1Safe Tcl(n) Tcl Built-In Commands Safe Tcl(n)
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8 safe - Creating and manipulating safe interpreters
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11 ::safe::interpCreate ?child? ?options...?
12 ::safe::interpInit child ?options...?
14 ::safe::interpConfigure child ?options...?
16 ::safe::interpDelete child
18 ::safe::interpAddToAccessPath child directory
20 ::safe::interpFindInAccessPath child directory
22 ::safe::setLogCmd ?cmd arg...?
24 OPTIONS
26 ?-accessPath pathList? ?-statics boolean? ?-noStatics? ?-nested bool‐
27 ean? ?-nestedLoadOk? ?-deleteHook script?
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31 Safe Tcl is a mechanism for executing untrusted Tcl scripts safely and
32 for providing mediated access by such scripts to potentially dangerous
33 functionality.
34 Safe Tcl ensures that untrusted Tcl scripts cannot harm the hosting ap‐
36 plication. It prevents integrity and privacy attacks. Untrusted Tcl
37 scripts are prevented from corrupting the state of the hosting applica‐
38 tion or computer. Untrusted scripts are also prevented from disclosing
39 information stored on the hosting computer or in the hosting applica‐
40 tion to any party.
41 Safe Tcl allows a parent interpreter to create safe, restricted inter‐
43 preters that contain a set of predefined aliases for the source, load,
44 file, encoding, and exit commands and are able to use the auto-loading
45 and package mechanisms.
46 No knowledge of the file system structure is leaked to the safe inter‐
48 preter, because it has access only to a virtualized path containing to‐
49 kens. When the safe interpreter requests to source a file, it uses the
50 token in the virtual path as part of the file name to source; the par‐
51 ent interpreter transparently translates the token into a real direc‐
52 tory name and executes the requested operation (see the section SECU‐
53 RITY below for details). Different levels of security can be selected
54 by using the optional flags of the commands described below.
55 All commands provided in the parent interpreter by Safe Tcl reside in
57 the safe namespace.
58
60 The following commands are provided in the parent interpreter:
61 ::safe::interpCreate ?child? ?options...?
63 Creates a safe interpreter, installs the aliases described in
64 the section ALIASES and initializes the auto-loading and package
65 mechanism as specified by the supplied options. See the OPTIONS
66 section below for a description of the optional arguments. If
67 the child argument is omitted, a name will be generated.
68 ::safe::interpCreate always returns the interpreter name.
69 The interpreter name child may include namespace separators, but
71 may not have leading or trailing namespace separators, or excess
72 colon characters in namespace separators. The interpreter name
73 is qualified relative to the global namespace ::, not the name‐
74 space in which the ::safe::interpCreate command is evaluated.
75 ::safe::interpInit child ?options...?
77 This command is similar to interpCreate except it that does not
78 create the safe interpreter. child must have been created by
79 some other means, like interp create -safe. The interpreter
80 name child may include namespace separators, subject to the same
81 restrictions as for interpCreate.
82 ::safe::interpConfigure child ?options...?
84 If no options are given, returns the settings for all options
85 for the named safe interpreter as a list of options and their
86 current values for that child. If a single additional argument
87 is provided, it will return a list of 2 elements name and value
88 where name is the full name of that option and value the current
89 value for that option and the child. If more than two addi‐
90 tional arguments are provided, it will reconfigure the safe in‐
91 terpreter and change each and only the provided options. See
92 the section on OPTIONS below for options description. Example
93 of use:
94 # Create new interp with the same configuration as "$i0":
96 set i1 [safe::interpCreate {*}[safe::interpConfigure $i0]]
97 # Get the current deleteHook
99 set dh [safe::interpConfigure $i0 -del]
100 # Change (only) the statics loading ok attribute of an
102 # interp and its deleteHook (leaving the rest unchanged):
103 safe::interpConfigure $i0 -delete {foo bar} -statics 0
104 ::safe::interpDelete child
106 Deletes the safe interpreter and cleans up the corresponding
107 parent interpreter data structures. If a deleteHook script was
108 specified for this interpreter it is evaluated before the inter‐
109 preter is deleted, with the name of the interpreter as an addi‐
110 tional argument.
111 ::safe::interpFindInAccessPath child directory
113 This command finds and returns the token for the real directory
114 directory in the safe interpreter's current virtual access path.
