Hi Ricardo,
Thank you for the patch.
On Wed, Dec 14, 2022 at 05:31:55PM +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@vger.kernel.org Fixes: e5225c820c05 ("media: uvcvideo: Send a control event when a Control Change interrupt arrives") Reviewed-by: Yunke Cao yunkec@chromium.org Signed-off-by: Ricardo Ribalda ribalda@chromium.org
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@chromium.org To: Sergey Senozhatsky senozhatsky@chromium.org To: Max Staudt mstaudt@google.com To: Laurent Pinchart laurent.pinchart@ideasonboard.com To: Mauro Carvalho Chehab mchehab@kernel.org Cc: linux-media@vger.kernel.org Cc: linux-kernel@vger.kernel.org
Changes in v4:
- Replace bool with atomic_t to avoid compiler reordering
- First complete the async work and then kill the urb to avoid race (Thanks Laurent!)
- Link to v3: https://lore.kernel.org/r/20221212-uvc-race-v3-0-954efc752c9a@chromium.org
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@chromium.org
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@chromium.org
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..1be6897a7d6d 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 (atomic_read(&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..4a95850cdc1b 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;
- atomic_set(&dev->flush_status, 0); return usb_submit_urb(dev->int_urb, flags);
} void uvc_status_stop(struct uvc_device *dev) {
- struct uvc_ctrl_work *w = &dev->async_ctrl;
- atomic_set(&dev->flush_status, 1);
Note that atomic_read() and atomic_set() do no imply any memory barrier on most architectures, as far as I can tell. They essentially become READ_ONCE() and WRITE_ONCE() calls, which guarantee that the compiler will not merge or split loads or stores, or reorder them with respect to load and stores to the *same* memory location, but nothing else. I think you need to add proper barriers, and you can then probably also drop usage of atomic_t.
- if (cancel_work_sync(&w->work))
uvc_ctrl_status_event(w->chain, w->ctrl, w->data);
This should get rid of the main race (possibly save the barrier issue), but it's not the most efficient option, and can still be problematic. Consider the following case:
CPU0 CPU1 ---- ----
void uvc_status_stop(struct uvc_device *dev) uvc_ctrl_status_event_async() { { ... atomic_set(&dev->flush_status, 1); ... if (cancel_work_sync()) ... schedule_work(); usb_kill_urb(); } }
The usb_kill_urb() call ensures that uvc_ctrl_status_event_async() completes before uvc_status_stop() returns, but there will still be work scheduled in that case. uvc_ctrl_status_event_work() will be run later, and as flush_status is set to 1, the function will not resubmit the URB. That fixes the main race, but leaves the asynchronous control status event handling for after uvc_status_stop() returns, which isn't great.
Now, if we consider that uvc_status_start() could be called shortly after uvc_status_stop(), we may get in a situation where uvc_status_start() will reset flush_status to 0 before uvc_ctrl_status_event_async() runs. Both uvc_ctrl_status_event_async() and uvc_status_start() will thus submit the same URB.
You can't fix this by first killing the URB and then cancelling the work, as there would then be a risk that uvc_ctrl_status_event_work() would be running in parallel, going past the flush_status check before flush_status gets set to 1 in uvc_status_stop(), and submitting the URB after usb_kill_urb() is called.
I think a good fix would be to check flush_status in uvc_ctrl_status_event_async() and avoid the schedule_work() call in that case. You could then also move the atomic_set(..., 0) call from uvc_status_start() to the end of uvc_status_stop() (again with proper barriers).
Could you please check the memory barriers and test the above proposal (after double-checking it of course, I may have missed something) ?
} diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h index df93db259312..1274691f157f 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;
- atomic_t flush_status; struct input_dev *input; char input_phys[64];