


Safe(3p)         Perl Programmers Reference Guide        Safe(3p)


NAME
       Safe - Compile and execute code in restricted compartments

SYNOPSIS
         use Safe;

         $compartment = new Safe;

         $compartment->permit(qw(time sort :browse));

         $result = $compartment->reval($unsafe_code);


DESCRIPTION
       The Safe extension module allows the creation of compart-
       ments in which perl code can be evaluated. Each compart-
       ment has

       a new namespace
               The "root" of the namespace (i.e. "main::") is
               changed to a different package and code evaluated
               in the compartment cannot refer to variables out-
               side this namespace, even with run-time glob
               lookups and other tricks.

               Code which is compiled outside the compartment can
               choose to place variables into (or share variables
               with) the compartment's namespace and only that
               data will be visible to code evaluated in the com-
               partment.

               By default, the only variables shared with com-
               partments are the "underscore" variables $_ and @_
               (and, technically, the less frequently used %_,
               the _ filehandle and so on). This is because oth-
               erwise perl operators which default to $_ will not
               work and neither will the assignment of arguments
               to @_ on subroutine entry.

       an operator mask
               Each compartment has an associated "operator
               mask". Recall that perl code is compiled into an
               internal format before execution.  Evaluating perl
               code (e.g. via "eval" or "do 'file'") causes the
               code to be compiled into an internal format and
               then, provided there was no error in the compila-
               tion, executed.  Code evaluated in a compartment
               compiles subject to the compartment's operator
               mask. Attempting to evaluate code in a compartment
               which contains a masked operator will cause the
               compilation to fail with an error. The code will
               not be executed.

               The default operator mask for a newly created



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Safe(3p)         Perl Programmers Reference Guide        Safe(3p)


               compartment is the ':default' optag.

               It is important that you read the Opcode(3) module
               documentation for more information, especially for
               detailed definitions of opnames, optags and
               opsets.

               Since it is only at the compilation stage that the
               operator mask applies, controlled access to poten-
               tially unsafe operations can be achieved by having
               a handle to a wrapper subroutine (written outside
               the compartment) placed into the compartment. For
               example,

                   $cpt = new Safe;
                   sub wrapper {
                       # vet arguments and perform potentially unsafe operations
                   }
                   $cpt->share('&wrapper');


WARNING
       The authors make no warranty, implied or otherwise, about
       the suitability of this software for safety or security
       purposes.

       The authors shall not in any case be liable for special,
       incidental, consequential, indirect or other similar dam-
       ages arising from the use of this software.

       Your mileage will vary. If in any doubt do not use it.

       RECENT CHANGES

       The interface to the Safe module has changed quite dramat-
       ically since version 1 (as supplied with Perl5.002). Study
       these pages carefully if you have code written to use Safe
       version 1 because you will need to makes changes.

       Methods in class Safe

       To create a new compartment, use

           $cpt = new Safe;

       Optional argument is (NAMESPACE), where NAMESPACE is the
       root namespace to use for the compartment (defaults to
       "Safe::Root0", incremented for each new compartment).

       Note that version 1.00 of the Safe module supported a sec-
       ond optional parameter, MASK.  That functionality has been
       withdrawn pending deeper consideration. Use the permit and
       deny methods described below.




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       The following methods can then be used on the compartment
       object returned by the above constructor. The object argu-
       ment is implicit in each case.

       permit (OP, ...)
               Permit the listed operators to be used when com-
               piling code in the compartment (in addition to any
               operators already permitted).

       permit_only (OP, ...)
               Permit only the listed operators to be used when
               compiling code in the compartment (no other opera-
               tors are permitted).

       deny (OP, ...)
               Deny the listed operators from being used when
               compiling code in the compartment (other operators
               may still be permitted).

       deny_only (OP, ...)
               Deny only the listed operators from being used
               when compiling code in the compartment (all other
               operators will be permitted).

       trap (OP, ...)
       untrap (OP, ...)
               The trap and untrap methods are synonyms for deny
               and permit respectfully.

       share (NAME, ...)
               This shares the variable(s) in the argument list
               with the compartment.  This is almost identical to
               exporting variables using the Exporter(3) module.

               Each NAME must be the name of a variable, typi-
               cally with the leading type identifier included. A
               bareword is treated as a function name.

               Examples of legal names are '$foo' for a scalar,
               '@foo' for an array, '%foo' for a hash, '&foo' or
               'foo' for a subroutine and '*foo' for a glob (i.e.
               all symbol table entries associated with "foo",
               including scalar, array, hash, sub and filehan-
               dle).

               Each NAME is assumed to be in the calling package.
               See share_from for an alternative method (which
               share uses).

       share_from (PACKAGE, ARRAYREF)
               This method is similar to share() but allows you
               to explicitly name the package that symbols should
               be shared from. The symbol names (including type
               characters) are supplied as an array reference.



