Re: [PATCH] virtio_balloon: don't push uninitialized buffers to stats virtqueue

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 22, 2017 at 04:10:27PM +0100, Ladi Prosek wrote:
> When init_vqs runs, virtio_balloon.stats is either uninitialized or
> contains stale values. The host updates its state with garbage data
> because it has no way of knowing that this is just a marker buffer
> used for signaling.
> 
> This patch initializes all tags with U16_MAX which is guaranteed to
> be ignored by the host.
> 
> The alternative fix would be to push an empty buffer in init_vqs but
> that's not easily done with the current virtio implementation and
> would still not eliminate the invariant that update_balloon_stats
> has to update all VIRTIO_BALLOON_S_NR fields.
> 
> Signed-off-by: Ladi Prosek <lprosek@redhat.com>
> ---
>  drivers/virtio/virtio_balloon.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 4e11915..d34f56f 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -423,12 +423,16 @@ static int init_vqs(struct virtio_balloon *vb)
>  	vb->deflate_vq = vqs[1];
>  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
>  		struct scatterlist sg;
> +		int idx;
>  		vb->stats_vq = vqs[2];
>  
>  		/*
>  		 * Prime this virtqueue with one buffer so the hypervisor can
>  		 * use it to signal us later (it can't be broken yet!).
>  		 */
> +		for (idx = 0; idx < VIRTIO_BALLOON_S_NR; idx++)
> +			update_stat(vb, idx, U16_MAX, 0);
> +

Can't we fill in actual values?

>  		sg_init_one(&sg, vb->stats, sizeof vb->stats);
>  		if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)
>  		    < 0)
> -- 
> 2.7.4