364 </dd>
 365 </li>
 366 <dt><strong><a name="item_cpu">CPU</a></strong>
 367
 368 <dd>
 369 <p>Causing infinite loops etc.</p>
 370 </dd>
 371 </li>
 372 <dt><strong><a name="item_snooping">Snooping</a></strong>
 373
 374 <dd>
 375 <p>Copying private information out of your system. Even something as
 376 simple as your user name is of value to others. Much useful information
 377 could be gleaned from your environment variables for example.</p>
 378 </dd>
 379 </li>
 380 <dt><strong><a name="item_signals">Signals</a></strong>
 381
 382 <dd>
 383 <p>Causing signals (especially SIGFPE and SIGALARM) to affect your process.</p>
 384 </dd>
 385 <dd>
 386 <p>Setting up a signal handler will need to be carefully considered
 387 and controlled.  What mask is in effect when a signal handler
 388 gets called?  If a user can get an imported function to get an
 389 exception and call the user's signal handler, does that user's
 390 restricted mask get re-instated before the handler is called?
 391 Does an imported handler get called with its original mask or
 392 the user's one?</p>
 393 </dd>
 394 </li>
 395 <dt><strong><a name="item_state_changes">State Changes</a></strong>
 396
 397 <dd>
 398 <p>Ops such as chdir obviously effect the process as a whole and not just
 399 the code in the compartment. Ops such as rand and srand have a similar
 400 but more subtle effect.</p>
 401 </dd>
 402 </li>
 403 </dl>
 404 <p>
 405 </p>
 406 <h2><a name="author">AUTHOR</a></h2>
 407 <p>Originally designed and implemented by Malcolm Beattie,
 408 <a href="mailto:mbeattie@sable.ox.ac.uk.">mbeattie@sable.ox.ac.uk.</a></p>
 409 <p>Reworked to use the Opcode module and other changes added by Tim Bunce
 410 &lt;<em><a href="mailto:Tim.Bunce@ig.co.uk">Tim.Bunce@ig.co.uk</a></em>&gt;.</p>
 411 <table border="0" width="100%" cellspacing="0" cellpadding="3">
 412 <tr><td class="block" style="background-color: #cccccc" valign="middle">
 413 <big><strong><span class="block">&nbsp;Safe - Compile and execute code in restricted compartments</span></strong></big>
 414 </td></tr>
 415 </table>
 416
 417 </body>
 418
 419 </html>