Re: [PATCH v2 1/2] media: uvcvideo: Fix race condition for meta buffer list

[Hans de Goede]

Hi Ricardo,

On 29-Jun-26 19:31, Ricardo Ribalda wrote:
> queue->irqueue contains a list of the buffers owned by the driver. The
> list is protected by queue->irqlock. uvc_queue_get_current_buffer()
> returns a pointer to the current buffer in that list, but does not
> remove the buffer from it. This can lead to race conditions.
> 
> Inspecting the code, it seems that the candidate for such race is
> uvc_queue_return_buffers(). For the capture queue, that function is
> called with the device streamoff, so no race can occur. On the other
> hand, the metadata queue, could trigger a race condition, because
> stop_streaming can be called with the device in any streaming state.
> 
> We can solve this issue introducing a flag, stream->meta.in_flight,
> protected with a spinlock. When there is a buffer in flight that can
> write into metadata the flag is raised, notifying the stop streaming
> that it needs to wait.
> 
> Reported-by: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-media/20250630141707.GG20333@xxxxxxxxxxxxxxxxxxxxxxxxxx/
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 088ead255245 ("media: uvcvideo: Add a metadata device node")
> Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>

First of all thank you for looking into fixing this.

I'm sorry, but this feels more like a band-aid then a proper fix.

How about adding a started bool to struct uvc_streaming which gets
set to 1 by uvc_video_start_streaming() and 0 by uvc_video_stop_streaming().

And then call uvc_video_stop_streaming() from either 
uvc_stop_streaming_video() or uvc_stop_streaming_meta()
depending on which one gets called first ?

With a mutex protecting the started bool and being held
over calling uvc_video_stop_streaming() ?

So stop the actual hw streaming when either of the
2 possible /dev/video0 nodes gets its vb2_ops.stop_streaming
callback called?

And to this before draining the buffer queue.

That seems cleaner then this approach?

Regards,

Hans

p.s.

1. It is tempting to also apply the same approach to
vb2_ops.start_streaming, but allowing the meta queue to be
the one to start streaming will likely cause issues. E.g.
the streaming code assumes having a meta-queue active is
optional, but not the other way around.

TL;DR: vb2_ops.start_streaming should stay as is.

2. While looking into this I noticed that struct uvc_streaming
already has an active member, but unless I'm missing something
that ever only gets initialized to 0. So I think that can be
dropped. (If you re-use this please change it to a bool, no
need to have it atomic while protected by a mutex).



> ---
>  drivers/media/usb/uvc/uvc_queue.c | 14 ++++++++++++++
>  drivers/media/usb/uvc/uvc_video.c | 30 +++++++++++++++++++++++++++++-
>  drivers/media/usb/uvc/uvcvideo.h  |  2 ++
>  3 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_queue.c b/drivers/media/usb/uvc/uvc_queue.c
> index 3c002c8f442f..af9dbfcf6f53 100644
> --- a/drivers/media/usb/uvc/uvc_queue.c
> +++ b/drivers/media/usb/uvc/uvc_queue.c
> @@ -209,10 +209,24 @@ static void uvc_stop_streaming_video(struct vb2_queue *vq)
>  static void uvc_stop_streaming_meta(struct vb2_queue *vq)
>  {
>  	struct uvc_video_queue *queue = vb2_get_drv_priv(vq);
> +	struct uvc_streaming *stream = queue->stream;
>  
>  	lockdep_assert_irqs_enabled();
>  
> +	spin_lock_irq(&stream->meta.irqlock);
> +	while (stream->meta.in_flight) {
> +		spin_unlock_irq(&stream->meta.irqlock);
> +		schedule();
> +		spin_lock_irq(&stream->meta.irqlock);
> +	}
> +	stream->meta.in_flight = true;
> +	spin_unlock_irq(&stream->meta.irqlock);
> +
>  	uvc_queue_return_buffers(queue, UVC_BUF_STATE_ERROR);
> +
> +	scoped_guard(spinlock_irq, &stream->meta.irqlock) {
> +		stream->meta.in_flight = false;
> +	}
>  }
>  
>  static const struct vb2_ops uvc_queue_qops = {
> diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> index fc3536a4399f..f6b55b3a3308 100644
> --- a/drivers/media/usb/uvc/uvc_video.c
> +++ b/drivers/media/usb/uvc/uvc_video.c
> @@ -1732,6 +1732,26 @@ static void uvc_video_encode_bulk(struct uvc_urb *uvc_urb,
>  	urb->transfer_buffer_length = stream->urb_size - len;
>  }
>  
> +static struct uvc_buffer *
> +uvc_video_get_current_meta_buffer(struct uvc_streaming *stream)
> +{
> +	struct uvc_video_queue *queue = &stream->meta.queue;
> +	struct uvc_buffer *buf;
> +
> +	buf = uvc_queue_get_current_buffer(queue);
> +	if (!buf)
> +		return NULL;
> +
> +	guard(spinlock_irqsave)(&stream->meta.irqlock);
> +
> +	if (stream->meta.in_flight)
> +		return NULL;
> +
> +	stream->meta.in_flight = true;
> +
> +	return buf;
> +}
> +
>  static void uvc_video_complete(struct urb *urb)
>  {
>  	struct uvc_urb *uvc_urb = urb->context;
> @@ -1767,7 +1787,7 @@ static void uvc_video_complete(struct urb *urb)
>  	buf = uvc_queue_get_current_buffer(queue);
>  
>  	if (vb2_qmeta)
> -		buf_meta = uvc_queue_get_current_buffer(qmeta);
> +		buf_meta = uvc_video_get_current_meta_buffer(stream);
>  
>  	/* Re-initialise the URB async work. */
>  	uvc_urb->async_operations = 0;
> @@ -1778,6 +1798,12 @@ static void uvc_video_complete(struct urb *urb)
>  	 */
>  	stream->decode(uvc_urb, buf, buf_meta);
>  
> +	if (buf_meta) {
> +		scoped_guard(spinlock_irqsave, &stream->meta.irqlock) {
> +			stream->meta.in_flight = false;
> +		}
> +	}
> +
>  	/* If no async work is needed, resubmit the URB immediately. */
>  	if (!uvc_urb->async_operations) {
>  		ret = usb_submit_urb(uvc_urb->urb, GFP_ATOMIC);
> @@ -2330,6 +2356,8 @@ int uvc_video_init(struct uvc_streaming *stream)
>  	for_each_uvc_urb(uvc_urb, stream)
>  		INIT_WORK(&uvc_urb->work, uvc_video_copy_data_work);
>  
> +	spin_lock_init(&stream->meta.irqlock);
> +
>  	return 0;
>  }
>  
> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
> index b6bcee4a222f..6f1a3381d392 100644
> --- a/drivers/media/usb/uvc/uvcvideo.h
> +++ b/drivers/media/usb/uvc/uvcvideo.h
> @@ -484,6 +484,8 @@ struct uvc_streaming {
>  		struct uvc_video_queue queue;
>  		u32 format;
>  		u32 buffersize;
> +		bool in_flight;
> +		spinlock_t irqlock; /* Protects in_flight. */
>  	} meta;
>  
>  	/* Context data used by the bulk completion handler. */
>