From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

Backing may be zero after failed bdrv_append in mirror_start_job,
which leads to SIGSEGV.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20170929152255.5431-1-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/mirror.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/mirror.c b/block/mirror.c
index 3b6f0c5772..153758ca9f 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -1041,6 +1041,10 @@ static int coroutine_fn bdrv_mirror_top_pwritev(BlockDriverState *bs,
 
 static int coroutine_fn bdrv_mirror_top_flush(BlockDriverState *bs)
 {
+    if (bs->backing == NULL) {
+        /* we can be here after failed bdrv_append in mirror_start_job */
+        return 0;
+    }
     return bdrv_co_flush(bs->backing->bs);
 }
 
-- 
2.13.6