Forwarded: [PATCH] mm/workingset: fix NULL pointer dereference in lru_gen_test_recent

[syzbot <syzbot+e008db2ac01e282550ee@syzkaller.appspotmail.com>]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com.

***

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 | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/workingset.c b/mm/workingset.c
index e9f05634747a..8166793b38dc 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -272,6 +272,8 @@ 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))
+		WRITE_ONCE(memcg, root_mem_cgroup);
 	*lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
 	max_seq = READ_ONCE((*lruvec)->lrugen.max_seq);
-- 
2.43.0