[PATCH v2] mm: list_lru: fix NULL deref when a folio moves to the root memcg

[Shakeel Butt <shakeel.butt@linux.dev>]

folio_memcg_list_lru_alloc() makes sure that a folio's memcg has its
own lists in a list_lru, such as the THP deferred split queue. It looks
up the folio's memcg twice: a fast path under RCU checks if the lists
exist, and a slow path takes a reference and allocates them. The fast
path returns early for the root memcg, which uses the global lists.
The slow path does not check for the root.

The memcg can change between the two lookups: when a cgroup is
removed, its folios are moved to the parent memcg. If the folio is
moved up to the root in between, the slow path gets the root, and
__memcg_list_lru_alloc() dereferences the root's parent, which is NULL.

Users see a NULL pointer dereference oops while a THP is allocated,
such as in a page fault, in khugepaged or at swap-in. tmpfs huge pages
are affected too, and for them the oops can also happen in reclaim,
for example in kswapd. The task that hits the oops is killed, or the
machine panics if panic_on_oops is set.

The race is narrow: the folio has to be moved up to the root right
between the two lookups. For example, its cgroup is directly below the
root and is removed, or has its memory controller disabled, at that
moment.

Fix it by checking memcg_list_lru_allocated() again in the slow path,
like the fast path does. It returns true for the root, which needs no
lists of its own.

Fixes: ae64f07a6a40 ("mm: list_lru: introduce folio_memcg_list_lru_alloc()")
Reported-by: Chris Mason <mason@kernel.org>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
Changes in v2:
- Check memcg_list_lru_allocated() instead of mem_cgroup_is_root(), like
  the fast path does, and update the comment to match (Muchun).
- Link to v1: https://lore.kernel.org/20261112024140.648088-1-shakeel.butt@linux.dev

 mm/list_lru.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/list_lru.c b/mm/list_lru.c
index d1d256822ccc..ca8de3793166 100644
--- a/mm/list_lru.c
+++ b/mm/list_lru.c
@@ -645,9 +645,16 @@ int folio_memcg_list_lru_alloc(struct folio *folio, struct list_lru *lru,
 	if (likely(res))
 		return 0;
 
-	/* Allocation may block, pin the memcg */
+	/*
+	 * Allocation may block, so pin the memcg. If the folio's cgroup
+	 * was removed after the check above, the folio has moved up to a
+	 * parent memcg. That memcg may already have its lists, or may be
+	 * the root, which uses the global lists and has no parent. So
+	 * check again before allocating.
+	 */
 	memcg = get_mem_cgroup_from_folio(folio);
-	res = __memcg_list_lru_alloc(memcg, lru, gfp);
+	if (!memcg_list_lru_allocated(memcg, lru))
+		res = __memcg_list_lru_alloc(memcg, lru, gfp);
 	mem_cgroup_put(memcg);
 	return res;
 }
-- 
2.53.0-Meta