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

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

On Fri, Nov 26, 2021 at 10:45:43AM +0800, Jason Wang wrote:
> 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

No:


The driver MUST NOT make assumptions about data in device-writable buffers
beyond the first \field{len} bytes, and SHOULD ignore this data.



> >
> >
> > > 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
> >