Re: [PATCH 2/3] usb: gadget: uvc: cleanup request when not in correct state

[Avichal Rakesh <arakesh@google.com>]

On 9/15/23 16:32, Michael Grzeschik wrote:
> Hi Avichal
> 
> On Mon, Sep 11, 2023 at 09:52:22PM -0700, Avichal Rakesh wrote:
>> On 9/10/23 17:24, Michael Grzeschik wrote:
>>> The uvc_video_enable function of the uvc-gadget driver is dequeing and
>>> immediately deallocs all requests on its disable codepath. This is not
>>> save since the dequeue function is async and does not ensure that the
>>> requests are left unlinked in the controller driver.
>>>
>>> By adding the ep_free_request into the completion path of the requests
>>> we ensure that the request will be properly deallocated.
>>>
>>> Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
>>> ---
>>>  drivers/usb/gadget/function/uvc_video.c | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
>>> index 4b6e854e30c58c..52e3666b51f743 100644
>>> --- a/drivers/usb/gadget/function/uvc_video.c
>>> +++ b/drivers/usb/gadget/function/uvc_video.c
>>> @@ -256,6 +256,12 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req)
>>>      struct uvc_device *uvc = video->uvc;
>>>      unsigned long flags;
>>>
>>> +    if (uvc->state == UVC_STATE_CONNECTED) {
>>> +        usb_ep_free_request(video->ep, ureq->req);
>> nit: You can probably just call usb_ep_free_request with req instead of ureq->req.
> 
> Thanks, thats a good point.
> 
>>> +        ureq->req = NULL;
>>> +        return;
>>> +    }
>>> +
>>>      switch (req->status) {
>>>      case 0:
>>>          break;
>>
>> Perhaps I am missing something here, but I am not sure how this alone
>> fixes the use-after-free issue. uvcg_video_enable still deallocates
>> _all_ usb_requests right after calling usb_ep_dequeue, so it is still
>> possible that an unreturned request is deallocated, and now it is
>> possible that the complete callback accesses a deallocated ureq :(
> 
> Since the issue I saw was usually coming from the list_del_entry_valid check in
> the list_del_entry of the giveback function, the issue was probably just not
> triggered anymore as the complete function did exit early.
> 
> So this fix alone is actually bogus without a second patch I had in the stack.
> The second patch I am refering should change the actual overall issue:
> 
> https://lore.kernel.org/linux-usb/20230915233113.2903645-1-m.grzeschik@pengutronix.de/T/#u
> 
> This early list_del and this patch here should ensure that the
> concurrent functions are not handling already freed memory.

Oh, the patch linked above is interesting. It effectively force removes the dwc3_request
from whatever list it belongs to? So if DWC3's interrupt handler is delayed past 
UVC gadget's ep_free_request call, then it won't see the requests in its cancelled 
list at all. However, this setup is still prone to errors. For example, there is now
a chance that gadget_ep_free_request is called twice for one request. A scheduling
like the following might cause double kfree:

1. uvcg_video_enable calls usb_ep_dequeue for all usb_requests
2. While the usb_ep_dequeues are being processed, dwc3's interrupt handler starts 
   calling the complete callbacks.
3. The complete callback calls gadget_ep_free_request (calling kfree as a result)
4. Meanwhile, uvcg_video_enable has moved to uvc_video_free_requests which also
   calls gadget_ep_free_request (calling kfree).

There is currently (even in your patches) no synchronization between calls to 
gadget_ep_free_request via complete callback and uvcg_video_enable, which will 
inevitably call usb_ep_free_request twice for one request. 

Does that make sense, or am I misunderstanding some part of the patch?

- Avi.