[PATCH 4.2.y-ckt 111/268] virtio_balloon: fix race by fill and leak

[Kamal Mostafa]

4.2.8-ckt3 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Minchan Kim <minchan at kernel.org>

commit f68b992bbb474641881932c61c92dcfa6f5b3689 upstream.

During my compaction-related stuff, I encountered a bug
with ballooning.

With repeated inflating and deflating cycle, guest memory(
ie, cat /proc/meminfo | grep MemTotal) is decreased and
couldn't be recovered.

The reason is balloon_lock doesn't cover release_pages_balloon
so struct virtio_balloon fields could be overwritten by race
of fill_balloon(e,g, vb->*pfns could be critical).

This patch fixes it in my test.

Signed-off-by: Minchan Kim <minchan at kernel.org>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/virtio/virtio_balloon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 82e80e0..18fd1b2 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -205,8 +205,8 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
 	 */
 	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);
+	mutex_unlock(&vb->balloon_lock);
 	return num_freed_pages;
 }
 
-- 
1.9.1