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

[Luiz Capitulino <lcapitulino@redhat.com>]

The balloon_page_dequeue() function 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.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---

PS: I didn't get this in practice. I found it by code review. On the other
    hand, automatic-ballooning was able to put such invalid requests in
    the virtqueue and QEMU would explode...

PPS: Very lightly tested

 drivers/virtio/virtio_balloon.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index bd3ae32..71af7b5 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);
 }
-- 
1.8.1.4

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