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@xxxxxxxxxx>
> ---
> 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
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