Re: [PATCH v1 0/3] usb: gadget: uvc: stability fixes on STREAMOFF.

[Avichal Rakesh <arakesh@google.com>]

On 10/5/23 15:05, Michael Grzeschik wrote:
> Hi Avichal,
> 
> On Thu, Oct 05, 2023 at 11:30:32AM -0700, Avichal Rakesh wrote:
>> On 10/5/23 03:14, Michael Grzeschik wrote:
>>> On Thu, Oct 05, 2023 at 11:23:27AM +0300, Laurent Pinchart wrote:
>>>> On Tue, Oct 03, 2023 at 01:09:06PM +0200, Michael Grzeschik wrote:
>>>>> On Sat, Sep 30, 2023 at 11:48:18AM -0700, Avichal Rakesh wrote:
>>>>> > We have been seeing two main stability issues that uvc gadget driver
>>>>> > runs into when stopping streams:
>>>>> >  1. Attempting to queue usb_requests to a disabled usb_ep
>>>>> >  2. use-after-free issue for inflight usb_requests
>>>>> >
>>>>> > The three patches below fix the two issues above. Patch 1/3 fixes the
>>>>> > first issue, and Patch 2/3 and 3/3 fix the second issue.
>>>>> >
>>>>> > Avichal Rakesh (3):
>>>>> >   usb: gadget: uvc: prevent use of disabled endpoint
>>>>> >   usb: gadget: uvc: Allocate uvc_requests one at a time
>>>>> >   usb: gadget: uvc: Fix use-after-free for inflight usb_requests
>>>>> >
>>>>> > drivers/usb/gadget/function/f_uvc.c     |  11 +-
>>>>> > drivers/usb/gadget/function/f_uvc.h     |   2 +-
>>>>> > drivers/usb/gadget/function/uvc.h       |   6 +-
>>>>> > drivers/usb/gadget/function/uvc_v4l2.c  |  21 ++-
>>>>> > drivers/usb/gadget/function/uvc_video.c | 189 +++++++++++++++++-------
>>>>> > 5 files changed, 164 insertions(+), 65 deletions(-)
>>>>>
>>>>> These patches are not applying on gregkh/usb-testing since
>>>>> Greg did take my patches first. I have already rebased them.
>>>>
>>>> I think they got merged too soon :-( We could fix things on top, but
>>>> there's very little time to do so for v6.7.
>>>
>>> Agreed. I was jumping from one workaround to another one, since this
>>> is not easy to fix in a proper way. And still after this long discussion
>>> with Avichal I don't think we are there yet.
>>>
>>>
>>> So far the first two patches from Avichal look legit. But the overall
>>> Use-After-Free fix is yet to be done properly.
>>>
>>> The "abondoned" method he suggested is really bad to follow and will
>>> add too much complexity and will be hard to debug.
>>>
>>> IMHO it should be possible to introduce two cleanup pathes.
>>>
>>> One path would be in the uvc_cleanup_requests that will cleanup the
>>> requests that are actually not used in the controller and are registered
>>> in the req_free list.
>>>
>>> The second path would be the complete functions that are being run
>>> from the controller and will ensure that the cleanup will really free
>>> the requests from the controller after they were consumed.
>>>
>>> What do you think?
>>
>> I am not sure I follow. Patch 3/3 does exactly what you say here.
> 
> Yes, it was just to summ up what the latest state of the idea was,
> so Laurent does not read the whole thread in detail. Sorry for not
> being clear enough about that.

Whoops! Sorry about the misunderstanding!

> 
>> There are two cleanup paths:
>>  1. uvcg_video_disable cleans up only the requests in req_free, and
>>  2. complete handler cleans up the in-flight requests.
>>
>> The "abandoned" flag is simply to let the completion handler know
>> which requests to clean up and which ones to re-queue back to
>> the gadget driver.
> 
> What I don't get is, why in the case of shutdown there needs to
> be something re-queued back to the gadget driver. There should not
> need to be any sort of barrier flag for the requests. Just the
> complete handler running past a barrier where it knows that the
> whole device is stopped. So every call on complete should then clean
> that exact request it is touching currently.
> 
> I don't know where the extra complexity comes from.

A lot of this complexity comes from assuming a back to back
STREAMOFF -> STREAMON sequence is possible where the gadget driver
doesn't have the time to clean up all in-flight usb_requests.
However, looking through the usb gadget APIs again, and it 
looks like  usb_ep_disable enforces that all requests will 
be sent back to the gadget driver before it returns. 

So you're right:
With Patch 1/3 in place, I think we can just guard on uvc->state 
alone, because control requests are blocked until usb_ep_disable
is finished anyway. I'll upload v4 with the "is_abandoned"
flag removed and the checks simplified once I've verified the
fix locally.

That should also remove any bookkeeping issues that may have 
triggered the stack below.

Regards,
Avi.

> 
>> The other "complications" are around making sure we can trust
>> the values in an inherently racey situation. The reasoning
>> can admittedly be difficult to follow at a glance, which incidentally
>> is why I went with a simple to prove timed wait in the past
>> (https://lore.kernel.org/20230912041910.726442-3-arakesh@google.com).
>>
>> I am not suggesting we go back to a timed wait, but please do look
>> at the patch and let me know which parts don't make sense, or are
>> difficult to understand. We can add more documentation about our
>> assumptions there, or if you have a way to do this that you
>> think is simpler to reason about, then please let me know and I'll
>> be more than happy to use that!
> 
> I really try to spin my head around the idea of the is_abondoned flag
> you are using. Unfortunatly for now I am out to debug the issues I see
> with your series.
> 
> So I did try these patches you send. Yes the deadlock error is gone with
> v3. But the linked list is still running into cases where
> dwc3_gadget_giveback(complete) is touching requests that are already
> freed.
> 
> [   61.408715] ------------[ cut here ]------------
> [   61.413897] kernel BUG at lib/list_debug.c:56!
> ...
> [   61.590762] Call trace:
> [   61.596890]  __list_del_entry_valid+0xb8/0xe8
> [   61.603408]  dwc3_gadget_giveback+0x3c/0x1b0
> [   61.607594]  dwc3_remove_requests.part.0+0xcc/0x100
> [   61.612948]  __dwc3_gadget_ep_disable+0xbc/0x1b8
> [   61.621019]  dwc3_gadget_ep_disable+0x48/0x100
> [   61.627925]  usb_ep_disable+0x3c/0x138
> [   61.638230]  uvc_function_setup_continue+0x3c/0x60
> [   61.645040]  uvc_v4l2_streamoff+0x5c/0x80
> [   61.659812]  v4l_streamoff+0x40/0x60
> [   61.668950]  __video_do_ioctl+0x344/0x420
> [   61.679548]  video_usercopy+0x1d0/0x788
> [   61.685677]  video_ioctl2+0x40/0x70
> [   61.697439]  v4l2_ioctl+0x68/0xa0
> [   61.709200]  __arm64_sys_ioctl+0x304/0xda0
> [   61.720768]  invoke_syscall.constprop.0+0x70/0x130
>