[PATCH 45/55] mirror: Do not dereference invalid pointers

[Michael Roth <mdroth@linux.vnet.ibm.com>]

From: Max Reitz <mreitz@redhat.com>

mirror_exit_common() may be called twice (if it is called from
mirror_prepare() and fails, it will be called from mirror_abort()
again).

In such a case, many of the pointers in the MirrorBlockJob object will
already be freed.  This can be seen most reliably for s->target, which
is set to NULL (and then dereferenced by blk_bs()).

Cc: qemu-stable@nongnu.org
Fixes: 737efc1eda23b904fbe0e66b37715fb0e5c3e58b
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20191014153931.20699-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit f93c3add3a773e0e3f6277e5517583c4ad3a43c2)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
 block/mirror.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 9f5c59ece1..0e3f7923cf 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -618,11 +618,11 @@ static int mirror_exit_common(Job *job)
 {
     MirrorBlockJob *s = container_of(job, MirrorBlockJob, common.job);
     BlockJob *bjob = &s->common;
-    MirrorBDSOpaque *bs_opaque = s->mirror_top_bs->opaque;
+    MirrorBDSOpaque *bs_opaque;
     AioContext *replace_aio_context = NULL;
-    BlockDriverState *src = s->mirror_top_bs->backing->bs;
-    BlockDriverState *target_bs = blk_bs(s->target);
-    BlockDriverState *mirror_top_bs = s->mirror_top_bs;
+    BlockDriverState *src;
+    BlockDriverState *target_bs;
+    BlockDriverState *mirror_top_bs;
     Error *local_err = NULL;
     bool abort = job->ret < 0;
     int ret = 0;
@@ -632,6 +632,11 @@ static int mirror_exit_common(Job *job)
     }
     s->prepared = true;
 
+    mirror_top_bs = s->mirror_top_bs;
+    bs_opaque = mirror_top_bs->opaque;
+    src = mirror_top_bs->backing->bs;
+    target_bs = blk_bs(s->target);
+
     if (bdrv_chain_contains(src, target_bs)) {
         bdrv_unfreeze_backing_chain(mirror_top_bs, target_bs);
     }
-- 
2.17.1