The hardlockup detector's probe path (watchdog_hardlockup_probe()) can
be executed in a non-pinned context, such as during the asynchronous
retry mechanism (lockup_detector_delay_init) which runs in a standard
unbound workqueue.

In this context, the existing implementation of
hardlockup_detector_event_create() suffers from a race condition due to
potential task migration. It relies on is_percpu_thread() to ensure
CPU-locality, but worker threads in a global workqueue do not carry the
PF_PERCPU_THREAD flag, causing the WARN_ON() to trigger and violating
the assumption of stable per-cpu access.

If the task migrates during the probe:
1. It might set 'watchdog_ev' on one CPU but fail to clear it if the
   subsequent migration causes the cleanup logic to run on a different CPU.
2. This leaves a stale pointer to a freed perf_event in the original
   CPU's 'watchdog_ev' variable, leading to a use-after-free (UAF) when
   the watchdog is later enabled or reconfigured.

While this issue was prominently observed in downstream kernels (like
openEuler 4.19) where initialization timings are shifted to a post-SMP
phase, it represents a latent bug in the mainline asynchronous
initialization path.

Refactor hardlockup_detector_event_create() to be stateless by returning
the created perf_event pointer instead of directly modifying the per-cpu
'watchdog_ev' variable. This allows the probe logic to safely manage
the temporary event. Use cpu_hotplug_disable() during the probe to ensure
the target CPU remains valid throughout the check.

Fixes: 930d8f8dbab9 ("watchdog/perf: adapt the watchdog_perf interface for async model")
Signed-off-by: Shouxin Sun <sunshx@chinatelecom.cn>
Signed-off-by: Junnan Zhang <zhangjn11@chinatelecom.cn>
Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
Cc: Song Liu <song@kernel.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Jinchao Wang <wangjinchao600@gmail.com>
Cc: <stable@vger.kernel.org>
---
v5:
- Refine description: clarify it identifies a latent bug in the mainline
  asynchronous retry path where worker threads lack PF_PERCPU_THREAD.
v4:
- Add cpu_hotplug_disable() in watchdog_hardlockup_probe() to stabilize
  the probe CPU.
- Update description to explain the relevance of 4.19 logs.
v3:
- Refactor hardlockup_detector_event_create() to be stateless.
v2:
- Add Cc stable.

 kernel/watchdog_perf.c | 56 +++++++++++++++++++++++++-----------------
 1 file changed, 34 insertions(+), 22 deletions(-)

diff --git a/kernel/watchdog_perf.c b/kernel/watchdog_perf.c
index d3ca70e3c256..887b61c65c1b 100644
--- a/kernel/watchdog_perf.c
+++ b/kernel/watchdog_perf.c
@@ -17,6 +17,7 @@
 #include <linux/atomic.h>
 #include <linux/module.h>
 #include <linux/sched/debug.h>
+#include <linux/cpu.h>
 
 #include <asm/irq_regs.h>
 #include <linux/perf_event.h>
@@ -118,18 +119,11 @@ static void watchdog_overflow_callback(struct perf_event *event,
 	watchdog_hardlockup_check(smp_processor_id(), regs);
 }
 
-static int hardlockup_detector_event_create(void)
+static struct perf_event *hardlockup_detector_event_create(unsigned int cpu)
 {
-	unsigned int cpu;
 	struct perf_event_attr *wd_attr;
 	struct perf_event *evt;
 
-	/*
-	 * Preemption is not disabled because memory will be allocated.
-	 * Ensure CPU-locality by calling this in per-CPU kthread.
-	 */
-	WARN_ON(!is_percpu_thread());
-	cpu = raw_smp_processor_id();
 	wd_attr = &wd_hw_attr;
 	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
 
@@ -143,14 +137,7 @@ static int hardlockup_detector_event_create(void)
 						       watchdog_overflow_callback, NULL);
 	}
 
-	if (IS_ERR(evt)) {
-		pr_debug("Perf event create on CPU %d failed with %ld\n", cpu,
-			 PTR_ERR(evt));
-		return PTR_ERR(evt);
-	}
-	WARN_ONCE(this_cpu_read(watchdog_ev), "unexpected watchdog_ev leak");
-	this_cpu_write(watchdog_ev, evt);
-	return 0;
+	return evt;
 }
 
