From: Andrew Morton <akpm@linux-foundation.org>
To: Rik van Riel <riel@surriel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@kernel.org>,
Roman Gushchin <roman.gushchin@linux.dev>,
Shakeel Butt <shakeel.butt@linux.dev>,
Muchun Song <muchun.song@linux.dev>,
cgroups@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, kernel-team@meta.com
Subject: Re: [PATCH] mm/memcg: fix UAF in drain_all_stock() async work during offline
Date: Thu, 27 Aug 2026 15:35:27 -0700 [thread overview]
Message-ID: <20260827153527.e0e968a22717f558a22fb893@linux-foundation.org> (raw)
In-Reply-To: <20260827124211.3b94b103@fangorn>
On Thu, 27 Aug 2026 12:42:11 -0400 Rik van Riel <riel@surriel.com> wrote:
> drain_all_stock() queues drain work on remote CPUs via
> schedule_drain_work() -> queue_work_on(memcg_wq) and returns
> immediately without waiting. The worker, drain_local_memcg_stock()
> / drain_local_obj_stock(), dereferences per-CPU stock caches with
> READ_ONCE(stock->cached[i]) and does css_put() / obj_cgroup_put().
>
> mem_cgroup_css_offline() calls drain_all_stock(memcg) to
> optimize reclamation latency, but never flushes memcg_wq. If
> that races with cgroup removal, free can happen while workers
> are still pending, causing UAF. The drain work could also have
> been queued by somebody else before offline started (e.g. high
> throttling), not just by the offline path itself.
>
> Timeline illustrating the race:
>
> ...
>
> Fix by having the offline path wait for the workqueue to be
> done with the memcg, before freeing the memcg.
>
> Found through a code audit with kres.
>
> Fixes: 591edfb10a94 ("mm: drain memcg stocks on css offlining")
> Cc: stable@vger.kernel.org
> Assisted-by: Hermes:muse-spark-1.2 kres
Sashiko might have found another thing in there:
https://sashiko.dev/#/patchset/20260827124211.3b94b103@fangorn
next prev parent reply other threads:[~2026-08-27 22:35 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-08-27 16:42 [PATCH] mm/memcg: fix UAF in drain_all_stock() async work during offline Rik van Riel
2026-08-27 16:57 ` Shakeel Butt
2026-08-27 17:14 ` Johannes Weiner
2026-08-27 22:35 ` Andrew Morton [this message]
2026-08-28 2:33 ` Rik van Riel
2026-08-28 0:42 ` kernel test robot
2026-08-28 3:22 ` kernel test robot
2026-08-28 10:25 ` kernel test robot
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