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

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 <[email protected]>.

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 '<[email protected]>' --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 <[email protected]>
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.

index 50e8d04ab661..2e5b89d7d866 100644
@@ -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.

Please see the attached patch for 5.4.y

Oleg.

From a3dbb5447bc9a8f9c04ffa5381b0a0bd77b1bdd5 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <[email protected]>
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()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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.

---
 kernel/time/posix-cpu-timers.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index eacb0ca30193..c77433047a9d 100644
@@ -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

[ Sasha's backport helper bot ]

Hi,

✅ All tests passed successfully. No issues detected. No action required from the submitter.

The upstream commit SHA1 provided is correct: f90fff1e152dedf52b932240ebbd670d83330eca

Status in newer kernel trees:
6.15.y | Present (different SHA1: d65f6c68f7b1) 6.12.y | Present (different SHA1: 18a3e65f32ed) 6.6.y | Present (different SHA1: 8db5813e9ad7) 6.1.y | Present (different SHA1: 61fa08967f27) 5.15.y | Present (different SHA1: f6e90a3258e0) 5.10.y | Present (different SHA1: 1c179c7c3b82)

Note: The patch differs from the upstream commit:
---
1: f90fff1e152de ! 1: 933e5cbcf266c posix-cpu-timers: fix race between handle_posix_cpu_timers() and posix_cpu_timer_del()
@@ Metadata
Author: Oleg Nesterov <[email protected]>

## Commit message ##
- posix-cpu-timers: fix race between handle_posix_cpu_timers() and posix_cpu_timer_del()
+ FAILED: patch "[PATCH] posix-cpu-timers: fix race between handle_posix_cpu_timers()" failed to apply to 5.4-stable tree
+
+ On 06/17, [email protected] wrote:
+ >
+ > The patch below does not apply to the 5.4-stable tree.
+
+ Please see the attached patch for 5.4.y
+
+ Oleg.
+
+ From a3dbb5447bc9a8f9c04ffa5381b0a0bd77b1bdd5 Mon Sep 17 00:00:00 2001
+ From: Oleg Nesterov <[email protected]>
+ 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()
+ MIME-Version: 1.0
+ Content-Type: text/plain; charset=UTF-8
+ Content-Transfer-Encoding: 8bit
+
+ 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
@@ kernel/time/posix-cpu-timers.c: void run_posix_cpu_timers(void)
+ 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.
+ * The fast path checks that there are no expired thread or thread
+ * group timers. If that's so, just return.
---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-5.4.y        |  Success    |  Success   |