mm/kfence: add reboot notifier to disable KFENCE on shutdown

During system shutdown, KFENCE can cause IPI synchronization issues if it
remains active through the reboot process.  To prevent this, register a
reboot notifier that disables KFENCE and cancels any pending timer work
early in the shutdown sequence.

This is only necessary when CONFIG_KFENCE_STATIC_KEYS is enabled, as this
configuration sends IPIs that can interfere with shutdown.  Without static
keys, no IPIs are generated and KFENCE can safely remain active.

The notifier uses maximum priority (INT_MAX) to ensure KFENCE shuts down
before other subsystems that might still depend on stable memory
allocation behavior.

This fixes a late kexec CSD lockup[1] when kfence is trying to IPI a CPU
that is busy in a IRQ-disabled context printing characters to the console.

Link: https://lkml.kernel.org/r/20251127-kfence-v2-1-daeccb5ef9aa@debian.org
Link: https://lkml.kernel.org/r/20251126-kfence-v1-1-5a6e1d7c681c@debian.org
Link: https://lore.kernel.org/all/sqwajvt7utnt463tzxgwu2yctyn5m6bjwrslsnupfexeml6hkd@v6sqmpbu3vvu/ [1]
Fixes: 0ce20dd ("mm: add Kernel Electric-Fence infrastructure")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: leitao

mm/kfence/core.c

@@ -26,6 +26,7 @@
 #include <linux/panic_notifier.h>
 #include <linux/random.h>
 #include <linux/rcupdate.h>
+#include <linux/reboot.h>
 #include <linux/sched/clock.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
@@ -820,6 +821,25 @@ static struct notifier_block kfence_check_canary_notifier = {
 static struct delayed_work kfence_timer;
 
 #ifdef CONFIG_KFENCE_STATIC_KEYS
+static int kfence_reboot_callback(struct notifier_block *nb,
+				  unsigned long action, void *data)
+{
+	/*
+	 * Disable kfence to avoid static keys IPI synchronization during
+	 * late shutdown/kexec
+	 */
+	WRITE_ONCE(kfence_enabled, false);
+	/* Cancel any pending timer work */
+	cancel_delayed_work_sync(&kfence_timer);
+
+	return NOTIFY_OK;
+}
+
+static struct notifier_block kfence_reboot_notifier = {
+	.notifier_call = kfence_reboot_callback,
+	.priority = INT_MAX, /* Run early to stop timers ASAP */
+};
+
 /* Wait queue to wake up allocation-gate timer task. */
 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
 
@@ -901,6 +921,10 @@ static void kfence_init_enable(void)
 	if (kfence_check_on_panic)
 		atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
 
+#ifdef CONFIG_KFENCE_STATIC_KEYS
+	register_reboot_notifier(&kfence_reboot_notifier);
+#endif
+
 	WRITE_ONCE(kfence_enabled, true);
 	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);