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

[Laurent Pinchart]

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 ?

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