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

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

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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

Syzbot reported a general protection fault in lru_gen_test_recent() when
accessing invalid memory addresses:

  Oops: general protection fault in lru_gen_test_recent+0xfc/0x370
  KASAN: probably user-memory-access in range [0x0000000000004e00-0x0000000000004e07]
  RIP: 0010:lru_gen_test_recent+0xfc/0x370

The crash occurs when unpack_shadow() extracts a pglist_data pointer from
a shadow entry. The pgdat can be NULL when NODE_DATA(nid) returns NULL for
an invalid or offlined NUMA node ID stored in the shadow entry.

The existing code doesn't check for NULL pgdat before passing it to
mem_cgroup_lruvec(), which can lead to crashes when dereferencing the
invalid pointer.

Fix this by checking if pgdat is NULL and setting lruvec to NULL before
returning false. The caller in lru_gen_refault() will then skip processing
via the check "if (lruvec != folio_lruvec(folio)) goto unlock", preventing
use of the invalid lruvec.

Reported-by: syzbot+e008db2ac01e282550ee@syzkaller.appspot.com
Link: https://syzkaller.appspot.com/bug?extid=e008db2ac01e282550ee
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
---
 mm/workingset.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index e9f05634747a..b63948f4e91a 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -270,7 +270,10 @@ static bool lru_gen_test_recent(void *shadow, struct lruvec **lruvec,
 	struct pglist_data *pgdat;
 
 	unpack_shadow(shadow, &memcg_id, &pgdat, token, workingset);
-
+	if (unlikely(!pgdat)) {
+		*lruvec = NULL;
+		return false;
+	}
 	memcg = mem_cgroup_from_id(memcg_id);
 	*lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
-- 
2.43.0