Both migration thread or background snapshot thread will take a refcount of
the migration object at the entrace of the thread function.

That makes sense, because it protects the object from being freed by the
main thread in migration_shutdown() later, but it might still race with it
if the thread is scheduled too late.  Consider the case right after
pthread_create() happened, VM shuts down with the object released, but
right after that the migration thread finally got created, referencing
MigrationState* in the opaque pointer which is already freed.

The only 100% safe way to make sure it won't get freed is taking the
refcount right before the thread is created, meanwhile when BQL is held.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20241024213056.1395400-2-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index bcb735869b..de80d64dda 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3488,7 +3488,6 @@ static void *migration_thread(void *opaque)
 
     rcu_register_thread();
 
-    object_ref(OBJECT(s));
     update_iteration_initial_status(s);
 
     if (!multifd_send_setup()) {
@@ -3626,7 +3625,6 @@ static void *bg_migration_thread(void *opaque)
     int ret;
 
     rcu_register_thread();
-    object_ref(OBJECT(s));
 
     migration_rate_set(RATE_LIMIT_DISABLED);
 
@@ -3838,6 +3836,14 @@ void migrate_fd_connect(MigrationState *s, Error *error_in)
         }
     }
 
+    /*
+     * Take a refcount to make sure the migration object won't get freed by
+     * the main thread already in migration_shutdown().
+     *
+     * The refcount will be released at the end of the thread function.
+     */
+    object_ref(OBJECT(s));
+
     if (migrate_background_snapshot()) {
         qemu_thread_create(&s->thread, MIGRATION_THREAD_SNAPSHOT,
                 bg_migration_thread, s, QEMU_THREAD_JOINABLE);
-- 
2.45.0