Linux Virtualization - Jan 2019 - [RFC PATCH 1/1] s390/virtio: handle find on invalid queue gracefully

On Wed,  2 Jan 2019 18:50:20 +0100
Halil Pasic <pasic at linux.ibm.com> wrote:
> A queue with a capacity of zero is clearly not a valid virtio queue.
> Some emulators report zero queue size if queried with an invalid queue
> index. Instead of crashing in this case let us just return -EINVAL. To
> make that work properly, let us fix the notifier cleanup logic as well.
> 
> Signed-off-by: Halil Pasic <pasic at linux.ibm.com>
> ---
> 
> This patch is motivated by commit 86a5597 "virtio-balloon:
> VIRTIO_BALLOON_F_FREE_PAGE_HINT" (Wei Wang, 2018-08-27) which
triggered
> the described scenario.  The emulator in question is the current QEMU.
> The problem we run into is the underflow in the following loop
> in  __vring_new_virtqueue():
> for (i = 0; i < vring.num-1; i++)
> 	vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1)
> Namely vring.num is an unsigned int.
> 
> RFC because I'm not sure about -EINVAL being a good choice, and about
> us caring about what happens if a virtio driver misbehaves like described.
For virtio-pci, the spec says that a zero queue size means that the
queue is unavailable. I don't think we have specified that explicitly
for virtio-ccw, but it does make sense.

virtio-pci returns -ENOENT in that case, which might be a good choice
here as well.
> 
> ---
>  drivers/s390/virtio/virtio_ccw.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/s390/virtio/virtio_ccw.c
b/drivers/s390/virtio/virtio_ccw.c
> index fc9dbad476c0..147927ed4fca 100644
> --- a/drivers/s390/virtio/virtio_ccw.c
> +++ b/drivers/s390/virtio/virtio_ccw.c
> @@ -272,6 +272,8 @@ static void virtio_ccw_drop_indicators(struct
virtio_ccw_device *vcdev)
>  {
>  	struct virtio_ccw_vq_info *info;
>  
> +	if (!vcdev->airq_info)
> +		return;
Which case is this guarding against? names[i] was NULL for every index?
>  	list_for_each_entry(info, &vcdev->virtqueues, node)
>  		drop_airq_indicator(info->vq, vcdev->airq_info);
>  }
> @@ -514,6 +516,10 @@ static struct virtqueue *virtio_ccw_setup_vq(struct
virtio_device *vdev,
>  		err = info->num;
>  		goto out_err;
>  	}
> +	if (info->num == 0) {
> +		err = -EINVAL;
> +		goto out_err;
> +	}
>  	size = PAGE_ALIGN(vring_size(info->num, KVM_VIRTIO_CCW_RING_ALIGN));
>  	info->queue = alloc_pages_exact(size, GFP_KERNEL | __GFP_ZERO);
>  	if (info->queue == NULL) {