Re: [PATCH] media: uvc: Avoid cyclic entity chains due to malformed USB descriptors

[Will Deacon]

Hi Laurent,

On Wed, Oct 02, 2019 at 04:09:13PM +0300, Laurent Pinchart wrote:
> Thank you for the patch.

And thank you for the quick response.

> On Wed, Oct 02, 2019 at 12:27:53PM +0100, Will Deacon wrote:
> > I don't have a way to reproduce the original issue, so this change is
> > based purely on inspection. Considering I'm not familiar with USB nor
> > UVC, I may well have missed something!
> 
> I may also be missing something, I haven't touched this code for a long
> time :-)

Actually, that is pretty helpful because it will make backporting easier
if we get to that :)

> uvc_scan_chain_entity(), at the end of the function, adds the entity to
> the list of entities in the chain with
> 
> 	list_add_tail(&entity->chain, &chain->entities);

Yes.

> uvc_scan_chain_forward() is then called (from uvc_scan_chain()), and
> iterates over all entities connected to the entity being scanned.
> 
> 	while (1) {
> 		forward = uvc_entity_by_reference(chain->dev, entity->id,
> 			forward);

Yes.

> At this point forward may be equal to entity, if entity references
> itself.

Correct -- that's indicative of a malformed entity which we want to reject,
right?

> 		if (forward == NULL)
> 			break;
> 		if (forward == prev)
> 			continue;
> 		if (forward->chain.next || forward->chain.prev) {
> 			uvc_trace(UVC_TRACE_DESCR, "Found reference to "
> 				"entity %d already in chain.\n", forward->id);
> 			return -EINVAL;
> 		}
> 
> But then this check should trigger, as forward == entity and entity is
> in the chain's list of entities.

Right, but this code is added by my patch, no? In mainline, the code only
has the first two checks, which both end up comparing against NULL the first
time around:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/usb/uvc/uvc_driver.c#n1489

Or are you referring to somewhere else?

Will