Re: [PATCH] drm/virtio: add timeout to virtqueue wait to avoid hung task

[syzbot <syzbot@syzkaller.appspotmail.com>]

> virtio_gpu_queue_ctrl_sgs() and virtio_gpu_queue_cursor() use
> wait_event() without timeout when waiting for virtqueue space. If the
> host device stops processing commands, these waits block indefinitely.
> Since callers may hold DRM locks, this can make the entire system
> unresponsive.
>
> Replace wait_event() with wait_event_timeout() using a 5-second timeout,
> consistent with the existing timeout pattern in the driver. On timeout,
> clean up and return -ENODEV, following the same error path as
> drm_dev_enter() failure.
>
> Reported-by: syzbot+d6dd6f86d3aaf7eebe7406e45c1c6e549453f224@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?id=d6dd6f86d3aaf7eebe7406e45c1c6e549453f224
> Reported-by: syzbot+908bd910da5dd79b88de4cf7baf376cc873a922e@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?id=908bd910da5dd79b88de4cf7baf376cc873a922e
> Signed-off-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
> ---
>  drivers/gpu/drm/virtio/virtgpu_vq.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
> index 67865810a2e7..05e816a0ae0b 100644
> --- a/drivers/gpu/drm/virtio/virtgpu_vq.c
> +++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
> @@ -396,7 +396,16 @@ static int virtio_gpu_queue_ctrl_sgs(struct virtio_gpu_device *vgdev,
>  	if (vq->num_free < elemcnt) {
>  		spin_unlock(&vgdev->ctrlq.qlock);
>  		virtio_gpu_notify(vgdev);
> -		wait_event(vgdev->ctrlq.ack_queue, vq->num_free >= elemcnt);
> +		if (!wait_event_timeout(vgdev->ctrlq.ack_queue,
> +					vq->num_free >= elemcnt,
> +					5 * HZ)) {
> +			/* The device did not respond */
> +			if (fence && vbuf->objs)
> +				virtio_gpu_array_unlock_resv(vbuf->objs);
> +			free_vbuf(vgdev, vbuf);
> +			drm_dev_exit(idx);
> +			return -ENODEV;
> +		}
>  		goto again;
>  	}
>  
> @@ -566,7 +575,14 @@ static void virtio_gpu_queue_cursor(struct virtio_gpu_device *vgdev,
>  	ret = virtqueue_add_sgs(vq, sgs, outcnt, 0, vbuf, GFP_ATOMIC);
>  	if (ret == -ENOSPC) {
>  		spin_unlock(&vgdev->cursorq.qlock);
> -		wait_event(vgdev->cursorq.ack_queue, vq->num_free >= outcnt);
> +		if (!wait_event_timeout(vgdev->cursorq.ack_queue,
> +					vq->num_free >= outcnt,
> +					5 * HZ)) {
> +			/* The device did not respond */
> +			free_vbuf(vgdev, vbuf);
> +			drm_dev_exit(idx);
> +			return;
> +		}
>  		spin_lock(&vgdev->cursorq.qlock);
>  		goto retry;
>  	} else {
>
> ---
> base-commit: 5d6919055dec134de3c40167a490f33c74c12581
> change-id: 20260512-virtio-gpu_wait_event-e0cdf8675b7c
>
> Best regards,
> -- 
> Ryosuke Yasuoka <ryasuoka@redhat.com>
>

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