mm: fix NULL ptr dereference in move_pages

commit 6e8b09ea upstream.

Commit 3268c63e ("mm: fix move/migrate_pages() race on task struct") has
added an odd construct where 'mm' is checked for being NULL, and if it is,
it would get dereferenced anyways by mput()ing it.
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

mm/migrate.c

@@ -1412,14 +1412,14 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid, unsigned long, nr_pages,
 	mm = get_task_mm(task);
 	put_task_struct(task);
 
-	if (mm) {
+	if (!mm)
+		return -EINVAL;
+
 	if (nodes)
 		err = do_pages_move(mm, task_nodes, nr_pages, pages,
 				    nodes, status, flags);
 	else
 		err = do_pages_stat(mm, nr_pages, pages, status);
-	} else
-		err = -EINVAL;
 
 	mmput(mm);
 	return err;