Re: [PATCH v3] media: uvcvideo: Fix race condition with usb_kill_urb

[Ricardo Ribalda]

Hi Laurent

On Wed, 14 Dec 2022 at 13:41, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> Hi Ricardo,
>
> Thank you for the patch.
>
> On Wed, Dec 14, 2022 at 12:18:52PM +0100, Ricardo Ribalda wrote:
> > usb_kill_urb warranties that all the handlers are finished when it
> > returns, but does not protect against threads that might be handling
> > asynchronously the urb.
> >
> > For UVC, the function uvc_ctrl_status_event_async() takes care of
> > control changes asynchronously.
> >
> >  If the code is executed in the following order:
> >
> > CPU 0                                 CPU 1
> > =====                                         =====
> > uvc_status_complete()
> >                                       uvc_status_stop()
> > uvc_ctrl_status_event_work()
> >                                       uvc_status_start() -> FAIL
>
>
>
> > Then uvc_status_start will keep failing and this error will be shown:
> >
> > <4>[    5.540139] URB 0000000000000000 submitted while active
> > drivers/usb/core/urb.c:378 usb_submit_urb+0x4c3/0x528
> >
> > Let's improve the current situation, by not re-submiting the urb if
> > we are stopping the status event. Also process the queued work
> > (if any) during stop.
> >
> > CPU 0                                 CPU 1
> > =====                                         =====
> > uvc_status_complete()
> >                                       uvc_status_stop()
> >                                       uvc_status_start()
> > uvc_ctrl_status_event_work() -> FAIL
> >
> > Hopefully, with the usb layer protection this should be enough to cover
> > all the cases.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: e5225c820c05 ("media: uvcvideo: Send a control event when a Control Change interrupt arrives")
> > Reviewed-by: Yunke Cao <yunkec@xxxxxxxxxxxx>
> > Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> > ---
> > uvc: Fix race condition on uvc
> >
> > Make sure that all the async work is finished when we stop the status urb.
> >
> > To: Yunke Cao <yunkec@xxxxxxxxxxxx>
> > To: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> > To: Max Staudt <mstaudt@xxxxxxxxxx>
> > To: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> > To: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
> > Cc: linux-media@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> > Changes in v3:
> > - Remove the patch for dev->status, makes more sense in another series, and makes
> >   the zero day less nervous.
> > - Update reviewed-by (thanks Yunke!).
> > - Link to v2: https://lore.kernel.org/r/20221212-uvc-race-v2-0-54496cc3b8ab@xxxxxxxxxxxx
> >
> > Changes in v2:
> > - Add a patch for not kalloc dev->status
> > - Redo the logic mechanism, so it also works with suspend (Thanks Yunke!)
> > - Link to v1: https://lore.kernel.org/r/20221212-uvc-race-v1-0-c52e1783c31d@xxxxxxxxxxxx
> > ---
> >  drivers/media/usb/uvc/uvc_ctrl.c   | 3 +++
> >  drivers/media/usb/uvc/uvc_status.c | 6 ++++++
> >  drivers/media/usb/uvc/uvcvideo.h   | 1 +
> >  3 files changed, 10 insertions(+)
> >
> > diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> > index c95a2229f4fa..5160facc8e20 100644
> > --- a/drivers/media/usb/uvc/uvc_ctrl.c
> > +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> > @@ -1442,6 +1442,9 @@ static void uvc_ctrl_status_event_work(struct work_struct *work)
> >
> >       uvc_ctrl_status_event(w->chain, w->ctrl, w->data);
> >
> > +     if (dev->flush_status)
> > +             return;
> > +
> >       /* Resubmit the URB. */
> >       w->urb->interval = dev->int_ep->desc.bInterval;
> >       ret = usb_submit_urb(w->urb, GFP_KERNEL);
> > diff --git a/drivers/media/usb/uvc/uvc_status.c b/drivers/media/usb/uvc/uvc_status.c
> > index 7518ffce22ed..09a5802dc974 100644
> > --- a/drivers/media/usb/uvc/uvc_status.c
> > +++ b/drivers/media/usb/uvc/uvc_status.c
> > @@ -304,10 +304,16 @@ int uvc_status_start(struct uvc_device *dev, gfp_t flags)
> >       if (dev->int_urb == NULL)
> >               return 0;
> >
> > +     dev->flush_status = false;
> >       return usb_submit_urb(dev->int_urb, flags);
> >  }
> >
> >  void uvc_status_stop(struct uvc_device *dev)
> >  {
> > +     struct uvc_ctrl_work *w = &dev->async_ctrl;
> > +
> > +     dev->flush_status = true;
> >       usb_kill_urb(dev->int_urb);
>
> Isn't this still racy ?

Indeed...

I could add a mutex just for flush_status

what do you think?

>
> CPU0                                    CPU1
> ====                                    ====
>
> uvc_status_stop()                       uvc_ctrl_status_event_work()
> {                                       {
>                                                 if (dev->flush_status)
>                                                         return;
>
>         dev->flush_status = true;
>         usb_kill_urb(dev->int_urb);
>
>                                                 /* Resubmit the URB. */
>                                                 w->urb->interval = dev->int_ep->desc.bInterval;
>                                                 ret = usb_submit_urb(w->urb, GFP_KERNEL);
>                                         }
>
>         if (cancel_work_sync(&w->work))
>                 uvc_ctrl_status_event(w->chain, w->ctrl, w->data);
> }
>
> uvc_status_start() will then return an error, right ?
>
> > +     if (cancel_work_sync(&w->work))
> > +             uvc_ctrl_status_event(w->chain, w->ctrl, w->data);
> >  }
> > diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> > index df93db259312..6a9b72d6789e 100644
> > --- a/drivers/media/usb/uvc/uvcvideo.h
> > +++ b/drivers/media/usb/uvc/uvcvideo.h
> > @@ -560,6 +560,7 @@ struct uvc_device {
> >       struct usb_host_endpoint *int_ep;
> >       struct urb *int_urb;
> >       u8 *status;
> > +     bool flush_status;
> >       struct input_dev *input;
> >       char input_phys[64];
> >
> >
> > ---
> > base-commit: 0ec5a38bf8499f403f81cb81a0e3a60887d1993c
> > change-id: 20221212-uvc-race-09276ea68bf8
>
> --
> Regards,
>
> Laurent Pinchart



-- 
Ricardo Ribalda