 /**
@@ -159,17 +146,26 @@ static int hardlockup_detector_event_create(void)
  */
 void watchdog_hardlockup_enable(unsigned int cpu)
 {
+	struct perf_event *evt;
+
 	WARN_ON_ONCE(cpu != smp_processor_id());
 
-	if (hardlockup_detector_event_create())
+	evt = hardlockup_detector_event_create(cpu);
+	if (IS_ERR(evt)) {
+		pr_debug("Perf event create on CPU %d failed with %ld\n", cpu,
+			 PTR_ERR(evt));
 		return;
+	}
 
 	/* use original value for check */
 	if (!atomic_fetch_inc(&watchdog_cpus))
 		pr_info("Enabled. Permanently consumes one hw-PMU counter.\n");
 
+	WARN_ONCE(this_cpu_read(watchdog_ev), "unexpected watchdog_ev leak");
+	this_cpu_write(watchdog_ev, evt);
+
 	watchdog_init_timestamp();
-	perf_event_enable(this_cpu_read(watchdog_ev));
+	perf_event_enable(evt);
 }
 
 /**
@@ -263,19 +259,35 @@ bool __weak __init arch_perf_nmi_is_available(void)
  */
 int __init watchdog_hardlockup_probe(void)
 {
+	struct perf_event *evt;
+	unsigned int cpu;
 	int ret;
 
 	if (!arch_perf_nmi_is_available())
 		return -ENODEV;
 
-	ret = hardlockup_detector_event_create();
+	if (!hw_nmi_get_sample_period(watchdog_thresh))
+		return -EINVAL;
 
-	if (ret) {
+	/*
+	 * Test hardware PMU availability by creating a temporary perf event.
+	 * The requested CPU is arbitrary; preemption is not disabled, so
+	 * raw_smp_processor_id() is used. Surround with cpu_hotplug_disable()
+	 * to ensure the arbitrarily chosen CPU remains online during the check.
+	 * The event is released immediately.
+	 */
+	cpu_hotplug_disable();
+	cpu = raw_smp_processor_id();
+	evt = hardlockup_detector_event_create(cpu);
+	if (IS_ERR(evt)) {
 		pr_info("Perf NMI watchdog permanently disabled\n");
+		ret = PTR_ERR(evt);
 	} else {
-		perf_event_release_kernel(this_cpu_read(watchdog_ev));
-		this_cpu_write(watchdog_ev, NULL);
+		perf_event_release_kernel(evt);
+		ret = 0;
 	}
+	cpu_hotplug_enable();
+
 	return ret;
 }
 
-- 
2.51.0

On Mon 2026-01-26 21:22:24, Qiliang Yuan wrote:
> The hardlockup detector's probe path (watchdog_hardlockup_probe()) can
> be executed in a non-pinned context, such as during the asynchronous
> retry mechanism (lockup_detector_delay_init) which runs in a standard
> unbound workqueue.

[...]

> Refactor hardlockup_detector_event_create() to be stateless by returning
> the created perf_event pointer instead of directly modifying the per-cpu
> 'watchdog_ev' variable. This allows the probe logic to safely manage
> the temporary event. Use cpu_hotplug_disable() during the probe to ensure
> the target CPU remains valid throughout the check.
> 
> Fixes: 930d8f8dbab9 ("watchdog/perf: adapt the watchdog_perf interface for async model")
> Signed-off-by: Shouxin Sun <sunshx@chinatelecom.cn>
> Signed-off-by: Junnan Zhang <zhangjn11@chinatelecom.cn>
> Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
> Signed-off-by: Qiliang Yuan <yuanql9@chinatelecom.cn>
> Cc: Song Liu <song@kernel.org>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Jinchao Wang <wangjinchao600@gmail.com>
> Cc: <stable@vger.kernel.org>

Please, do not remove people from Cc, especially when you send new
versions on such a rapid speed.

I was on Cc only for this version. There were no replies. I started
review just to realize that another 4 versions were sent within
a week and they got some proper review and v9 already ended in
linux-next...

Best Regards,
Petr