On Thu, 24 Oct 2019, Andrey Konovalov wrote: > On Thu, Oct 24, 2019 at 7:57 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Thu, 24 Oct 2019, Andrey Konovalov wrote: > > > > > > Is this really the sort of thing we need to catch? It isn't a bug in > > > > any existing kernel code, as far as I know. Maybe only gadgetfs and > > > > configfs need to worry about it. > > > > > > Hi Alan, > > > > > > Do you mean that the gadget driver must ensure that the max packet > > > size in the endpoint descriptor is not zero? Do HCDs rely on that? I > > > can add this check into the driver we use for USB fuzzing. > > > > Well, if there are any gadget drivers in the kernel which do set an > > endpoint's maxpacket size to 0, they should be fixed. I'm not aware of > > any. > > > > Of course, gadget drivers in userspace are always suspect. That's why > > I suggested having gadgetfs and configfs perform this check. Even so > > it's not really a _security_ risk, because only the superuser is > > allowed to run a userspace gadget driver. (Although obviously it is > > better to have a clean failure than to crash the system when a buggy > > program runs with superuser privileges.) > > > > Yes, HCDs do depend on endpoints having reasonable maxpacket values. I > > suppose the core should check for this. Currently we check for values > > that are too large or invalid in other ways (like high-speed bulk > > endpoints with maxpacket != 512), but we don't check for 0. > > Oh, I think I've confused the terms here. I meant to ask about UDCs. > The question is whether it's OK to try and emulate a gadget with > maxpacket = 0 on a board with a hardware UDC? Or can it cause issues? > The fact that HCDs must ensure correct maxpacket values of course > makes sense. It doesn't make any sense to have an endpoint with maxpacket = 0 -- either real or emulated. The USB spec doesn't prohibit them (probably an oversight), but such endpoints would be useless since it would not be possible to transfer any data to/from them. And as you surmised, it wouldn't be at all surprising for UDC drivers to crash (much like dummy-hcd does) when faced with an endpoint having maxpacket = 0. Best to rule out the possibility entirely. Alan Stern
