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

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Nov 11, 2026 at 10:12:31PM -0800, Shakeel Butt wrote:
> 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>

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>