Tcl Tcl Built-In Commands - Safe manual page

Safe Tcl is a mechanism for executing untrusted Tcl scripts safely and for providing mediated access by such scripts to potentially dangerous functionality.

The Safe Base ensures that untrusted Tcl scripts cannot harm the hosting application. The Safe Base prevents integrity and privacy attacks. Untrusted Tcl scripts are prevented from corrupting the state of the hosting application or computer. Untrusted scripts are also prevented from disclosing information stored on the hosting computer or in the hosting application to any party.

The Safe Base allows a master interpreter to create safe, restricted interpreters that contain a set of predefined aliases for the source, load, file and exit commands and are able to use the auto-loading and package mechanisms.

No knowledge of the file system structure is leaked to the safe interpreter, because it has access only to a virtualized path containing tokens. When the safe interpreter requests to source a file, it uses the token in the virtual path as part of the file name to source; the master interpreter translates the token into a real directory name and executes the requested operation. Different levels of security can be selected by using the optional flags of the commands described below.

All commands provided in the master interpreter by the Safe Base reside in the safe namespace. ::safe::interpCreate creates a new safe interpreter with options, described in the section OPTIONS. The return value is the name of the new safe interpreter created. ::safe::interpInit is similar to ::safe::interpCreate except that it requires as its first argument the name of a safe interpreter that was previously created directly using the interp command. ::safe::interpDelete deletes the interpreter named by its argument. ::safe::interpConfigure can be used to set or get options for the named safe interpreters; the options are described in the section OPTIONS.

A virtual path is maintained in the master interpreter for each safe interpreter created by ::safe::interpCreate or initialized by ::safe::interpInit. The path maps tokens accessible in the safe interpreter into real path names on the local file system. This prevents safe interpreters from gaining knowledge about the structure of the file system of the host on which the interpeter is executing. When a token is used in a safe interpreter in a request to source or load a file, the token is translated to a real path name and the file to be sourced or loaded is located on the file system. The safe interpreter never gains knowledge of the actual path name under which the file is stored on the file system. Commands are provided in the master interpreter to manipulate the virtual path for a safe interpreter. ::safe::interpConfigure can be used to set a new path for a safe interpreter. ::safe::interpAddToAccessPath adds a directory to the virtual path for the named safe interpreter and returns the token by which that directory will be accessible in the safe interpreter. ::safe::interpFindInAccessPath finds the requested directory in the virtual path for the named safe interpreter and returns the token by which that directory can be accessed in the safe interpreter. If the path is not found, an error is raised.

::safe::setLogCommand installs a script to be called when interesting life cycle events happen. This script will be called with one argument, a string describing the event.

ALIASES

The following aliases are provided in a safe interpreter:

source fileName: The requested file, a Tcl source file, is sourced into the safe interpreter if it is found. The source alias can only source files from directories in the virtual path for the safe interpreter. The source alias requires the safe interpreter to use one of the token names in its virtual path to denote the directory in which the file to be sourced can be found. See the section on SECURITY for more discussion of restrictions on valid filenames.
load fileName: The requested file, a shared object file, in dynamically loaded into the safe interpreter if it is found. The filename must contain a token name mentioned in the virtual path for the safe interpreter for it to be found successfully. Additionally, the shared object file must contain a safe entry point; see the manual page for the load command for more details.
file ?options?: The file alias provides access to a safe subset of the subcommands of the file command; it allows only dirname, join, extension, root, tail, pathname and split subcommands. For more details on what these subcommands do see the manual page for the file command.
exit: The calling interpreter is deleted and its computation is stopped, but the Tcl process in which this interpreter exists is not terminated.

COMMANDS

The following commands are provided in the master interpreter:

::safe::interpCreate ?slave? ?options...?

Creates a safe interpreter, installs the aliases described in the section ALIASES and initializes the auto-loading and package mechanism as specified by the supplied options. See the OPTIONS section below for a description of the common optional arguments. If the slave argument is omitted, a name will be generated. ::safe::interpCreate always returns the interpreter name.

::safe::interpInit slave ?options...?

This command is similar to interpCreate except it that does not create the safe interpreter. slave must have been created by some other means, like ::interp create -safe.

::safe::interpConfigure slave ?options...?

If no options are given, returns the settings for all options for the named safe interpreter. If options are supplied, sets the options for the named safe interpreter. See the section on OPTIONS below.

::safe::interpDelete slave

Deletes the safe interpreter and cleans up the corresponding master interpreter data structures. If a deletehook script was specified for this interpreter it is evaluated before the interpreter is deleted, with the name of the interpreter as an additional argument.

