Safe Tcl ensures that untrusted Tcl scripts cannot harm the hosting application. It 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.
Safe Tcl allows a parent interpreter to create safe, restricted interpreters that contain a set of predefined aliases for the source, load, file, encoding, 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 parent interpreter transparently translates the token into a real directory name and executes the requested operation (see the section SECURITY below for details). Different levels of security can be selected by using the optional flags of the commands described below.
All commands provided in the parent interpreter by Safe Tcl reside in the safe namespace.
The interpreter name child may include namespace separators, but may not have leading or trailing namespace separators, or excess colon characters in namespace separators. The interpreter name is qualified relative to the global namespace ::, not the namespace in which the ::safe::interpCreate command is evaluated.
# Create new interp with the same configuration as "$i0":
set i1 [safe::interpCreate {*}[safe::interpConfigure $i0]]
# Get the current deleteHook
set dh [safe::interpConfigure $i0 -del]
# Change (only) the statics loading ok attribute of an
# interp and its deleteHook (leaving the rest unchanged):
safe::interpConfigure $i0 -delete {foo bar} -statics 0
$child eval [list set tk_library \
[::safe::interpFindInAccessPath $name $tk_library]]
$child eval [list set tk_library \
[::safe::interpAddToAccessPath $name $tk_library]]
Example of use:
::safe::setLogCmd puts stderr
Below is the output of a sample session in which a safe interpreter attempted to source a file not found in its virtual access path. Note that the safe interpreter only received an error message saying that the file was not found:
NOTICE for child interp10 : Created
NOTICE for child interp10 : Setting accessPath=(/foo/bar) staticsok=1 nestedok=0 deletehook=()
NOTICE for child interp10 : auto_path in interp10 has been set to {$p(:0:)}
ERROR for child interp10 : /foo/bar/init.tcl: no such file or directory
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 safe subsets of file and encoding. The safe interpreter can also auto-load code and it can request that packages be loaded.
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 that 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 parent interpreter. This virtual path system is maintained in the parent interpreter for each safe interpreter created by ::safe::interpCreate or initialized by ::safe::interpInit and the path maps tokens accessible in the safe interpreter into real path names on the local file system thus preventing safe interpreters from gaining knowledge about the structure of the file system of the host on which the interpreter is executing. The only valid file names arguments for the source and load aliases provided to the child are path in the form of [file join token filename] (i.e. when using the native file path formats: token/filename on Unix and token\filename on Windows), where token is representing one of the directories of the accessPath list and filename is one file in that directory (no sub directories access are allowed).
When a token is used in a safe interpreter in a request to source or load a file, the token is checked and 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.
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 restricts access to files meeting the following constraints: the file name must fourteen characters or shorter, must not contain more than one dot (“.”), must end up with the extension (“.tcl”) or be called (“tclIndex”.)
Each element of the initial access path list will be assigned a token that will be set in the child auto_path and the first element of that list will be set as the tcl_library for that child.
If the access path argument is not given to ::safe::interpCreate or ::safe::interpInit or is the empty list, the default behavior is to let the child access the same packages as the parent has access to (Or to be more precise: only packages written in Tcl (which by definition cannot be dangerous as they run in the child interpreter) and C extensions that provides a _SafeInit entry point). For that purpose, the parent's auto_path will be used to construct the child access path. In order that the child successfully loads the Tcl library files (which includes the auto-loading mechanism itself) the tcl_library will be added or moved to the first position if necessary, in the child access path, so the child tcl_library will be the same as the parent's (its real path will still be invisible to the child though). In order that auto-loading works the same for the child and the parent in this by default case, the first-level sub directories of each directory in the parent auto_path will also be added (if not already included) to the child access path. 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.
When the accessPath is changed after the first creation or initialization (i.e. through interpConfigure -accessPath list), an auto_reset is automatically evaluated in the safe interpreter to synchronize its auto_index with the new token list.
If you wish to use Safe Base interpreters with "Sync Mode" off, evaluate the command
safe::setSyncMode 0
Use ::safe::interpCreate or ::safe::interpInit to create an interpreter with the properties that you require. The simplest way is not to specify -accessPath or -autoPath, which means the safe interpreter will use the same paths as the parent interpreter. However, if -accessPath is specified, then -autoPath must also be specified, or else it will be set to {}.
