1Safe(3pm) Perl Programmers Reference Guide Safe(3pm)
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6 Safe - Compile and execute code in restricted compartments
7
9 use Safe;
10 $compartment = new Safe;
12 $compartment->permit(qw(time sort :browse));
14 $result = $compartment->reval($unsafe_code);
16
18 The Safe extension module allows the creation of compartments in which
19 perl code can be evaluated. Each compartment has
20 a new namespace
22 The "root" of the namespace (i.e. "main::") is changed to a
23 different package and code evaluated in the compartment cannot
24 refer to variables outside this namespace, even with run-time
25 glob lookups and other tricks.
26 Code which is compiled outside the compartment can choose to
28 place variables into (or share variables with) the
29 compartment's namespace and only that data will be visible to
30 code evaluated in the compartment.
31 By default, the only variables shared with compartments are the
33 "underscore" variables $_ and @_ (and, technically, the less
34 frequently used %_, the _ filehandle and so on). This is
35 because otherwise perl operators which default to $_ will not
36 work and neither will the assignment of arguments to @_ on
37 subroutine entry.
38 an operator mask
40 Each compartment has an associated "operator mask". Recall that
41 perl code is compiled into an internal format before execution.
42 Evaluating perl code (e.g. via "eval" or "do 'file'") causes
43 the code to be compiled into an internal format and then,
44 provided there was no error in the compilation, executed. Code
45 evaluated in a compartment compiles subject to the
46 compartment's operator mask. Attempting to evaluate code in a
47 compartment which contains a masked operator will cause the
48 compilation to fail with an error. The code will not be
49 executed.
50 The default operator mask for a newly created compartment is
52 the ':default' optag.
53 It is important that you read the Opcode module documentation
55 for more information, especially for detailed definitions of
56 opnames, optags and opsets.
57 Since it is only at the compilation stage that the operator
59 mask applies, controlled access to potentially unsafe
60 operations can be achieved by having a handle to a wrapper
61 subroutine (written outside the compartment) placed into the
62 compartment. For example,
63 $cpt = new Safe;
65 sub wrapper {
66 # vet arguments and perform potentially unsafe operations
67 }
68 $cpt->share('&wrapper');
69
71 The Safe module does not implement an effective sandbox for evaluating
72 untrusted code with the perl interpreter.
73 Bugs in the perl interpreter that could be abused to bypass Safe
75 restrictions are not treated as vulnerabilities. See perlsecpolicy for
76 additional information.
77 The authors make no warranty, implied or otherwise, about the
79 suitability of this software for safety or security purposes.
80 The authors shall not in any case be liable for special, incidental,
82 consequential, indirect or other similar damages arising from the use
83 of this software.
84 Your mileage will vary. If in any doubt do not use it.
86
88 To create a new compartment, use
89 $cpt = new Safe;
91 Optional argument is (NAMESPACE), where NAMESPACE is the root namespace
93 to use for the compartment (defaults to "Safe::Root0", incremented for
94 each new compartment).
95 Note that version 1.00 of the Safe module supported a second optional
97 parameter, MASK. That functionality has been withdrawn pending deeper
98 consideration. Use the permit and deny methods described below.
99 The following methods can then be used on the compartment object
101 returned by the above constructor. The object argument is implicit in
102 each case.
103 permit (OP, ...)
105 Permit the listed operators to be used when compiling code in the
106 compartment (in addition to any operators already permitted).
107 You can list opcodes by names, or use a tag name; see "Predefined
109 Opcode Tags" in Opcode.
110 permit_only (OP, ...)
112 Permit only the listed operators to be used when compiling code in the
113 compartment (no other operators are permitted).
114 deny (OP, ...)
116 Deny the listed operators from being used when compiling code in the
117 compartment (other operators may still be permitted).
118 deny_only (OP, ...)
120 Deny only the listed operators from being used when compiling code in
121 the compartment (all other operators will be permitted, so you probably
122 don't want to use this method).
123 trap (OP, ...), untrap (OP, ...)
125 The trap and untrap methods are synonyms for deny and permit
126 respectfully.
127 share (NAME, ...)
129 This shares the variable(s) in the argument list with the compartment.
130 This is almost identical to exporting variables using the Exporter
131 module.
132 Each NAME must be the name of a non-lexical variable, typically with
134 the leading type identifier included. A bareword is treated as a
135 function name.
136 Examples of legal names are '$foo' for a scalar, '@foo' for an array,
138 '%foo' for a hash, '&foo' or 'foo' for a subroutine and '*foo' for a
139 glob (i.e. all symbol table entries associated with "foo", including
140 scalar, array, hash, sub and filehandle).
141 Each NAME is assumed to be in the calling package. See share_from for
143 an alternative method (which "share" uses).
144 share_from (PACKAGE, ARRAYREF)
146 This method is similar to share() but allows you to explicitly name the
147 package that symbols should be shared from. The symbol names (including
148 type characters) are supplied as an array reference.
149 $safe->share_from('main', [ '$foo', '%bar', 'func' ]);
151 Names can include package names, which are relative to the specified
153 PACKAGE. So these two calls have the same effect:
154 $safe->share_from('Scalar::Util', [ 'reftype' ]);
156 $safe->share_from('main', [ 'Scalar::Util::reftype' ]);
157 varglob (VARNAME)
159 This returns a glob reference for the symbol table entry of VARNAME in
160 the package of the compartment. VARNAME must be the name of a variable
161 without any leading type marker. For example:
162 ${$cpt->varglob('foo')} = "Hello world";
164 has the same effect as:
166 $cpt = new Safe 'Root';
168 $Root::foo = "Hello world";
169 but avoids the need to know $cpt's package name.
