Ubuntu Manpage: Safe Base - A mechanism for creating and manipulating safe interpreters

NAME

       Safe Base - A mechanism for creating and manipulating safe interpreters

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 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, 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 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 additional 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 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

::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 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 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. If this option is not specified, or if it is given as the empty
list, the safe interpreter 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 evaluated 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 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, 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
manual page for the load command for more details.

file ?subCmd args...?
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.

encoding ?subCmd args...?
The encoding alias provides access to a safe subset of the subcommands of the
encoding command; it disallows setting of the system encoding, but allows all
other subcommands including system 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 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 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  virtual  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
       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 slave 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 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 cannot be dangerous as they run in the
       slave interpreter) and C extensions that provides  a  _SafeInit  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  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.

KEYWORDS

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