The value of -autoPath will be that required to access tclIndex and pkgIndex.tcl files according to the same rules as an unsafe interpreter (see pkg_mkIndex(n) and library(n)).
With "Sync Mode" on, the option -autoPath is undefined, and the Safe Base sets the child's ::auto_path to a tokenized form of the access path. In addition to the directories present if "Safe Mode" is off, the ::auto_path includes the numerous subdirectories and module paths that belong to the access path.
In order to preserve compatibility with existing code, this synchronization of the ::auto_path and access path ("Sync Mode" on) is still the default. However, the Safe Base offers the option of limiting the safe interpreter's ::auto_path to the much shorter list of directories that is necessary for it to perform its function ("Sync Mode" off). Use the command ::safe::setSyncMode to choose the mode before creating any Safe Base interpreters.
In either mode, the most convenient way to initialize a safe interpreter is to call ::safe::interpCreate or ::safe::interpInit without the -accessPath or -autoPath options (or with the -accessPath option set to the empty list), which will give the safe interpreter the same access as the parent interpreter to packages, modules, and autoloader files. With "Sync Mode" off, the Safe Base will set the value of -autoPath to the parent's ::auto_path, and will set the child's ::auto_path to a tokenized form of the parent's ::auto_path.
With "Sync Mode" off, if a value is specified for -autoPath, even the empty list, in a call to ::safe::interpCreate, ::safe::interpInit, or ::safe::interpConfigure, it will be tokenized and used as the safe interpreter's ::auto_path. Any directories that do not also belong to the access path cannot be tokenized and will be silently ignored. However, the value of -autoPath will remain as specified, and will be used to re-tokenize the child's ::auto_path if ::safe::interpConfigure is called to change the value of -accessPath.
With "Sync Mode" off, if the access path is reset to the values in the parent interpreter by calling ::safe::interpConfigure with arguments -accessPath {}, then the ::auto_path will also be reset unless the argument -autoPath is supplied to specify a different value.
With "Sync Mode" off, if a non-empty value of -accessPath is supplied, the safe interpreter's ::auto_path will be set to {} (by ::safe::interpCreate, ::safe::interpInit) or left unchanged (by ::safe::interpConfigure). If the same command specifies a new value for -autoPath, it will be applied after the -accessPath argument has been processed.
Examples of use with "Sync Mode" off: any of these commands will set the ::auto_path to a tokenized form of its value in the parent interpreter:
safe::interpCreate foo
safe::interpCreate foo -accessPath {}
safe::interpInit bar
safe::interpInit bar -accessPath {}
safe::interpConfigure foo -accessPath {}
Example of use with "Sync Mode" off: when initializing a safe interpreter with a non-empty access path, the ::auto_path will be set to {} unless its own value is also specified:
safe::interpCreate foo -accessPath {
/usr/local/TclHome/lib/tcl9.0
/usr/local/TclHome/lib/tcl9.0/http1.0
/usr/local/TclHome/lib/tcl9.0/opt0.4
/usr/local/TclHome/lib/tcl9.0/msgs
/usr/local/TclHome/lib/tcl9.0/encoding
/usr/local/TclHome/lib
}
# The child's ::auto_path must be given a suitable value:
safe::interpConfigure foo -autoPath {
/usr/local/TclHome/lib/tcl9.0
/usr/local/TclHome/lib
}
# The two commands can be combined:
safe::interpCreate foo -accessPath {
/usr/local/TclHome/lib/tcl9.0
/usr/local/TclHome/lib/tcl9.0/http1.0
/usr/local/TclHome/lib/tcl9.0/opt0.4
/usr/local/TclHome/lib/tcl9.0/msgs
/usr/local/TclHome/lib/tcl9.0/encoding
/usr/local/TclHome/lib
} -autoPath {
/usr/local/TclHome/lib/tcl9.0
/usr/local/TclHome/lib
}
Example of use with "Sync Mode" off: the command safe::interpAddToAccessPath does not change the safe interpreter's ::auto_path, and so any necessary change must be made by the script:
safe::interpAddToAccessPath foo /usr/local/TclHome/lib/extras/Img1.4.11 lassign [safe::interpConfigure foo -autoPath] DUM childAutoPath lappend childAutoPath /usr/local/TclHome/lib/extras/Img1.4.11 safe::interpConfigure foo -autoPath $childAutoPath