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

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.4.y
git checkout FETCH_HEAD
git cherry-pick -x f90fff1e152dedf52b932240ebbd670d83330eca
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025061744-precinct-rubble-45c9@gregkh' --subject-prefix 'PATCH 5.4.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f90fff1e152dedf52b932240ebbd670d83330eca Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Fri, 13 Jun 2025 19:26:50 +0200
Subject: [PATCH] posix-cpu-timers: fix race between handle_posix_cpu_timers()
 and posix_cpu_timer_del()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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>

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.