linux-kernel - Re: [PATCH v2] sched/fair: Clear ->h_load

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Message-ID: <CAKfTPtC-L3R6iYA=boxQGKVafC_UhBihYq6n6qTJ6hk4Q76OZg@mail.gmail.com>
Date: Fri, 24 Oct 2025 09:08:26 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peng Wang <peng_wang@...ux.alibaba.com>
Cc: bsegall@...gle.com, dietmar.eggemann@....com, juri.lelli@...hat.com, 
	linux-kernel@...r.kernel.org, mgorman@...e.de, mingo@...hat.com, 
	peterz@...radead.org, rostedt@...dmis.org, vdavydov.dev@...il.com, 
	vschneid@...hat.com, stable@...r.kernel.org
Subject: Re: [PATCH v2] sched/fair: Clear ->h_load_next when unregistering cgroup

On Thu, 23 Oct 2025 at 08:29, Peng Wang <peng_wang@...ux.alibaba.com> wrote:
>
> An invalid pointer dereference bug was reported on arm64 cpu, and has
> not yet been seen on x86. A partial oops looks like:
>
>  Call trace:
>   update_cfs_rq_h_load+0x80/0xb0
>   wake_affine+0x158/0x168
>   select_task_rq_fair+0x364/0x3a8
>   try_to_wake_up+0x154/0x648
>   wake_up_q+0x68/0xd0
>   futex_wake_op+0x280/0x4c8
>   do_futex+0x198/0x1c0
>   __arm64_sys_futex+0x11c/0x198
>
> Link: https://lore.kernel.org/all/20251013071820.1531295-1-CruzZhao@linux.alibaba.com/
>
> We found that the task_group corresponding to the problematic se
> is not in the parent task_group’s children list, indicating that
> h_load_next points to an invalid address. Consider the following
> cgroup and task hierarchy:
>
>          A
>         / \
>        /   \
>       B     E
>      / \    |
>     /   \   t2
>    C     D
>    |     |
>    t0    t1
>
> Here follows a timing sequence that may be responsible for triggering
> the problem:
>
> CPU X                   CPU Y                   CPU Z
> wakeup t0
> set list A->B->C
> traverse A->B->C
> t0 exits
> destroy C
>                         wakeup t2
>                         set list A->E           wakeup t1
>                                                 set list A->B->D
>                         traverse A->B->C
>                         panic
>
> CPU Z sets ->h_load_next list to A->B->D, but due to arm64 weaker memory
> ordering, Y may observe A->B before it sees B->D, then in this time window,
> it can traverse A->B->C and reach an invalid se.
>
> We can avoid stale pointer accesses by clearing ->h_load_next when
> unregistering cgroup.
>
> Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
> Fixes: 685207963be9 ("sched: Move h_load calculation to task_h_load()")
> Cc: <stable@...r.kernel.org>
> Co-developed-by: Cruz Zhao <CruzZhao@...ux.alibaba.com>
> Signed-off-by: Cruz Zhao <CruzZhao@...ux.alibaba.com>
> Signed-off-by: Peng Wang <peng_wang@...ux.alibaba.com>
> ---
>  kernel/sched/fair.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cee1793e8277..a5fce15093d3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13427,6 +13427,14 @@ void unregister_fair_sched_group(struct task_group *tg)
>                                 list_del_leaf_cfs_rq(cfs_rq);
>                         }
>                         remove_entity_load_avg(se);
> +                       /*
> +                        * Clear parent's h_load_next if it points to the
> +                        * sched_entity being freed to avoid stale pointer.
> +                        */
> +                       struct cfs_rq *parent_cfs_rq = cfs_rq_of(se);

Move the declaration at the beg of the if (se) {

> +
> +                       if (READ_ONCE(parent_cfs_rq->h_load_next) == se)
> +                               WRITE_ONCE(parent_cfs_rq->h_load_next, NULL);
>                 }
>
>                 /*
> --
> 2.27.0
>