[PATCH] mm/shrinker: avoid out-of-bounds read in set_shrinker_bit()

[David Carlier <devnexen@gmail.com>]

set_shrinker_bit() reads info->unit[shrinker_id_to_index(shrinker_id)]
before checking shrinker_id against info->map_nr_max, so an id past the
currently visible map_nr_max reads past the unit[] array before the
WARN_ON_ONCE() catches it.

Move the load into the bounded branch.

Fixes: 307bececcd12 ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 mm/shrinker.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/shrinker.c b/mm/shrinker.c
index 76b3f750cf65..49256f81199f 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -197,12 +197,13 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
 {
 	if (shrinker_id >= 0 && memcg && !mem_cgroup_is_root(memcg)) {
 		struct shrinker_info *info;
-		struct shrinker_info_unit *unit;
 
 		rcu_read_lock();
 		info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
-		unit = info->unit[shrinker_id_to_index(shrinker_id)];
 		if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
+			struct shrinker_info_unit *unit;
+
+			unit = info->unit[shrinker_id_to_index(shrinker_id)];
 			/* Pairs with smp mb in shrink_slab() */
 			smp_mb__before_atomic();
 			set_bit(shrinker_id_to_offset(shrinker_id), unit->map);
-- 
2.53.0