virtio_balloon: leak_balloon(): only tell host if we got pages deflated

balloon_page_dequeue() can return NULL.  If it does for the first page
being freed then leak_balloon() will create a scatter list with len=0.
Which in turn seems to generate an invalid virtio request.

I didn't get this in practice, I found it by code review.  On the other
hand, such an invalid virtio request will cause errors in QEMU and
fill_balloon() also performs the same check implemented by this commit.

This bug was introduced in e2250429.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org # 3.9

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index bd3ae324a1a2..71af7b5abe01 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -191,7 +191,8 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
          * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
          * is true, we *have* to do it in this order
          */
-        tell_host(vb, vb->deflate_vq);
+        if (vb->num_pfns != 0)
+                tell_host(vb, vb->deflate_vq);
         mutex_unlock(&vb->balloon_lock);
         release_pages_by_pfn(vb->pfns, vb->num_pfns);
 }