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

[Jason Wang]

On Fri, Nov 26, 2021 at 12:14 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> 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.

The problem is, if we write 0, the driver may assume there's no change
on those pages?

Thanks

>
>
> > 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
>