171 reval (STRING, STRICT)
173 This evaluates STRING as perl code inside the compartment.
174 The code can only see the compartment's namespace (as returned by the
176 root method). The compartment's root package appears to be the "main::"
177 package to the code inside the compartment.
178 Any attempt by the code in STRING to use an operator which is not
180 permitted by the compartment will cause an error (at run-time of the
181 main program but at compile-time for the code in STRING). The error is
182 of the form "'%s' trapped by operation mask...".
183 If an operation is trapped in this way, then the code in STRING will
185 not be executed. If such a trapped operation occurs or any other
186 compile-time or return error, then $@ is set to the error message, just
187 as with an eval().
188 If there is no error, then the method returns the value of the last
190 expression evaluated, or a return statement may be used, just as with
191 subroutines and eval(). The context (list or scalar) is determined by
192 the caller as usual.
193 If the return value of reval() is (or contains) any code reference,
195 those code references are wrapped to be themselves executed always in
196 the compartment. See "wrap_code_refs_within".
197 The formerly undocumented STRICT argument sets strictness: if true 'use
199 strict;' is used, otherwise it uses 'no strict;'. Note: if STRICT is
200 omitted 'no strict;' is the default.
201 Some points to note:
203 If the entereval op is permitted then the code can use eval "..." to
205 'hide' code which might use denied ops. This is not a major problem
206 since when the code tries to execute the eval it will fail because the
207 opmask is still in effect. However this technique would allow clever,
208 and possibly harmful, code to 'probe' the boundaries of what is
209 possible.
210 Any string eval which is executed by code executing in a compartment,
212 or by code called from code executing in a compartment, will be eval'd
213 in the namespace of the compartment. This is potentially a serious
214 problem.
215 Consider a function foo() in package pkg compiled outside a compartment
217 but shared with it. Assume the compartment has a root package called
218 'Root'. If foo() contains an eval statement like eval '$foo = 1' then,
219 normally, $pkg::foo will be set to 1. If foo() is called from the
220 compartment (by whatever means) then instead of setting $pkg::foo, the
221 eval will actually set $Root::pkg::foo.
222 This can easily be demonstrated by using a module, such as the Socket
224 module, which uses eval "..." as part of an AUTOLOAD function. You can
225 'use' the module outside the compartment and share an (autoloaded)
226 function with the compartment. If an autoload is triggered by code in
227 the compartment, or by any code anywhere that is called by any means
228 from the compartment, then the eval in the Socket module's AUTOLOAD
229 function happens in the namespace of the compartment. Any variables
230 created or used by the eval'd code are now under the control of the
231 code in the compartment.
232 A similar effect applies to all runtime symbol lookups in code called
234 from a compartment but not compiled within it.
235 rdo (FILENAME)
237 This evaluates the contents of file FILENAME inside the compartment.
238 It uses the same rules as perl's built-in "do" to locate the file,
239 poossibly using @INC.
240 See above documentation on the reval method for further details.
242 root (NAMESPACE)
244 This method returns the name of the package that is the root of the
245 compartment's namespace.
246 Note that this behaviour differs from version 1.00 of the Safe module
248 where the root module could be used to change the namespace. That
249 functionality has been withdrawn pending deeper consideration.
250 mask (MASK)
252 This is a get-or-set method for the compartment's operator mask.
253 With no MASK argument present, it returns the current operator mask of
255 the compartment.
256 With the MASK argument present, it sets the operator mask for the
258 compartment (equivalent to calling the deny_only method).
259 wrap_code_ref (CODEREF)
261 Returns a reference to an anonymous subroutine that, when executed,
262 will call CODEREF with the Safe compartment 'in effect'. In other
263 words, with the package namespace adjusted and the opmask enabled.
264 Note that the opmask doesn't affect the already compiled code, it only
266 affects any further compilation that the already compiled code may try
267 to perform.
268 This is particularly useful when applied to code references returned
270 from reval().
271 (It also provides a kind of workaround for RT#60374: "Safe.pm sort {}
273 bug with -Dusethreads". See
274 <https://rt.perl.org/rt3//Public/Bug/Display.html?id=60374> for much
275 more detail.)
276 wrap_code_refs_within (...)
278 Wraps any CODE references found within the arguments by replacing each
279 with the result of calling "wrap_code_ref" on the CODE reference. Any
280 ARRAY or HASH references in the arguments are inspected recursively.
281 Returns nothing.
283
285 This section is just an outline of some of the things code in a
286 compartment might do (intentionally or unintentionally) which can have
287 an effect outside the compartment.
288 Memory Consuming all (or nearly all) available memory.
290 CPU Causing infinite loops etc.
292 Snooping
294 Copying private information out of your system. Even something
295 as simple as your user name is of value to others. Much useful
296 information could be gleaned from your environment variables
297 for example.
298 Signals Causing signals (especially SIGFPE and SIGALARM) to affect your
300 process.
301 Setting up a signal handler will need to be carefully
303 considered and controlled. What mask is in effect when a
304 signal handler gets called? If a user can get an imported
305 function to get an exception and call the user's signal
306 handler, does that user's restricted mask get re-instated
307 before the handler is called? Does an imported handler get
308 called with its original mask or the user's one?
309 State Changes
311 Ops such as chdir obviously effect the process as a whole and
312 not just the code in the compartment. Ops such as rand and
313 srand have a similar but more subtle effect.
314
316 Originally designed and implemented by Malcolm Beattie.
317 Reworked to use the Opcode module and other changes added by Tim Bunce.
319 Currently maintained by the Perl 5 Porters, <perl5-porters@perl.org>.
321perl v5.32.1 2021-03-31 Safe(3pm)