block/mirror: Fix use-after-free

If @bs does not have any parents, the only reference to @mirror_top_bs
will be held by the BlockJob object after the bdrv_unref() following
block_job_create(). However, if block_job_create() fails, this reference
will not exist and @mirror_top_bs will have been deleted when we
goto fail.

The issue comes back at all later entries to the fail label: We delete
the BlockJob object before rolling back our changes to the node graph.
This means that we will delete @mirror_top_bs in the process.

All in all, whenever @bs does not have any parents and we go down the
fail path we will dereference @mirror_top_bs after it has been deleted.

Fix this by invoking bdrv_unref() only when block_job_create() was
successful and by bdrv_ref()'ing @mirror_top_bs in the fail path before
deleting the BlockJob object. Finally, bdrv_unref() it at the end of the
fail path after we actually no longer need it.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/mirror.c

@@ -1150,7 +1150,7 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
     mirror_top_bs->total_sectors = bs->total_sectors;
 
     /* bdrv_append takes ownership of the mirror_top_bs reference, need to keep
-     * it alive until block_job_create() even if bs has no parent. */
+     * it alive until block_job_create() succeeds even if bs has no parent. */
     bdrv_ref(mirror_top_bs);
     bdrv_drained_begin(bs);
     bdrv_append(mirror_top_bs, bs, &local_err);
@@ -1168,10 +1168,12 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
                          BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
                          BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD, speed,
                          creation_flags, cb, opaque, errp);
-    bdrv_unref(mirror_top_bs);
     if (!s) {
         goto fail;
     }
+    /* The block job now has a reference to this node */
+    bdrv_unref(mirror_top_bs);
+
     s->source = bs;
     s->mirror_top_bs = mirror_top_bs;
 
@@ -1242,6 +1244,10 @@ static void mirror_start_job(const char *job_id, BlockDriverState *bs,
 
 fail:
     if (s) {
+        /* Make sure this BDS does not go away until we have completed the graph
+         * changes below */
+        bdrv_ref(mirror_top_bs);
+
         g_free(s->replaces);
         blk_unref(s->target);
         block_job_unref(&s->common);
@@ -1250,6 +1256,8 @@ fail:
     bdrv_child_try_set_perm(mirror_top_bs->backing, 0, BLK_PERM_ALL,
                             &error_abort);
     bdrv_replace_node(mirror_top_bs, backing_bs(mirror_top_bs), &error_abort);
+
+    bdrv_unref(mirror_top_bs);
 }
 
 void mirror_start(const char *job_id, BlockDriverState *bs,