block-backend: avoid bdrv_unregister_buf() NULL pointer deref

bdrv_*() APIs expect a valid BlockDriverState. Calling them with bs=NULL
leads to undefined behavior.

Jonathan Cameron reported this following NULL pointer dereference when a
VM with a virtio-blk device and a memory-backend-file object is
terminated:
1. qemu_cleanup() closes all drives, setting blk->root to NULL
2. qemu_cleanup() calls user_creatable_cleanup(), which results in a RAM
   block notifier callback because the memory-backend-file is destroyed.
3. blk_unregister_buf() is called by virtio-blk's BlockRamRegistrar
   notifier callback and undefined behavior occurs.

Fixes: baf4226 ("virtio-blk: use BDRV_REQ_REGISTERED_BUF optimization hint")
Co-authored-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221121211923.1993171-1-stefanha@redhat.com>

block/block-backend.c

@@ -2576,14 +2576,25 @@ static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter)
 
 bool blk_register_buf(BlockBackend *blk, void *host, size_t size, Error **errp)
 {
+    BlockDriverState *bs = blk_bs(blk);
+
     GLOBAL_STATE_CODE();
-    return bdrv_register_buf(blk_bs(blk), host, size, errp);
+
+    if (bs) {
+        return bdrv_register_buf(bs, host, size, errp);
+    }
+    return true;
 }
 
 void blk_unregister_buf(BlockBackend *blk, void *host, size_t size)
 {
+    BlockDriverState *bs = blk_bs(blk);
+
     GLOBAL_STATE_CODE();
-    bdrv_unregister_buf(blk_bs(blk), host, size);
+
+    if (bs) {
+        bdrv_unregister_buf(bs, host, size);
+    }
 }
 
 int coroutine_fn blk_co_copy_range(BlockBackend *blk_in, int64_t off_in,