Re: copy.c handling for RLS is insecure

Noah,

* Noah Misch (noah(at)leadboat(dot)com) wrote:
> On Tue, Dec 02, 2014 at 11:32:27AM -0500, Stephen Frost wrote:
> > * Robert Haas (robertmhaas(at)gmail(dot)com) wrote:
> > > On Thu, Nov 27, 2014 at 2:03 AM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> > > > Alright, I've done the change to use the RangeVar from CopyStmt, but
> > > > also added a check wherein we verify that the relation's OID returned
> > > > from the planned query is the same as the relation's OID that we did the
> > > > RLS check on- if they're different, we throw an error. Please let me
> > > > know if there are any remaining concerns.
>
> Here is the check in question (added in commit 143b39c):
>
> plan = planner(query, 0, NULL);
>
> /*
> * If we were passed in a relid, make sure we got the same one back
> * after planning out the query. It's possible that it changed
> * between when we checked the policies on the table and decided to
> * use a query and now.
> */
> if (queryRelId != InvalidOid)
> {
> Oid relid = linitial_oid(plan->relationOids);
>
> /*
> * There should only be one relationOid in this case, since we
> * will only get here when we have changed the command for the
> * user from a "COPY relation TO" to "COPY (SELECT * FROM
> * relation) TO", to allow row level security policies to be
> * applied.
> */
> Assert(list_length(plan->relationOids) == 1);
>
> if (relid != queryRelId)
> ereport(ERROR,
> (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> errmsg("relation referenced by COPY statement has changed")));
> }
>
> > > That's clearly an improvement, but I'm not sure it's water-tight.
> > > What if the name that originally referenced a table ended up
> > > referencing a view? Then you could get
> > > list_length(plan->relationOids) != 1.
> >
> > I'll test it out and see what happens. Certainly a good question and
> > if there's an issue there then I'll get it addressed.
>
> Yes, it can be made to reference a view and trip the assertion.

Ok. We can certainly make that assertion be a run-time consideration
instead, though I'm not thrilled with that being the only safe-guard.

> > > (And, in that case, I also wonder if you could get
> > > eval_const_expressions() to do evil things on your behalf while
> > > planning.)
> >
> > If it can be made to reference a view then there's an issue as the view
> > might include a function call itself which is provided by the attacker..
>
> Indeed. As the parenthetical remark supposed, the check happens too late to
> prevent a security breach. planner() has run eval_const_expressions(),
> executing code of the view owner's choosing.

Right.

> > Clearly, if we found a relation originally then we need that same
> > relation with the same OID after the conversion to a query.
>
> That is necessary but not sufficient. CREATE RULE can convert a table to a
> view without changing the OID, thereby fooling the check. Test procedure:

Ugh, yes, rules would cause a problem for this..

> -- as superuser (or createrole)
> create user blackhat;
> create user alice;
>
> -- as blackhat
> begin;
> create table exploit_rls_copy (c int);
> alter table exploit_rls_copy enable row level security;
> grant select on exploit_rls_copy to public;
> commit;
>
> -- as alice
> -- first, set breakpoint on BeginCopy
> copy exploit_rls_copy to stdout;
>
> -- as blackhat
> begin;
> create or replace function leak() returns int immutable as $$begin
> raise notice 'in leak()'; return 7; end$$ language plpgsql;
> create rule "_RETURN" as on select to exploit_rls_copy do instead
> select leak() as c from (values (0)) dummy;
> commit;
>
> -- Release breakpoint. leak() function call happens. After that, assertion
> -- fires if enabled. ERROR does not fire in any case.

Thanks for pointing this out. I'll look into it.

Stephen