migration: Fix possible race when shutting down to_dst_file

It's not safe to call qemu_file_shutdown() on the to_dst_file without
first checking for the file's presence under the lock. The cleanup of
this file happens at postcopy_pause() and migrate_fd_cleanup() which
are not necessarily running in the same thread as migrate_fd_cancel().

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-5-farosas@suse.de>

migration/migration.c

@@ -1246,7 +1246,7 @@ static void migrate_fd_error(MigrationState *s, const Error *error)
 static void migrate_fd_cancel(MigrationState *s)
 {
     int old_state ;
-    QEMUFile *f = migrate_get_current()->to_dst_file;
+
     trace_migrate_fd_cancel();
 
     WITH_QEMU_LOCK_GUARD(&s->qemu_file_lock) {
@@ -1272,11 +1272,13 @@ static void migrate_fd_cancel(MigrationState *s)
      * If we're unlucky the migration code might be stuck somewhere in a
      * send/write while the network has failed and is waiting to timeout;
      * if we've got shutdown(2) available then we can force it to quit.
-     * The outgoing qemu file gets closed in migrate_fd_cleanup that is
-     * called in a bh, so there is no race against this cancel.
      */
-    if (s->state == MIGRATION_STATUS_CANCELLING && f) {
-        qemu_file_shutdown(f);
+    if (s->state == MIGRATION_STATUS_CANCELLING) {
+        WITH_QEMU_LOCK_GUARD(&s->qemu_file_lock) {
+            if (s->to_dst_file) {
+                qemu_file_shutdown(s->to_dst_file);
+            }
+        }
     }
     if (s->state == MIGRATION_STATUS_CANCELLING && s->block_inactive) {
         Error *local_err = NULL;
@@ -1536,12 +1538,14 @@ void qmp_migrate_pause(Error **errp)
 {
     MigrationState *ms = migrate_get_current();
     MigrationIncomingState *mis = migration_incoming_get_current();
-    int ret;
+    int ret = 0;
 
     if (ms->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
         /* Source side, during postcopy */
         qemu_mutex_lock(&ms->qemu_file_lock);
-        ret = qemu_file_shutdown(ms->to_dst_file);
+        if (ms->to_dst_file) {
+            ret = qemu_file_shutdown(ms->to_dst_file);
+        }
         qemu_mutex_unlock(&ms->qemu_file_lock);
         if (ret) {
             error_setg(errp, "Failed to pause source migration");