path: root/doc/safe.n

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.so man.macros
.TH "Safe Tcl" n 8.0 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
safe \- Creating and manipulating safe interpreters
.SH SYNOPSIS
\fB::safe::interpCreate\fR ?\fIslave\fR? ?\fIoptions...\fR?
.sp
\fB::safe::interpInit\fR \fIslave\fR ?\fIoptions...\fR?
.sp
\fB::safe::interpConfigure\fR \fIslave\fR ?\fIoptions...\fR?
.sp
\fB::safe::interpDelete\fR \fIslave\fR
.sp
\fB::safe::interpAddToAccessPath\fR \fIslave\fR \fIdirectory\fR
.sp
\fB::safe::interpFindInAccessPath\fR \fIslave\fR \fIdirectory\fR
.sp
\fB::safe::setLogCmd\fR ?\fIcmd arg...\fR?
.SS OPTIONS
.PP
?\fB\-accessPath\fR \fIpathList\fR?
?\fB\-statics\fR \fIboolean\fR? ?\fB\-noStatics\fR?
?\fB\-nested\fR \fIboolean\fR? ?\fB\-nestedLoadOk\fR?
?\fB\-deleteHook\fR \fIscript\fR?
.BE
.SH DESCRIPTION
Safe Tcl is a mechanism for executing untrusted Tcl scripts
safely and for providing mediated access by such scripts to
potentially dangerous functionality.
.PP
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.
.PP
Safe Tcl allows a master interpreter to create safe, restricted
interpreters that contain a set of predefined aliases for the \fBsource\fR,
\fBload\fR, \fBfile\fR, \fBencoding\fR, and \fBexit\fR commands and
are able to use the auto-loading and package mechanisms.
.PP
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 \fBSECURITY\fR below for details).
Different levels of security can be selected by using the optional flags
of the commands described below.
.PP
All commands provided in the master interpreter by Safe Tcl reside in
the \fBsafe\fR namespace.
.SH COMMANDS
The following commands are provided in the master interpreter:
.TP
\fB::safe::interpCreate\fR ?\fIslave\fR? ?\fIoptions...\fR?
Creates a safe interpreter, installs the aliases described in the section
\fBALIASES\fR and initializes the auto-loading and package mechanism as
specified by the supplied \fIoptions\fR.
See the \fBOPTIONS\fR section below for a description of the
optional arguments.
If the \fIslave\fR argument is omitted, a name will be generated.
\fB::safe::interpCreate\fR always returns the interpreter name.
.TP
\fB::safe::interpInit\fR \fIslave\fR ?\fIoptions...\fR?
This command is similar to \fBinterpCreate\fR except it that does not
create the safe interpreter. \fIslave\fR must have been created by some
other means, like \fBinterp create\fR \fB\-safe\fR.
.TP
\fB::safe::interpConfigure\fR \fIslave\fR ?\fIoptions...\fR?
If no \fIoptions\fR are given, returns the settings for all options for the
named safe interpreter as a list of options and their current values
for that \fIslave\fR. 
If a single additional argument is provided,
it will return a list of 2 elements \fIname\fR and \fIvalue\fR where
\fIname\fR is the full name of that option and \fIvalue\fR the current value
for that option and the \fIslave\fR.
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 \fBOPTIONS\fR below for options description.
Example of use:
.RS
.PP
.CS
# 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
.CE
.RE
.TP
\fB::safe::interpDelete\fR \fIslave\fR
Deletes the safe interpreter and cleans up the corresponding  
master interpreter data structures.
If a \fIdeleteHook\fR script was specified for this interpreter it is
evaluated before the interpreter is deleted, with the name of the
interpreter as an additional argument.
.TP
\fB::safe::interpFindInAccessPath\fR \fIslave\fR \fIdirectory\fR
This command finds and returns the token for the real directory
\fIdirectory\fR in the safe interpreter's current virtual access path.
It generates an error if the directory is not found.
Example of use:
.RS
.PP
.CS
$slave eval [list set tk_library \e
      [::safe::interpFindInAccessPath $name $tk_library]]
.CE
.RE
.TP
\fB::safe::interpAddToAccessPath\fR \fIslave\fR \fIdirectory\fR
This command adds \fIdirectory\fR 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:
.RS
.PP
.CS
$slave eval [list set tk_library \e
      [::safe::interpAddToAccessPath $name $tk_library]]
.CE
.RE
.TP
\fB::safe::setLogCmd\fR ?\fIcmd arg...\fR?
This command installs a script that will be called when interesting
life cycle events occur for a safe interpreter.
When called with no arguments, it returns the currently installed script.
When called with one argument, an empty string, the currently installed
script is removed and logging is turned off.
The script will be invoked with one additional argument, a string
describing the event of interest.
The main purpose is to help in debugging safe interpreters.
Using this facility you can get complete error messages while the safe
interpreter gets only generic error messages.
This prevents a safe interpreter from seeing messages about failures
and other events that might contain sensitive information such as real
directory names.
.RS
.PP
Example of use:
.PP
.CS
::safe::setLogCmd puts stderr
.CE
.PP
Below is the output of a sample session in which a safe interpreter
attempted to source a file not found in its virtual access path.
