[PATCH 3.2 104/106] mm: fix NULL ptr dereference in move_pages

[Ben Hutchings]

3.2.93-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Sasha Levin <levinsasha928@xxxxxxxxx>

commit 6e8b09eaf268bceac0c62e389b4bc0cb83dfb8e5 upstream.

Commit 3268c63 ("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@xxxxxxxxx>
Cc: Dave Hansen <dave@xxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mel@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Christoph Lameter <cl@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/migrate.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1412,14 +1412,14 @@ SYSCALL_DEFINE6(move_pages, pid_t, pid,
 	mm = get_task_mm(task);
 	put_task_struct(task);
 
-	if (mm) {
-		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;
+	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);
 
 	mmput(mm);
 	return err;