NAME

Safe - Safe extension module for Perl

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 $_ and @_. 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 evaulated in a compartment compiles subject to the compartment's operator mask. Attempting to evaulate code in a compartment which contains a masked operator will cause the compilation to fail with an error. The code will not be executed.

By default, the operator mask for a newly created compartment masks out all operations which give "access to the system" in some sense. This includes masking off operators such as system, open, chown, and shmget but does not mask off operators such as print, sysread and <HANDLE>. Those file operators are allowed since for the code in the compartment to have access to a filehandle, the code outside the compartment must have explicitly placed the filehandle variable inside the compartment.

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');

Operator masks

An operator mask exists at user-level as a string of bytes of length MAXO, each of which is either 0x00 or 0x01. Here, MAXO is the number of operators in the current version of perl. The subroutine MAXO() (available for export by package Safe) returns the number of operators in the version of perl at the time the Safe extension was built.

If the Safe module is used as a dynamic extension then it should not be used with a future version of perl which has a different number of operators. The presence of a 0x01 byte at offset n of the string indicates that operator number n should be masked (i.e. disallowed). The Safe extension makes available routines for converting from operator names to operator numbers (and vice versa) and for converting from a list of operator names to the corresponding mask (and vice versa).

Methods in class Safe

To create a new compartment, use

$cpt = new Safe;

Optional arguments are (NAMESPACE, MASK), where

NAMESPACE

is the root namespace to use for the compartment (defaults to "Safe::Root000000000", auto-incremented for each new compartment); and

MASK

is the operator mask to use (defaults to a fairly restrictive set).

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

root (NAMESPACE)

This is a get-or-set method for the compartment's namespace. With the NAMESPACE argument present, it sets the root namespace for the compartment. With no NAMESPACE argument present, it returns the current root namespace of the compartment.

mask (MASK)

This is a get-or-set method for the compartment's operator mask. With the MASK argument present, it sets the operator mask for the compartment. With no MASK argument present, it returns the current operator mask of the compartment.

trap (OP, ...)

This sets bits in the compartment's operator mask corresponding to each operator named in the list of arguments. Each OP can be either the name of an operation or its number. See opcode.h or opcode.pl in the main perl distribution for a canonical list of operator names.

untrap (OP, ...)

This resets bits in the compartment's operator mask corresponding to each operator named in the list of arguments. Each OP can be either the name of an operation or its number. See opcode.h or opcode.pl in the main perl distribution for a canonical list of operator names.

share (VARNAME, ...)

This shares the variable(s) in the argument list with the compartment. Each VARNAME must be the name of a variable with a leading type identifier included. Examples of legal variable names are '$foo' for a scalar, '@foo' for an array, '%foo' for a hash, '&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).

varglob (VARNAME)

This returns a glob 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). Any attempt by code in STRING to use an operator which is in the compartment's mask 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. This method returns the error string produced by evaluating the code (i.e. what an ordinary eval puts in $@). If there was no error in the code, then a blank string is returned.

The complicated namespace swapping that goes on at the lowest level of the Safe extension makes it hard/impossible to mimic the return behaviour of the ordinary eval command.

rdo (FILENAME)

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

Subroutines in package Safe

The Safe package contains subroutines for manipulating operator names and operator masks. All are available for export by the package. The canonical list of operator names is the contents of the array op_name defined and initialised in file opcode.h of the Perl source distribution.

ops_to_mask (OP, ...)

This takes a list of operator names and returns an operator mask with precisely those operators masked.

mask_to_ops (MASK)

This takes an operator mask and returns a list of operator names corresponding to those operators which are masked in MASK.

opcode (OP, ...)

This takes a list of operator names and returns the corresponding list of opcodes (which can then be used as byte offsets into a mask).

opname (OP, ...)

This takes a list of opcodes and returns the corresponding list of operator names.

fullmask

This just returns a mask which has all operators masked. It returns the string "\1" x MAXO().

emptymask

This just returns a mask which has all operators unmasked. It returns the string "\0" x MAXO(). This is useful if you want a compartment to make use of the namespace protection features but do not want the default restrictive mask.

MAXO

This returns the number of operators (and hence the length of an operator mask). Note that this returns a constant defined in a file in the perl distribution source and hence reflects the number of operators in the version of perl in use at the time the Safe extension was built. If the Safe extension is used as a dynamic module then it is possible for it to be loaded (and run) by a newer version of perl which may, potentially, have a different number of operators. The Safe extension is not (currently) able to cope with this situation and may not fail safe.

op_mask

This returns the operator mask which is actually in effect at the time the invocation to the subroutine is compiled. In general, this is probably not terribly useful.

AUTHOR

Malcolm Beattie, mbeattie@sable.ox.ac.uk.