block: Fix locking in media change monitor commands

blk_insert_bs() requires that the caller holds the AioContext lock for
the node to be inserted. Since commit c066e808e1, neglecting to do so
causes a crash when the child has to be moved to a different AioContext
to attach it to the BlockBackend.

This fixes qmp_blockdev_insert_anon_medium(), which is called for the
QMP commands 'blockdev-insert-medium' and 'blockdev-change-medium', to
correctly take the lock.

Cc: qemu-stable@nongnu.org
Fixes: https://issues.redhat.com/browse/RHEL-3922
Fixes: c066e808e1
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20231013153302.39234-2-kwolf@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qapi-sysemu.c

@@ -237,6 +237,7 @@ static void qmp_blockdev_insert_anon_medium(BlockBackend *blk,
                                             BlockDriverState *bs, Error **errp)
 {
     Error *local_err = NULL;
+    AioContext *ctx;
     bool has_device;
     int ret;
 
@@ -258,7 +259,11 @@ static void qmp_blockdev_insert_anon_medium(BlockBackend *blk,
         return;
     }
 
+    ctx = bdrv_get_aio_context(bs);
+    aio_context_acquire(ctx);
     ret = blk_insert_bs(blk, bs, errp);
+    aio_context_release(ctx);
+
     if (ret < 0) {
         return;
     }