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Safe Tcl(n) Tcl Built-In Commands Safe Tcl(n)
NAME
Safe Base - A mechanism for creating and manipulating safe inter-
preters.
SYNOPSIS
::safe::interpCreate ?slave? ?options...?
::safe::interpInit slave ?options...?
::safe::interpConfigure slave ?options...?
::safe::interpDelete slave
::safe::interpAddToAccessPath slave directory
::safe::interpFindInAccessPath slave directory
::safe::setLogCmd ?cmd arg...?
OPTIONS
?-accessPath pathList? ?-statics boolean? ?-noStatics? ?-nested
boolean? ?-nestedLoadOk? ?-deleteHook script?
DESCRIPTION
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 host-
ing 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, 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 inter-
preter, 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 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 master interpreter by the Safe Base reside
in the safe namespace:
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 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 as a list of options and their
current values for that slave. If a single additional argument
is provided, it will return a list of 2 elements name and value
where name is the full name of that option and value the current
value for that option and the slave. If more than two addi-
tional arguments are provided, it will reconfigure the safe
interpreter and change each and only the provided options. See
the section on OPTIONS below for options description. Example
of use:
# Create a new interp with the same configuration as "$i0" : set
i1 [eval 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::interp-
Configure $i0 -delete {foo bar} -statics 0 ;
::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 inter-
preter is deleted, with the name of the interpreter as an addi-
tional 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 $tk_library]]
::safe::setLogCmd ?cmd arg...?
This command installs a script that will be called when inter-
esting life cycle events occur for a safe interpreter. When
called with no arguments, it returns the currently installed
script. When called with one argument, an empty string, the
currently installed script is removed and logging is turned off.
The script will be invoked with one additional argument, a
string describing the event of interest. The main purpose is to
help in debugging safe interpreters. Using this facility you
can get complete error messages while the safe interpreter gets
only generic error messages. This prevents a safe interpreter
from seeing messages about failures and other events that might
contain sensitive information such as real directory names.
Example of use: ::safe::setLogCmd puts stderr Below is the out-
put of a sample session in which a safe interpreter attempted to
source a file not found in its virtual access path. Note that
the safe interpreter only received an error message saying that
the file was not found: NOTICE for slave interp10 : Created
NOTICE for slave interp10 : Setting accessPath=(/foo/bar) stat-
icsok=1 nestedok=0 deletehook=() NOTICE for slave interp10 :
auto_path in interp10 has been set to {$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. If this option is not
specified, or if it is given as the empty list, the safe inter-
preter will use the same directories as its master for auto-
loading. See the section SECURITY below for more detail about
virtual paths, tokens and access control.
-statics boolean
This option specifies if the safe interpreter will be allowed to
load statically linked packages (like load {} Tk). The default
value is true : safe interpreters are allowed to load statically
linked packages.
-noStatics
This option is a convenience shortcut for -statics false and
thus specifies that the safe interpreter will not be allowed to
load statically linked packages.
-nested boolean
This option specifies if the safe interpreter will be allowed to
load packages into its own sub-interpreters. The default value
is false : safe interpreters are not allowed to load packages
into their own sub-interpreters.
-nestedLoadOk
This option is a convenience shortcut for -nested true and thus
specifies the safe interpreter will be allowed to load packages
into its own sub-interpreters.
-deleteHook script
When this option is given a non-empty script, it will be evalu-
ated in the master with the name of the safe interpreter as an
additional argument just before actually deleting the safe
interpreter. Giving an empty value removes any currently
installed deletion hook script for that safe interpreter. The
default value ({}) is not to have any deletion call back.
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 inter-
preter. The source alias requires the safe interpreter to use
one of the token names in its virtual path to denote the direc-
tory in which the file to be sourced can be found. See the sec-
tion on SECURITY for more discussion of restrictions on valid
filenames.
load fileName
The requested file, a shared object file, is 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 man-
ual page for the load command for more details.
file ?subCmd args...?
The file alias provides access to a safe subset of the subcom-
mands of the file command; it allows only dirname, join, exten-
sion, root, tail, pathname and split subcommands. For more
details on what these subcommands do see the manual page for the
file command.
encoding ?subCmd args...?
The encoding alias provides access to a safe subset of the sub-
commands of the encoding command; it disallows setting of the
system encoding, but allows all other subcommands including sys-
tem to check the current encoding.
exit The calling interpreter is deleted and its computation is
stopped, but the Tcl process in which this interpreter exists is
not terminated.
SECURITY
The Safe Base does not attempt to completely prevent annoyance and
denial of service attacks. These forms of attack prevent the applica-
tion 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 aggravating, 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 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 master interpreter. This vir-
tual path system is maintained in the master 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 prevent-
ing 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 pro-
vided to the slave are path in the form of [file join token filename]
(i.e. when using the native file path formats: token/filename on Unix,
token\filename on Windows, and token:filename on the Mac), where token
is representing one of the directories of the accessPath list and file-
name 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 meet-
ing 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 slave auto_path and the first element of that
list will be set as the tcl_library for that slave.
If the access path argument is not given or is the empty list, the
default behavior is to let the slave access the same packages as the
master has access to (Or to be more precise: only packages written in
Tcl (which by definition can't be dangerous as they run in the slave
interpreter) and C extensions that provides a Safe_Init entry point).
For that purpose, the master's auto_path will be used to construct the
slave access path. In order that the slave 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 slave access path, so the slave tcl_library will be the same as
the master's (its real path will still be invisible to the slave
though). In order that auto-loading works the same for the slave and
the master in this by default case, the first-level sub directories of
each directory in the master auto_path will also be added (if not
already included) to the slave 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 initializa-
tion (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.
SEE ALSO
interp(n), library(n), load(n), package(n), source(n), unknown(n)
KEYWORDS
alias, auto-loading, auto_mkindex, load, master interpreter, safe
interpreter, slave interpreter, source
Tcl 8.0 Safe Tcl(n)
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