migration/block-dirty-bitmap: make incoming disabled bitmaps busy

Incoming enabled bitmaps are busy, because we do
bdrv_dirty_bitmap_create_successor() for them. But disabled bitmaps
being migrated are not marked busy, and user can remove them during the
incoming migration. Then we may crash in cancel_incoming_locked() when
try to remove the bitmap that was already removed by user, like this:

 #0  qemu_mutex_lock_impl (mutex=0x5593d88c50d1, file=0x559680554b20
   "../block/dirty-bitmap.c", line=64) at ../util/qemu-thread-posix.c:77
 #1  bdrv_dirty_bitmaps_lock (bs=0x5593d88c0ee9)
   at ../block/dirty-bitmap.c:64
 qemu#2  bdrv_release_dirty_bitmap (bitmap=0x5596810e9570)
   at ../block/dirty-bitmap.c:362
 qemu#3  cancel_incoming_locked (s=0x559680be8208 <dbm_state+40>)
   at ../migration/block-dirty-bitmap.c:918
 qemu#4  dirty_bitmap_load (f=0x559681d02b10, opaque=0x559680be81e0
   <dbm_state>, version_id=1) at ../migration/block-dirty-bitmap.c:1194
 qemu#5  vmstate_load (f=0x559681d02b10, se=0x559680fb5810)
   at ../migration/savevm.c:908
 qemu#6  qemu_loadvm_section_part_end (f=0x559681d02b10,
   mis=0x559680fb4a30) at ../migration/savevm.c:2473
 qemu#7  qemu_loadvm_state_main (f=0x559681d02b10, mis=0x559680fb4a30)
   at ../migration/savevm.c:2626
 qemu#8  postcopy_ram_listen_thread (opaque=0x0)
   at ../migration/savevm.c:1871
 qemu#9  qemu_thread_start (args=0x5596817ccd10)
   at ../util/qemu-thread-posix.c:521
 qemu#10 start_thread () at /lib64/libpthread.so.0
 qemu#11 clone () at /lib64/libc.so.6

Note bs pointer taken from bitmap: it's definitely bad aligned. That's
because we are in use after free, bitmap is already freed.

So, let's make disabled bitmaps (being migrated) busy during incoming
migration.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20210322094906.5079-2-vsementsov@virtuozzo.com>

Author: Vladimir Sementsov-Ogievskiy

migration/block-dirty-bitmap.c

@@ -839,6 +839,8 @@ static int dirty_bitmap_load_start(QEMUFile *f, DBMLoadState *s)
             error_report_err(local_err);
             return -EINVAL;
         }
+    } else {
+        bdrv_dirty_bitmap_set_busy(s->bitmap, true);
     }
 
     b = g_new(LoadBitmapState, 1);
@@ -914,6 +916,8 @@ static void cancel_incoming_locked(DBMLoadState *s)
         assert(!s->before_vm_start_handled || !b->migrated);
         if (bdrv_dirty_bitmap_has_successor(b->bitmap)) {
             bdrv_reclaim_dirty_bitmap(b->bitmap, &error_abort);
+        } else {
+            bdrv_dirty_bitmap_set_busy(b->bitmap, false);
         }
         bdrv_release_dirty_bitmap(b->bitmap);
     }
@@ -951,6 +955,8 @@ static void dirty_bitmap_load_complete(QEMUFile *f, DBMLoadState *s)
 
     if (bdrv_dirty_bitmap_has_successor(s->bitmap)) {
         bdrv_reclaim_dirty_bitmap(s->bitmap, &error_abort);
+    } else {
+        bdrv_dirty_bitmap_set_busy(s->bitmap, false);
     }
 
     for (item = s->bitmaps; item; item = g_slist_next(item)) {