mirror: Do not dereference invalid pointers

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: 737efc1eSigned-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20191014153931.20699-2-mreitz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/mirror.c

@@ -620,11 +620,11 @@ 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;
@@ -634,6 +634,11 @@ 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);
     }