Note that the safe interpreter only received an error message saying that
the file was not found:
.PP
.CS
NOTICE for slave interp10 : Created
NOTICE for slave interp10 : Setting accessPath=(/foo/bar) staticsok=1 nestedok=0 deletehook=()
NOTICE for slave interp10 : auto_path in interp10 has been set to {$p(:0:)}
ERROR for slave interp10 : /foo/bar/init.tcl: no such file or directory
.CE
.RE
.SS OPTIONS
The following options are common to 
\fB::safe::interpCreate\fR, \fB::safe::interpInit\fR, 
and \fB::safe::interpConfigure\fR.
Any option name can be abbreviated to its minimal 
non-ambiguous name.
Option names are not case sensitive.
.TP 
\fB\-accessPath\fR \fIdirectoryList\fR
This option sets the list of directories from which the safe interpreter
can \fBsource\fR and \fBload\fR 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 \fBSECURITY\fR below for more detail about virtual paths, 
tokens and access control.
.TP
\fB\-statics\fR \fIboolean\fR
This option specifies if the safe interpreter will be allowed
to load statically linked packages (like \fBload {} Tk\fR).
The default value is \fBtrue\fR : 
safe interpreters are allowed to load statically linked packages.
.TP
\fB\-noStatics\fR
This option is a convenience shortcut for \fB\-statics false\fR and
thus specifies that the safe interpreter will not be allowed
to load statically linked packages.
.TP
\fB\-nested\fR \fIboolean\fR
This option specifies if the safe interpreter will be allowed
to load packages into its own sub-interpreters.
The default value is \fBfalse\fR : 
safe interpreters are not allowed to load packages into
their own sub-interpreters.
.TP
\fB\-nestedLoadOk\fR
This option is a convenience shortcut for \fB\-nested true\fR and
thus specifies the safe interpreter will be allowed
to load packages into its own sub-interpreters.
.TP 
\fB\-deleteHook\fR \fIscript\fR
When this option is given a non-empty \fIscript\fR, 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 (\fB{}\fR) is not to have any deletion call back.
.SH ALIASES
The following aliases are provided in a safe interpreter:
.TP
\fBsource\fR \fIfileName\fR
The requested file, a Tcl source file, is sourced into the safe interpreter
if it is found.
The \fBsource\fR alias can only source files from directories in
the virtual path for the safe interpreter. The \fBsource\fR 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 \fBSECURITY\fR for more discussion of restrictions on
valid filenames.
.TP
\fBload\fR \fIfileName\fR
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 \fBload\fR command for more details.
.TP
\fBfile\fR ?\fIsubCmd args...\fR?
The \fBfile\fR alias provides access to a safe subset of the subcommands of
the \fBfile\fR command; it allows only \fBdirname\fR, \fBjoin\fR,
\fBextension\fR, \fBroot\fR, \fBtail\fR, \fBpathname\fR and \fBsplit\fR
subcommands. For more details on what these subcommands do see the manual
page for the \fBfile\fR command.
.TP
\fBencoding\fR ?\fIsubCmd args...\fR?
The \fBencoding\fR alias provides access to a safe subset of the
subcommands of the \fBencoding\fR command;  it disallows setting of
the system encoding, but allows all other subcommands including
\fBsystem\fR to check the current encoding.
.TP
\fBexit\fR
The calling interpreter is deleted and its computation is stopped, but the
Tcl process in which this interpreter exists is not terminated.
.SH SECURITY
Safe Tcl 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 Safe Tcl
is to prevent.
.PP
The commands available in a safe interpreter, in addition to
the safe set as defined in \fBinterp\fR manual page, are mediated aliases
for \fBsource\fR, \fBload\fR, \fBexit\fR, and safe subsets of
\fBfile\fR and \fBencoding\fR. The safe interpreter can also auto-load
code and it can request that packages be loaded.
.PP
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 \fB::safe::interpCreate\fR or initialized by
\fB::safe::interpInit\fR 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 \fBsource\fR and \fBload\fR aliases provided to the slave
are path in the form of 
\fB[file join \fItoken filename\fB]\fR (i.e. when using the
native file path formats: \fItoken\fB/\fIfilename\fR
on Unix and \fItoken\fB\e\fIfilename\fR on Windows),
where \fItoken\fR is representing one of the directories 
of the \fIaccessPath\fR list and \fIfilename\fR is
one file in that directory (no sub directories access are allowed).
.PP
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.
.PP
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 \fBsource\fR alias restricts access to files
meeting the following constraints: the file name must
fourteen characters or shorter, must not contain more than one dot
.PQ \fB.\fR "" ,
must end up with the extension
.PQ \fB.tcl\fR
or be called
.PQ \fBtclIndex\fR .
.PP
Each element of the initial access path
list will be assigned a token that will be set in
the slave \fBauto_path\fR and the first element of that list will be set as
the \fBtcl_library\fR for that slave.
.PP
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 
\fBauto_path\fR 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 \fBtcl_library\fR will be
added or moved to the first position if necessary, in the 
slave access path, so the slave
\fBtcl_library\fR 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 \fBauto_path\fR 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 \fB\-accessPath\fR flag
instead of relying on this default mechanism.
.PP
When the \fIaccessPath\fR is changed after the first creation or
initialization (i.e. through \fBinterpConfigure -accessPath \fR\fIlist\fR),
an \fBauto_reset\fR is automatically evaluated in the safe interpreter
to synchronize its \fBauto_index\fR with the new token list.
.SH "SEE ALSO"
interp(n), library(n), load(n), package(n), source(n), unknown(n)
.SH KEYWORDS
alias, auto\-loading, auto_mkindex, load, master interpreter, safe
interpreter, slave interpreter, source
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