Message-ID: <ZSMHeH6jNtXMRR2K@pengutronix.de>
Date:   Sun, 8 Oct 2023 21:48:08 +0200
From:   Michael Grzeschik <mgr@...gutronix.de>
To:     Avichal Rakesh <arakesh@...gle.com>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Daniel Scally <dan.scally@...asonboard.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        jchowdhary@...gle.com, etalvala@...gle.com,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/3] usb: gadget: uvc: stability fixes on STREAMOFF.

On Fri, Oct 06, 2023 at 04:48:19PM -0700, Avichal Rakesh wrote:
>On 10/6/23 15:53, Michael Grzeschik wrote:
>> On Fri, Oct 06, 2023 at 10:00:11AM -0700, Avichal Rakesh wrote:
>>>
>>>
>>> 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.
>>
>> Great!
>
>Uhh...apologies, I will have to take this back. I've been
>trying to use uvc->state as the condition for when completion
>handler should clean up usb_requests, and I cannot figure
>out a way to do so cleanly.
>
>The fundamental problem with using uvc->state is that it is
>not protected by any locks. So there is no real way to
>assert that its value has not changed between reading
>uvc->state and acting on it.
>
>Naively we can write something like the following in the
>completion handler:
>
>void uvc_video_complete(...) {
>    if (uvc->state != UVC_EVENT_STREAMING) {
>        usb_ep_free_request(....);
>    } else {
>        // handle usb_request normally
>    }
>}
>
>But without any locks, there are no guarantees that
>uvc->state didn't mutate immediately after the if
>condition was checked, and the complete handler is
>handling a request that it should've freed instead
>or vice-versa. This argument would hold for any logic
>we guard with uvc->state, making uvc->state effectively
>useless as a check for freeing memory.

Yes, this makes total sense. Since the above condition was also part of
the wait_event patch you created in the first place, I bet this issue
was there aswell and was probably causing the issues I saw while testing
it.


>We can work around it by either
>1. Locking uvc->state with some driver level lock
>   to ensure that we can trust the value of uvc->state
>   at least for a little while, or
>2. Using some other barrier condition that is protected by
>   another lock
>
>If we go with (1), we'd have to add a lock around every
>and every write to uvc->state, which isn't terrible, but
>would require more testing to ensure that it doesn't
>create any new deadlocks.
>
>For (2), with the realization that usb_ep_disable flushes
>all requests, we can add a barrier in uvc_video, protected by
>req_lock. That should simplify the logic a little bit and
>will hopefully be easier to reason about.
>
>I could of course be missing a simpler solution here,
>and am happy to be wrong. So please let me know if you
>have any other ideas on how to guarantee such a check.

For now, I have no better Idea. Idea (2) sounds like
a good compromise. But I will have to review that code
to really judge.

Thanks for the work!

Michael


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