linux-kernel - Re: [PATCH] posix-cpu-timers: fix race between handle_posix_cpu_timers() and posix_cpu_timer

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Message-ID: <87msabphoo.ffs@tglx>
Date: Fri, 13 Jun 2025 21:25:43 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Oleg Nesterov <oleg@...hat.com>, Benoît Sevens
 <bsevens@...gle.com>,
 Frederic Weisbecker <frederic@...nel.org>
Cc: Zander Work <zdw@...gle.com>, security@...nel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] posix-cpu-timers: fix race between
 handle_posix_cpu_timers() and posix_cpu_timer_del()

On Fri, Jun 13 2025 at 19:26, Oleg Nesterov wrote:
> If an exiting non-autoreaping task has already passed exit_notify() and
> calls handle_posix_cpu_timers() from IRQ, it can be reaped by its parent
> or debugger right after unlock_task_sighand().
>
> If a concurrent posix_cpu_timer_del() runs at that moment, it won't be
> able to detect timer->it.cpu.firing != 0: cpu_timer_task_rcu() and/or
> lock_task_sighand() will fail.
>
> Add the tsk->exit_state check into run_posix_cpu_timers() to fix this.
>
> This fix is not needed if CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y, because
> exit_task_work() is called before exit_notify(). But the check still
> makes sense, task_work_add(&tsk->posix_cputimers_work.work) will fail
> anyway in this case.
>
> Cc: stable@...r.kernel.org
> Reported-by: Benoît Sevens <bsevens@...gle.com>
> Fixes: 0bdd2ed4138e ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

Linus, I assume you take it directly or do you want me to play the
intermediary?

Thanks,

        tglx