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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 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.
METHODS
To create a new compartment, use
$cpt = new Safe;
Optional argument is (NAMESPACE), where NAMESPACE is the root namespace to use for the
compartment (defaults to "Safe::Root0", incremented for each new compartment).
Note that version 1.00 of the Safe module supported a second optional parameter, MASK.
That functionality has been withdrawn pending deeper consideration. Use the permit and
deny methods described below.
The following methods can then be used on the compartment object returned by the above
constructor. The object argument is implicit in each case.
permit (OP, ...)
Permit the listed operators to be used when compiling code in the compartment (in addition
to any operators already permitted).
You can list opcodes by names, or use a tag name; see "Predefined Opcode Tags" in Opcode.
permit_only (OP, ...)
Permit only the listed operators to be used when compiling code in the compartment (no
other operators are permitted).
deny (OP, ...)
Deny the listed operators from being used when compiling code in the compartment (other
operators may still be permitted).
deny_only (OP, ...)
Deny only the listed operators from being used when compiling code in the compartment (all
other operators will be permitted, so you probably don't want to use this method).
trap (OP, ...), untrap (OP, ...)
The trap and untrap methods are synonyms for deny and permit respectfully.
share (NAME, ...)
This shares the variable(s) in the argument list with the compartment. This is almost
identical to exporting variables using the Exporter module.
Each NAME must be the name of a non-lexical variable, typically with the leading type
identifier included. A bareword is treated as a function name.
Examples of legal names are '$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' ]);
Names can include package names, which are relative to the specified PACKAGE. So these
two calls have the same effect:
$safe->share_from('Scalar::Util', [ 'reftype' ]);
$safe->share_from('main', [ 'Scalar::Util::reftype' ]);
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->varglob('foo')} = "Hello world";
has the same effect as:
$cpt = new Safe 'Root';
$Root::foo = "Hello world";
but avoids the need to know $cpt's package name.
reval (STRING, STRICT)
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 $@ is set
to the error message, just as with an eval().
If there is no error, then the method returns the value of the last expression evaluated,
or a return statement may be used, just as with subroutines and eval(). The context (list
or scalar) is determined by the caller as usual.
If the return value of reval() is (or contains) any code reference, those code references
are wrapped to be themselves executed always in the compartment. See
"wrap_code_refs_within".
The formerly undocumented STRICT argument sets strictness: if true 'use strict;' is used,
otherwise it uses 'no strict;'. Note: if STRICT is omitted 'no strict;' is the default.
Some points to note:
If the entereval op is permitted then the code can use eval "..." to 'hide' code which
might use denied ops. This is not a major problem since when the code tries to execute the
eval it will fail because the opmask is still in effect. However this technique would
allow clever, and possibly harmful, code to 'probe' the boundaries of what is possible.
Any string eval which is executed by code executing in a compartment, or by code called
from code executing in a compartment, will be eval'd in the namespace of the compartment.
This is potentially a serious problem.
Consider a function foo() in package pkg compiled outside a compartment but shared with
it. Assume the compartment has a root package called 'Root'. If foo() contains an eval
statement like eval '$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. It uses the same
rules as perl's built-in "do" to locate the file, poossibly using @INC.
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).
wrap_code_ref (CODEREF)
Returns a reference to an anonymous subroutine that, when executed, will call CODEREF with
the Safe compartment 'in effect'. In other words, with the package namespace adjusted and
the opmask enabled.
Note that the opmask doesn't affect the already compiled code, it only affects any further
compilation that the already compiled code may try to perform.
This is particularly useful when applied to code references returned from reval().
(It also provides a kind of workaround for RT#60374: "Safe.pm sort {} bug with
-Dusethreads". See <http://rt.perl.org/rt3//Public/Bug/Display.html?id=60374> for much
more detail.)
wrap_code_refs_within (...)
Wraps any CODE references found within the arguments by replacing each with the result of
calling "wrap_code_ref" on the CODE reference. Any ARRAY or HASH references in the
arguments are inspected recursively.
Returns nothing.
RISKS
This section is 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.
Reworked to use the Opcode module and other changes added by Tim Bunce.
Currently maintained by the Perl 5 Porters, <perl5-porters@perl.org>.