115 It generates an error if the directory is not found. Example of
116 use:
117 $child eval [list set tk_library \
119 [::safe::interpFindInAccessPath $name $tk_library]]
120 ::safe::interpAddToAccessPath child directory
122 This command adds directory to the virtual path maintained for
123 the safe interpreter in the parent, and returns the token that
124 can be used in the safe interpreter to obtain access to files in
125 that directory. If the directory is already in the virtual
126 path, it only returns the token without adding the directory to
127 the virtual path again. Example of use:
128 $child eval [list set tk_library \
130 [::safe::interpAddToAccessPath $name $tk_library]]
131 ::safe::setLogCmd ?cmd arg...?
133 This command installs a script that will be called when inter‐
134 esting life cycle events occur for a safe interpreter. When
135 called with no arguments, it returns the currently installed
136 script. When called with one argument, an empty string, the
137 currently installed script is removed and logging is turned off.
138 The script will be invoked with one additional argument, a
139 string describing the event of interest. The main purpose is to
140 help in debugging safe interpreters. Using this facility you
141 can get complete error messages while the safe interpreter gets
142 only generic error messages. This prevents a safe interpreter
143 from seeing messages about failures and other events that might
144 contain sensitive information such as real directory names.
145 Example of use:
147 ::safe::setLogCmd puts stderr
149 Below is the output of a sample session in which a safe inter‐
151 preter attempted to source a file not found in its virtual ac‐
152 cess path. Note that the safe interpreter only received an er‐
153 ror message saying that the file was not found:
154 NOTICE for child interp10 : Created
156 NOTICE for child interp10 : Setting accessPath=(/foo/bar) staticsok=1 nestedok=0 deletehook=()
157 NOTICE for child interp10 : auto_path in interp10 has been set to {$p(:0:)}
158 ERROR for child interp10 : /foo/bar/init.tcl: no such file or directory
159 OPTIONS
161 The following options are common to ::safe::interpCreate, ::safe::in‐
162 terpInit, and ::safe::interpConfigure. Any option name can be abbrevi‐
163 ated to its minimal non-ambiguous name. Option names are not case sen‐
164 sitive.
165 -accessPath directoryList
167 This option sets the list of directories from which the safe in‐
168 terpreter can source and load files. If this option is not
169 specified, or if it is given as the empty list, the safe inter‐
170 preter will use the same directories as its parent for auto-
171 loading. See the section SECURITY below for more detail about
172 virtual paths, tokens and access control.
173 -statics boolean
175 This option specifies if the safe interpreter will be allowed to
176 load statically linked packages (like load {} Tk). The default
177 value is true : safe interpreters are allowed to load statically
178 linked packages.
179 -noStatics
181 This option is a convenience shortcut for -statics false and
182 thus specifies that the safe interpreter will not be allowed to
183 load statically linked packages.
184 -nested boolean
186 This option specifies if the safe interpreter will be allowed to
187 load packages into its own sub-interpreters. The default value
188 is false : safe interpreters are not allowed to load packages
189 into their own sub-interpreters.
190 -nestedLoadOk
192 This option is a convenience shortcut for -nested true and thus
193 specifies the safe interpreter will be allowed to load packages
194 into its own sub-interpreters.
195 -deleteHook script
197 When this option is given a non-empty script, it will be evalu‐
198 ated in the parent with the name of the safe interpreter as an
199 additional argument just before actually deleting the safe in‐
200 terpreter. Giving an empty value removes any currently in‐
201 stalled deletion hook script for that safe interpreter. The de‐
202 fault value ({}) is not to have any deletion call back.
203
205 The following aliases are provided in a safe interpreter:
206 source fileName
208 The requested file, a Tcl source file, is sourced into the safe
209 interpreter if it is found. The source alias can only source
210 files from directories in the virtual path for the safe inter‐
211 preter. The source alias requires the safe interpreter to use
212 one of the token names in its virtual path to denote the direc‐
213 tory in which the file to be sourced can be found. See the sec‐
214 tion on SECURITY for more discussion of restrictions on valid
215 filenames.
216 load fileName
218 The requested file, a shared object file, is dynamically loaded
219 into the safe interpreter if it is found. The filename must
220 contain a token name mentioned in the virtual path for the safe
221 interpreter for it to be found successfully. Additionally, the
222 shared object file must contain a safe entry point; see the man‐
223 ual page for the load command for more details.
224 file ?subCmd args...?
