Re: [RESEND PATCH v2 2/2] mm/kmemleak: Fix UAF bug in kmemleak_scan()

[Waiman Long]

On 1/23/23 14:24, Catalin Marinas wrote:
> On Fri, Jan 20, 2023 at 05:54:28PM -0500, Waiman Long wrote:
> > On 1/20/23 14:18, Catalin Marinas wrote:
> > > >    /*
> > > > @@ -633,6 +642,7 @@ static void __create_object(unsigned long ptr, size_t size,
> > > >    	object->count = 0;			/* white color initially */
> > > >    	object->jiffies = jiffies;
> > > >    	object->checksum = 0;
> > > > +	object->del_state = 0;
> > > >    	/* task information */
> > > >    	if (in_hardirq()) {
> > > > @@ -1470,9 +1480,22 @@ static void kmemleak_cond_resched(struct kmemleak_object *object)
> > > >    	if (!get_object(object))
> > > >    		return;	/* Try next object */
> > > > +	raw_spin_lock_irq(&kmemleak_lock);
> > > > +	if (object->del_state & DELSTATE_REMOVED)
> > > > +		goto unlock_put;	/* Object removed */
> > > > +	object->del_state |= DELSTATE_NO_DELETE;
> > > > +	raw_spin_unlock_irq(&kmemleak_lock);
> > > > +
> > > >    	rcu_read_unlock();
> > > >    	cond_resched();
> > > >    	rcu_read_lock();
> > > > +
> > > > +	raw_spin_lock_irq(&kmemleak_lock);
> > > > +	if (object->del_state & DELSTATE_REMOVED)
> > > > +		list_del_rcu(&object->object_list);
> > > > +	object->del_state &= ~DELSTATE_NO_DELETE;
> > > > +unlock_put:
> > > > +	raw_spin_unlock_irq(&kmemleak_lock);
> > > >    	put_object(object);
> > > >    }
> > > I'm not sure this was the only problem. We do have the problem that the
> > > current object may be removed from the list, solved above, but another
> > > scenario I had in mind is the next object being released during this
> > > brief resched period. The RCU relies on object->next->next being valid
> > > but, with a brief rcu_read_unlock(), the object->next could be freed,
> > > reallocated, so object->next->next invalid.
> > Looking at the following scenario,
> >
> > object->next => A (removed)
> > A->next => B (removed)
> >
> > As object->next is pointing to A, A must still be allocated and not freed
> > yet. Now if B is also removed, there are 2 possible case.
> >
> > 1) B is removed from the list after the removal of A. In that case, it is
> > not possible that A is allocated, but B is freed.
> >
> > 2) B is removed before A. A->next can't pointed to B when it is being
> > removed. Due to weak memory ordering, it is possible that another cpu can
> > see A->next still pointing to B. In that case, I believe that it is still
> > within the grace period where neither A or B is freed.
> >
> > In fact, it is no different from a regular scanning of the object list
> > without ever called cond_resched().
> More like thinking out loud:
>
> The lockless RCU loop relies on object->next->next being valid within
> the grace period (A not freed). Due to weak memory ordering, the looping
> CPU may not observe the object->next update (removal of A) by another
> CPU, so it continues to loop over it. But since we do an
> rcu_read_unlock() in the middle of the loop, I don't think these
> assumptions are still valid, so A may be freed.
>
> What we need is that object->next reading for the following iteration
> either sees the updated object->next (B) or it sees A but the latter
> still around. I think this holds with the proposed
> kmemleak_cond_resched() since we now start a new grace period with
> rcu_read_lock() followed by taking and releasing kmemleak_lock. The
> latter would give us the memory ordering required since removing object
> A from the list does take the lock.
>
> So yeah, you are probably right, I just find it hard to get my head
> around ;). I still think it would be simpler with a single kmemleak_lock
> (no object->lock) but that's more involved than a simple fix.
>
> Assuming your (and my) reasoning above is correct:
>
> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>

I should have mentioned the fact that taking the kmemleak_lock will post 
some ordering guarantee since it is done after a new rcu_read_lock(). So 
yes, even if both A and B are removed from the object_list, they should 
still be around and not freed yet.

Thanks for your review.

Cheers,
Longman