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withdrawn pending deeper consideration.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