::safe::interpFindInAccessPath slave directory

This command finds and returns the token for the real directory directory in the safe interpreter's current virtual access path. It generates an error if the directory is not found. Example of use:

$slave eval [list set tk_library [::safe::interpFindInAccessPath $name $tk_library]]

::safe::interpAddToAccessPath slave directory

This command adds directory to the virtual path maintained for the safe interpreter in the master, and returns the token that can be used in the safe interpreter to obtain access to files in that directory. If the directory is already in the virtual path, it only returns the token without adding the directory to the virtual path again. Example of use:

$slave eval [list set tk_library [::safe::interpAddToAccessPath $name $tk_library]]

::safe::setLogCmd ?cmd arg...?

This command installs a script that will be called when interesting lifecycle events occur for a safe interpreter. When called with no arguments, it returns the currently installed script. When called with one argument, an empty string, the currently installed script is removed and logging is turned off. The script will be invoked with one additional argument, a string describing the event of interest. The main purpose is to help in debugging safe interpreters. Using this facility you can get complete error messages while the safe interpreter gets only generic error messages. This prevents a safe interpreter from seeing messages about failures and other events that might contain sensitive information such as real directory names.

Example of use:

::safe::setLogCmd puts stderr

Below is the output of a sample session in which a safe interpreter attempted to source a file not found in its virtual access path. Note that the safe interpreter only received an error message saying that the file was not found:

NOTICE for slave interp10 : Created
NOTICE for slave interp10 : Setting accessPath=(/foo/bar) staticsok=1 nestedok=0 deletehook=()
NOTICE for slave interp10 : auto_path in interp10 has been set to {$p(:0:)}
ERROR for slave interp10 : /foo/bar/init.tcl: no such file or directory

OPTIONS

The following options are common to ::safe::interpCreate, ::safe::interpInit, and ::safe::interpConfigure. Any option name can be abbreviated to its minimal non-ambiguous name. Option names are not case sensitive.

-accessPath ?directoryList?: This option sets the list of directories from which the safe interpreter can source and load files, and returns a list of tokens that will allow the safe interpreter access to these directories. If a value for directoryList is not given, or if it is given as the empty list, the safe interpreter will use the same directories than its master for auto-loading. See the section SECURITY below for more detail about virtual paths, tokens and access control.
-noStatics: This option specifies that the safe interpreter will not be allowed to load statically linked packages (like load {} Tk). The default is that safe interpreters are allowed to load statically linked packages.
-nestedLoadOk: This option specifies that the safe interpreter will be allowed to load packages into its own subinterpreters. The default is that safe interpreters are not allowed to load packages into their own subinterpreters.
-deleteHook ?script?: If script is given, it is evaluated in the master with the name of the safe interpreter as an additional argument just before deleting the safe interpreter. If no value is given for script any currently installed deletion hook script for that safe interpreter is removed; it will no longer be called when the interpreter is deleted. There is no deletion hook script installed by default.

SECURITY

The Safe Base does not attempt to completely prevent annoyance and denial of service attacks. These forms of attack prevent the application or user from temporarily using the computer to perform useful work, for example by consuming all available CPU time or all available screen real estate. These attacks, while agravating, are deemed to be of lesser importance in general than integrity and privacy attacks that the Safe Base is to prevent. The commands available in a safe interpreter, in addition to the safe set as defined in interp manual page, are mediated aliases for source, load, exit, and a safe subset of file. The safe interpreter can also auto-load code and it can request to load packages. Because some of these commands access the local file system, there is a potential for information leakage about its directory structure. To prevent this, commands which take file names as arguments in a safe interpreter use tokens instead of the real directory names. These tokens are translated to the real directory name while a request to, e.g., source a file is mediated by the master interpreter.

To further prevent potential information leakage from sensitive files that are accidentally included in the set of files that can be sourced by a safe interpreter, the source alias is restricted so that it can only source files with names that have the extension .tcl, that contain only one dot and that are forteen characters long or shorter.

The default value of the Tcl variable auto_path in a safe interpreter is a virtualized token list for the directories in the value of its master's auto_path variable and their immediate subdirectories. The first token in this list is also assigned to the Tcl varibale tcl_library in the safe interpreter. You can always specify a more restrictive path for which sub directories will never be searched by explicitly specifying your directory list with the -accessPath flag instead of relying on this default mechanism.

KEYWORDS

alias, auto-loading, auto_mkindex, load, master interpreter, safe interpreter, slave interpreter, source

Copyright © 1995-1996 Sun Microsystems, Inc.
Copyright © 1995-1997 Roger E. Critchlow Jr.