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

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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
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 mm/workingset.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index e9f05634747a..847580173fb0 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -272,8 +272,15 @@ 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);
+	pr_warn("DEBUG: memcg_id=%d memcg=%p root_mem_cgroup=%p\n",memcg_id, memcg, root_mem_cgroup);
+	if (!memcg) {
+		pr_warn("DEBUG: memcg is NULL, using root_mem_cgroup\n");
+		memcg = root_mem_cgroup;
+		pr_warn("DEBUG: after assignment memcg=%p\n", memcg);
+	}
+	pr_warn("DEBUG: about to call mem_cgroup_lruvec with memcg=%p\n", memcg);
 	*lruvec = mem_cgroup_lruvec(memcg, pgdat);
-
+	pr_warn("DEBUG: mem_cgroup_lruvec returned successfully\n");
 	max_seq = READ_ONCE((*lruvec)->lrugen.max_seq);
 	max_seq &= EVICTION_MASK >> LRU_REFS_WIDTH;
 
-- 
2.43.0