Re: [PATCH 2/2] virtio-balloon: correct used length

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

On Thu, Nov 25, 2021 at 10:20:46AM +0800, Jason Wang wrote:
> Spec said:
> 
> "and len the total of bytes written into the buffer."
> 
> For inflateq, deflateq and statsq, we don't process in_sg so the used
> length should be zero. For free_page_vq, since the pages could be
> changed in the destination, we should make all pages used for safety.

Yea, about that, I know I said it, but I was wrong, sorry.

Spec says this:

	\field{len} is particularly useful
	for drivers using untrusted buffers: if a driver does not know exactly
	how much has been written by the device, the driver would have to zero
	the buffer in advance to ensure no data leakage occurs.

	For example, a network driver may hand a received buffer directly to
	an unprivileged userspace application.  If the network device has not
	overwritten the bytes which were in that buffer, this could leak the
	contents of freed memory from other processes to the application.


In other words, device must guarantee that used length was
written into. Since we don't know that, we really should
write 0 there, and the fact we don't is a spec violation.


> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  hw/virtio/virtio-balloon.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index 17de2558cb..fb4426ac0c 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -231,7 +231,7 @@ static void balloon_stats_poll_cb(void *opaque)
>          return;
>      }
>  
> -    virtqueue_push(s->svq, s->stats_vq_elem, s->stats_vq_offset);
> +    virtqueue_push(s->svq, s->stats_vq_elem, 0);
>      virtio_notify(vdev, s->svq);
>      g_free(s->stats_vq_elem);
>      s->stats_vq_elem = NULL;
> @@ -438,7 +438,7 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
>              memory_region_unref(section.mr);
>          }
>  
> -        virtqueue_push(vq, elem, offset);
> +        virtqueue_push(vq, elem, 0);
>          virtio_notify(vdev, vq);
>          g_free(elem);
>          virtio_balloon_pbp_free(&pbp);
> @@ -510,6 +510,7 @@ static bool get_free_page_hints(VirtIOBalloon *dev)
>      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
>      VirtQueue *vq = dev->free_page_vq;
>      bool ret = true;
> +    size_t used = 0;
>      int i;
>  
>      while (dev->block_iothread) {
> @@ -548,11 +549,12 @@ static bool get_free_page_hints(VirtIOBalloon *dev)
>          for (i = 0; i < elem->in_num; i++) {
>              qemu_guest_free_page_hint(elem->in_sg[i].iov_base,
>                                        elem->in_sg[i].iov_len);
> +            used += elem->in_sg[i].iov_len;
>          }
>      }
>  
>  out:
> -    virtqueue_push(vq, elem, 1);
> +    virtqueue_push(vq, elem, used);
>      g_free(elem);
>      return ret;
>  }
> -- 
> 2.25.1