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

[Waiman Long <longman@redhat.com>]

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@arm.com>

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