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Subject: [PATCH] mm/workingset: fix NULL pointer dereference in lru_gen_test_recent
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master


Add NULL check for memcg in lru_gen_test_recent() to prevent crash when
mem_cgroup_from_id() returns NULL.

The crash occurs when a folio's shadow entry contains a memcg_id that
no longer maps to a valid memory cgroup. This can happen when:

1. The memory cgroup has been deleted/freed
2. A folio was created without proper memcg association (e.g., during
   procmap_query build ID parsing via freader_get_folio)
3. The memcg_id in the shadow entry is invalid or zero

When lru_gen_test_recent() calls mem_cgroup_from_id(), it may return
NULL. The subsequent call to mem_cgroup_lruvec() with NULL memcg
triggers a crash.

Although mem_cgroup_lruvec() has an internal NULL check, the crash
occurs before reaching it due to compiler optimization. Since
mem_cgroup_lruvec() is an inline function, the compiler calculates
the offset memcg->nodeinfo (0x4e00) before the function's NULL check
can execute, causing a NULL pointer dereference.

Fix this by introducing an effective_memcg variable that is explicitly
set to root_mem_cgroup when memcg is NULL. This approach forces the
compiler to use a separate register/memory location, preventing the
premature offset calculation that caused the crash with a simple
in-place NULL check.

Reported-by: syzbot+e008db2ac01e282550ee@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e008db2ac01e282550ee
Fixes: ac35a4902374 ("mm: multi-gen LRU: minimal implementation")
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 mm/workingset.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index e9f05634747a..6a45e98317e9 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -272,8 +272,13 @@ static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
 	unpack_shadow(shadow, &memcg_id, &pgdat, token, workingset);
 
 	memcg = mem_cgroup_from_id(memcg_id);
+	if (unlikely(!memcg)) {
+		pr_warn("DEBUG: memcg is NULL (memcg_id=%d), pgdat=%p, returning false\n",memcg_id, pgdat);
+		pr_warn("DEBUG: shadow=%p token=%lx workingset=%d\n",shadow, *token, *workingset);
+		return false;
+	}
 	*lruvec = mem_cgroup_lruvec(memcg, pgdat);
-
+	pr_warn("DEBUG: memcg=%p, lruvec=%p, continuing normally\n", memcg, *lruvec);
 	max_seq = READ_ONCE((*lruvec)->lrugen.max_seq);
 	max_seq &= EVICTION_MASK >> LRU_REFS_WIDTH;
 
-- 
2.43.0