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@vger.kernel.org
Reported-by: Benoît Sevens <bsevens@google.com>
Fixes: 0bdd2ed4138e ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/time/posix-cpu-timers.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 50e8d04ab661..2e5b89d7d866 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1405,6 +1405,15 @@ void run_posix_cpu_timers(void)
 
 	lockdep_assert_irqs_disabled();
 
+	/*
+	 * Ensure that release_task(tsk) can't happen while
+	 * handle_posix_cpu_timers() is running. Otherwise, a concurrent
+	 * posix_cpu_timer_del() may fail to lock_task_sighand(tsk) and
+	 * miss timer->it.cpu.firing != 0.
+	 */
+	if (tsk->exit_state)
+		return;
+
 	/*
 	 * If the actual expiry is deferred to task work context and the
 	 * work is already scheduled there is no point to do anything here.
-- 
2.25.1.362.g51ebf55

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@vger.kernel.org
> Reported-by: Benoît Sevens <bsevens@google.com>
> Fixes: 0bdd2ed4138e ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

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

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

Thanks,

        tglx

On Fri, 13 Jun 2025 at 12:25, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Linus, I assume you take it directly or do you want me to play the
> intermediary?

I already took it just because it was easy and I was following this
discussion anyway.

            Linus

On Fri, 13 Jun 2025 at 10:27, Oleg Nesterov <oleg@redhat.com> wrote:
>
> Add the tsk->exit_state check into run_posix_cpu_timers() to fix this.

Thanks, I'll just apply this directly since I've been cc'd on the
whole discussion anyway.

             Linus