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

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Message-ID: <20250613172650.GA26022@redhat.com>
Date: Fri, 13 Jun 2025 19:26:50 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Benoît Sevens <bsevens@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>
Cc: Zander Work <zdw@...gle.com>, security@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] posix-cpu-timers: fix race between handle_posix_cpu_timers()
 and posix_cpu_timer_del()

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>
---
 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