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Safe(3)
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 compartments 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 compartment.
By default, the only variables shared with compartments 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 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 potentially 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 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 dramatically 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
$cpt = new Safe;
Optional argument is (NAMESPACE), where NAMESPACE is the root namespace to
use for the compartment (defaults to "Safe::Root0", incremented for each
new compartment).
Note that version 1.00 of the Safe module supported a second optional
parameter, MASK. That functionality has been withdrawn pending deeper
consideration. Use the permit and deny methods described below.
The following methods can then be used on the compartment object returned
by the above constructor. The object argument is implicit in each case.
permit (OP, ...)
Permit the listed operators to be used when compiling code in the
compartment (in addition to any operators already permitted).
permit_only (OP, ...)
Permit only the listed operators to be used when compiling code in
the compartment (no other operators are permitted).
deny (OP, ...)
Deny the listed operators from being used when compiling code in
the compartment (other operators may still be permitted).
deny_only (OP, ...)
Deny only the listed operators from being used when compiling code
in the compartment (all other operators will be permitted).
trap (OP, ...)
untrap (OP, ...)
The trap and untrap methods are synonyms for deny and permit
respectfully.
share (NAME, ...)
This shares the variable(s) in the argument list with the
compartment. This is almost identical to exporting variables using
the Exporter module.
Each NAME must be the name of a non-lexical variable, typically
with the leading type identifier included. A bareword is treated as
a function name.
Examples of legal names are '$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)
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.
$safe->share_from('main', [ '$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 compartment.
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 $@ is set to the error
message, just as with an eval().
If there is no error, then the method returns the value of the last
expression evaluated, or a return statement may be used, just as
with subroutines and eval(). The context (list or scalar) is
determined by the caller as usual.
This behaviour differs from the beta distribution of the Safe
extension where earlier versions of perl made it hard to mimic the
return behaviour of the eval() command and the context was always
scalar.
Some points to note:
If the entereval op is permitted then the code can use eval "..."
to 'hide' code which might use denied ops. This is not a major
problem since when the code tries to execute the eval it will fail
because the opmask is still in effect. However this technique would
allow clever, and possibly harmful, code to 'probe' the boundaries
of what is possible.
Any string eval which is executed by code executing in a
compartment, or by code called from code executing in a
compartment, will be eval'd in the namespace of the compartment.
This is potentially a serious problem.
Consider a function foo() in package pkg compiled outside a
compartment but shared with it. Assume the compartment has a root
package called 'Root'. If foo() contains an eval statement like
eval '$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 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 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 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 compartment.
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 carefully 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 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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