block-migration: Ensure we don't crash during migration cleanup

We can fail the blk_insert_bs() at init_blk_migration(), leaving the
BlkMigDevState without a dirty_bitmap and BlockDriverState. Account
for the possibly missing elements when doing cleanup.

Fix the following crashes:

Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
0x0000555555ec83ef in bdrv_release_dirty_bitmap (bitmap=0x0) at ../block/dirty-bitmap.c:359
359         BlockDriverState *bs = bitmap->bs;
 #0  0x0000555555ec83ef in bdrv_release_dirty_bitmap (bitmap=0x0) at ../block/dirty-bitmap.c:359
 #1  0x0000555555bba331 in unset_dirty_tracking () at ../migration/block.c:371
 #2  0x0000555555bbad98 in block_migration_cleanup_bmds () at ../migration/block.c:681

Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
0x0000555555e971ff in bdrv_op_unblock (bs=0x0, op=BLOCK_OP_TYPE_BACKUP_SOURCE, reason=0x0) at ../block.c:7073
7073        QLIST_FOREACH_SAFE(blocker, &bs->op_blockers[op], list, next) {
 #0  0x0000555555e971ff in bdrv_op_unblock (bs=0x0, op=BLOCK_OP_TYPE_BACKUP_SOURCE, reason=0x0) at ../block.c:7073
 #1  0x0000555555e9734a in bdrv_op_unblock_all (bs=0x0, reason=0x0) at ../block.c:7095
 #2  0x0000555555bbae13 in block_migration_cleanup_bmds () at ../migration/block.c:690

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Message-id: 20230731203338.27581-1-farosas@suse.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit f187609)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

migration/block.c

@@ -368,7 +368,9 @@ static void unset_dirty_tracking(void)
     BlkMigDevState *bmds;
 
     QSIMPLEQ_FOREACH(bmds, &block_mig_state.bmds_list, entry) {
-        bdrv_release_dirty_bitmap(bmds->dirty_bitmap);
+        if (bmds->dirty_bitmap) {
+            bdrv_release_dirty_bitmap(bmds->dirty_bitmap);
+        }
     }
 }
 
@@ -676,13 +678,18 @@ static int64_t get_remaining_dirty(void)
 static void block_migration_cleanup_bmds(void)
 {
     BlkMigDevState *bmds;
+    BlockDriverState *bs;
     AioContext *ctx;
 
     unset_dirty_tracking();
 
     while ((bmds = QSIMPLEQ_FIRST(&block_mig_state.bmds_list)) != NULL) {
         QSIMPLEQ_REMOVE_HEAD(&block_mig_state.bmds_list, entry);
-        bdrv_op_unblock_all(blk_bs(bmds->blk), bmds->blocker);
+
+        bs = blk_bs(bmds->blk);
+        if (bs) {
+            bdrv_op_unblock_all(bs, bmds->blocker);
+        }
         error_free(bmds->blocker);
 
         /* Save ctx, because bmds->blk can disappear during blk_unref.  */