Re: [Qemu-devel] [PATCH] balloon: fix segfault and harden the stats queue

[Paolo Bonzini]

On 01/03/2016 12:14, Ladi Prosek wrote:
> The segfault here is triggered by the driver notifying the stats queue
> twice after adding a buffer to it. This effectively resets stats_vq_elem
> back to NULL and QEMU crashes on the next stats timer tick in
> balloon_stats_poll_cb.
> 
> This is a regression introduced in 51b19ebe4320f3dc, although admittedly
> the device assumed too much about the stats queue protocol even before
> that commit. This commit adds a few more checks and ensures that the one
> stats buffer gets deallocated on device reset.
> 
> Signed-off-by: Ladi Prosek <address@hidden>
> ---
>  hw/virtio/virtio-balloon.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index e9c30e9..e97d403 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -101,7 +101,7 @@ static void balloon_stats_poll_cb(void *opaque)
>      VirtIOBalloon *s = opaque;
>      VirtIODevice *vdev = VIRTIO_DEVICE(s);
>  
> -    if (!balloon_stats_supported(s)) {
> +    if (s->stats_vq_elem == NULL || !balloon_stats_supported(s)) {
>          /* re-schedule */
>          balloon_stats_change_timer(s, s->stats_poll_interval);
>          return;
> @@ -258,11 +258,20 @@ static void virtio_balloon_receive_stats(VirtIODevice 
> *vdev, VirtQueue *vq)
>      size_t offset = 0;
>      qemu_timeval tv;
>  
> -    s->stats_vq_elem = elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> +    elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
>      if (!elem) {
>          goto out;
>      }
>  
> +    if (s->stats_vq_elem != NULL) {
> +        /* This should never happen if the driver follows the spec. */
> +        virtqueue_push(vq, s->stats_vq_elem, 0);
> +        virtio_notify(vdev, vq);
> +        g_free(s->stats_vq_elem);
> +    }
> +
> +    s->stats_vq_elem = elem;
> +
>      /* Initialize the stats to get rid of any stale values.  This is only
>       * needed to handle the case where a guest supports fewer stats than it
>       * used to (ie. it has booted into an old kernel).
> @@ -458,6 +467,16 @@ static void virtio_balloon_device_unrealize(DeviceState 
> *dev, Error **errp)
>      virtio_cleanup(vdev);
>  }
>  
> +static void virtio_balloon_device_reset(VirtIODevice *vdev)
> +{
> +    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
> +
> +    if (s->stats_vq_elem != NULL) {
> +        g_free(s->stats_vq_elem);
> +        s->stats_vq_elem = NULL;
> +    }
> +}
> +
>  static void virtio_balloon_instance_init(Object *obj)
>  {
>      VirtIOBalloon *s = VIRTIO_BALLOON(obj);
> @@ -486,6 +505,7 @@ static void virtio_balloon_class_init(ObjectClass *klass, 
> void *data)
>      set_bit(DEVICE_CATEGORY_MISC, dc->categories);
>      vdc->realize = virtio_balloon_device_realize;
>      vdc->unrealize = virtio_balloon_device_unrealize;
> +    vdc->reset = virtio_balloon_device_reset;
>      vdc->get_config = virtio_balloon_get_config;
>      vdc->set_config = virtio_balloon_set_config;
>      vdc->get_features = virtio_balloon_get_features;
> 

Reviewed-by: Paolo Bonzini <address@hidden>

There seem to be no other instances of the bug.

Paolo