Re: FAILED: patch "[PATCH] posix-cpu-timers: fix race between handle_posix_cpu_timers()" failed to apply to 5.4-stable tree

[Oleg Nesterov <oleg@redhat.com>]

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On 06/17, gregkh@linuxfoundation.org wrote:
>
> The patch below does not apply to the 5.4-stable tree.

Please see the attached patch for 5.4.y

Oleg.

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From a3dbb5447bc9a8f9c04ffa5381b0a0bd77b1bdd5 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Tue, 17 Jun 2025 19:15:50 +0200
Subject: [PATCH 5.4.y] posix-cpu-timers: fix race between
 handle_posix_cpu_timers() and posix_cpu_timer_del()
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commit f90fff1e152dedf52b932240ebbd670d83330eca upstream.

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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 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 eacb0ca30193..c77433047a9d 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1119,6 +1119,15 @@ 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;
+
 	/*
 	 * The fast path checks that there are no expired thread or thread
 	 * group timers.  If that's so, just return.
-- 
2.25.1.362.g51ebf55