MirOS Manual: Safe(3p)


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 compartment
     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 outside
             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 compart-
             ment.

             By default, the only variables shared with compart-
             ments are the "underscore" variables $_ and @_ (and,
             technically, the less frequently used %_, the _
             filehandle and so on). This is because otherwise
             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 compilation, 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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             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 exam-
             ple,

                 $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 pur-
     poses.

     The authors shall not in any case be liable for special,
     incidental, consequential, indirect or other similar damages
     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 dramati-
     cally 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 second
     optional parameter, MASK.  That functionality has been with-
     drawn 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 compil-
             ing code in the compartment (in addition to any
             operators already permitted).

             You can list opcodes by names, or use a tag name;
             see "Predefined Opcode Tags" in Opcode.

     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 com-
             piling 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 opera-
             tors 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 module.

             Each NAME must be the name of a non-lexical vari-
             able, typically 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 filehandle).

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

     share_from (PACKAGE, ARRAYREF)

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             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 char-
             acters) are supplied as an array reference.

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

     varglob (VARNAME)
             This returns a glob reference for the symbol table
             entry of VARNAME in the package of the compartment.
             VARNAME must be the name of a variable without 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 compartment'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 operator
             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...".

             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 subroutines 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:

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             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 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 executing
             in a compartment, or by code called from code exe-
             cuting 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 (auto-
             loaded) function with the compartment. If an auto-
             load 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 created 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 look-
             ups in code called from a compartment but not com-
             piled 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

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             been withdrawn pending deeper consideration.

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

             With no MASK argument present, it returns the
             current operator mask of the compartment.

             With the MASK argument present, it sets the operator
             mask for the compartment (equivalent to calling 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 example.

     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 exception 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 sub-
             tle effect.

     AUTHOR

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

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

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