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

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.

Determined from code inspection.

Move the load into the bounded branch.

Link: https://lore.kernel.org/20260510183700.102475-1-devnexen@gmail.com
Fixes: 307bececcd12 ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Qi Zheng <qi.zheng@linux.dev>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shrinker.c b/mm/shrinker.c
index 76b3f750cf65521b5519db4d75f99dee60c885b1..49256f81199f18087240475d524ec8d855738e11 100644 (file)
--- 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);