226 The file alias provides access to a safe subset of the subcom‐
227 mands of the file command; it allows only dirname, join, exten‐
228 sion, root, tail, pathname and split subcommands. For more de‐
229 tails on what these subcommands do see the manual page for the
230 file command.
231 encoding ?subCmd args...?
233 The encoding alias provides access to a safe subset of the sub‐
234 commands of the encoding command; it disallows setting of the
235 system encoding, but allows all other subcommands including sys‐
236 tem to check the current encoding.
237 exit The calling interpreter is deleted and its computation is
239 stopped, but the Tcl process in which this interpreter exists is
240 not terminated.
241
243 Safe Tcl does not attempt to completely prevent annoyance and denial of
244 service attacks. These forms of attack prevent the application or user
245 from temporarily using the computer to perform useful work, for example
246 by consuming all available CPU time or all available screen real es‐
247 tate. These attacks, while aggravating, are deemed to be of lesser im‐
248 portance in general than integrity and privacy attacks that Safe Tcl is
249 to prevent.
250 The commands available in a safe interpreter, in addition to the safe
252 set as defined in interp manual page, are mediated aliases for source,
253 load, exit, and safe subsets of file and encoding. The safe interpreter
254 can also auto-load code and it can request that packages be loaded.
255 Because some of these commands access the local file system, there is a
257 potential for information leakage about its directory structure. To
258 prevent this, commands that take file names as arguments in a safe in‐
259 terpreter use tokens instead of the real directory names. These tokens
260 are translated to the real directory name while a request to, e.g.,
261 source a file is mediated by the parent interpreter. This virtual path
262 system is maintained in the parent interpreter for each safe inter‐
263 preter created by ::safe::interpCreate or initialized by ::safe::inter‐
264 pInit and the path maps tokens accessible in the safe interpreter into
265 real path names on the local file system thus preventing safe inter‐
266 preters from gaining knowledge about the structure of the file system
267 of the host on which the interpreter is executing. The only valid file
268 names arguments for the source and load aliases provided to the child
269 are path in the form of [file join token filename] (i.e. when using the
270 native file path formats: token/filename on Unix and token\filename on
271 Windows), where token is representing one of the directories of the ac‐
272 cessPath list and filename is one file in that directory (no sub direc‐
273 tories access are allowed).
274 When a token is used in a safe interpreter in a request to source or
276 load a file, the token is checked and translated to a real path name
277 and the file to be sourced or loaded is located on the file system.
278 The safe interpreter never gains knowledge of the actual path name un‐
279 der which the file is stored on the file system.
280 To further prevent potential information leakage from sensitive files
282 that are accidentally included in the set of files that can be sourced
283 by a safe interpreter, the source alias restricts access to files meet‐
284 ing the following constraints: the file name must fourteen characters
285 or shorter, must not contain more than one dot (“.”), must end up with
286 the extension (“.tcl”) or be called (“tclIndex”.)
287 Each element of the initial access path list will be assigned a token
289 that will be set in the child auto_path and the first element of that
290 list will be set as the tcl_library for that child.
291 If the access path argument is not given or is the empty list, the de‐
293 fault behavior is to let the child access the same packages as the par‐
294 ent has access to (Or to be more precise: only packages written in Tcl
295 (which by definition cannot be dangerous as they run in the child in‐
296 terpreter) and C extensions that provides a _SafeInit entry point). For
297 that purpose, the parent's auto_path will be used to construct the
298 child access path. In order that the child successfully loads the Tcl
299 library files (which includes the auto-loading mechanism itself) the
300 tcl_library will be added or moved to the first position if necessary,
301 in the child access path, so the child tcl_library will be the same as
302 the parent's (its real path will still be invisible to the child
303 though). In order that auto-loading works the same for the child and
304 the parent in this by default case, the first-level sub directories of
305 each directory in the parent auto_path will also be added (if not al‐
306 ready included) to the child access path. You can always specify a
307 more restrictive path for which sub directories will never be searched
308 by explicitly specifying your directory list with the -accessPath flag
309 instead of relying on this default mechanism.
310 When the accessPath is changed after the first creation or initializa‐
312 tion (i.e. through interpConfigure -accessPath list), an auto_reset is
313 automatically evaluated in the safe interpreter to synchronize its
314 auto_index with the new token list.
315
317 interp(n), library(n), load(n), package(n), source(n), unknown(n)
318
320 alias, auto-loading, auto_mkindex, load, parent interpreter, safe in‐
321 terpreter, child interpreter, source
322Tcl 8.0 Safe Tcl(n)