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                   $safe->share_from('main', [ '$foo', '%bar', 'func' ]);


       varglob (VARNAME)
               This returns a glob reference for the symbol table
               entry of VARNAME in the package of the compart-
               ment. VARNAME must be the name of a variable with-
               out any leading type marker. For example,

                   $cpt = new Safe 'Root';
                   $Root::foo = "Hello world";
                   # Equivalent version which doesn't need to know $cpt's package name:
                   ${$cpt->varglob('foo')} = "Hello world";


       reval (STRING)
               This evaluates STRING as perl code inside the com-
               partment.

               The code can only see the compartment's namespace
               (as returned by the root method). The compart-
               ment's root package appears to be the "main::"
               package to the code inside the compartment.

               Any attempt by the code in STRING to use an opera-
               tor which is not permitted by the compartment will
               cause an error (at run-time of the main program
               but at compile-time for the code in STRING).  The
               error is of the form "%s trapped by operation mask
               operation...".

               If an operation is trapped in this way, then the
               code in STRING will not be executed. If such a
               trapped operation occurs or any other compile-time
               or return error, then $@ is set to the error mes-
               sage, just as with an eval().

               If there is no error, then the method returns the
               value of the last expression evaluated, or a
               return statement may be used, just as with subrou-
               tines and eval(). The context (list or scalar) is
               determined by the caller as usual.

               This behaviour differs from the beta distribution
               of the Safe extension where earlier versions of
               perl made it hard to mimic the return behaviour of
               the eval() command and the context was always
               scalar.

               Some points to note:

               If the entereval op is permitted then the code can
               use eval "..." to 'hide' code which might use
               denied ops. This is not a major problem since when



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               the code tries to execute the eval it will fail
               because the opmask is still in effect. However
               this technique would allow clever, and possibly
               harmful, code to 'probe' the boundaries of what is
               possible.

               Any string eval which is executed by code execut-
               ing in a compartment, or by code called from code
               executing in a compartment, will be eval'd in the
               namespace of the compartment. This is potentially
               a serious problem.

               Consider a function foo() in package pkg compiled
               outside a compartment but shared with it. Assume
               the compartment has a root package called 'Root'.
               If foo() contains an eval statement like eval
               '$foo = 1' then, normally, $pkg::foo will be set
               to 1.  If foo() is called from the compartment (by
               whatever means) then instead of setting $pkg::foo,
               the eval will actually set $Root::pkg::foo.

               This can easily be demonstrated by using a module,
               such as the Socket module, which uses eval "..."
               as part of an AUTOLOAD function. You can 'use' the
               module outside the compartment and share an
               (autoloaded) function with the compartment. If an
               autoload is triggered by code in the compartment,
               or by any code anywhere that is called by any
               means from the compartment, then the eval in the
               Socket module's AUTOLOAD function happens in the
               namespace of the compartment. Any variables cre-
               ated or used by the eval'd code are now under the
               control of the code in the compartment.

               A similar effect applies to all runtime symbol
               lookups in code called from a compartment but not
               compiled within it.

       rdo (FILENAME)
               This evaluates the contents of file FILENAME
               inside the compartment.  See above documentation
               on the reval method for further details.

       root (NAMESPACE)
               This method returns the name of the package that
               is the root of the compartment's namespace.

               Note that this behaviour differs from version 1.00
               of the Safe module where the root module could be
               used to change the namespace. That functionality
               has been withdrawn pending deeper consideration.

       mask (MASK)
               This is a get-or-set method for the compartment's



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               operator mask.

               With no MASK argument present, it returns the cur-
               rent operator mask of the compartment.

               With the MASK argument present, it sets the opera-
               tor mask for the compartment (equivalent to call-
               ing the deny_only method).

       Some Safety Issues

       This section is currently just an outline of some of the
       things code in a compartment might do (intentionally or
       unintentionally) which can have an effect outside the com-
       partment.

       Memory  Consuming all (or nearly all) available memory.

       CPU     Causing infinite loops etc.

       Snooping
               Copying private information out of your system.
               Even something as simple as your user name is of
               value to others. Much useful information could be
               gleaned from your environment variables for exam-
               ple.

       Signals Causing signals (especially SIGFPE and SIGALARM)
               to affect your process.

               Setting up a signal handler will need to be care-
               fully considered and controlled.  What mask is in
               effect when a signal handler gets called?  If a
               user can get an imported function to get an excep-
               tion and call the user's signal handler, does that
               user's restricted mask get re-instated before the
               handler is called?  Does an imported handler get
               called with its original mask or the user's one?

       State Changes
               Ops such as chdir obviously effect the process as
               a whole and not just the code in the compartment.
               Ops such as rand and srand have a similar but more
               subtle effect.

       AUTHOR

       Originally designed and implemented by Malcolm Beattie,
       mbeattie@sable.ox.ac.uk.

       Reworked to use the Opcode module and other changes added
       by Tim Bunce <Tim.Bunce@ig.co.uk>.





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