Commit 9bc9ccbf authored Jan 16, 2026 by Breno Leitao Committed by Andrew Morton Jan 20, 2026

mm/kfence: fix potential deadlock in reboot notifier

The reboot notifier callback can deadlock when calling
cancel_delayed_work_sync() if toggle_allocation_gate() is blocked in
wait_event_idle() waiting for allocations, that might not happen on
shutdown path.

The issue is that cancel_delayed_work_sync() waits for the work to
complete, but the work is waiting for kfence_allocation_gate > 0 which
requires allocations to happen (each allocation is increased by 1) -
allocations that may have stopped during shutdown.

Fix this by:
1. Using cancel_delayed_work() (non-sync) to avoid blocking. Now the
   callback succeeds and return.
2. Adding wake_up() to unblock any waiting toggle_allocation_gate()
3. Adding !kfence_enabled to the wait condition so the wake succeeds

The static_branch_disable() IPI will still execute after the wake, but at
this early point in shutdown (reboot notifier runs with INT_MAX priority),
the system is still functional and CPUs can respond to IPIs.

Link: https://lkml.kernel.org/r/20260116-kfence_fix-v1-1-4165a055933f@debian.org


Fixes: ce2bba89 ("mm/kfence: add reboot notifier to disable KFENCE on shutdown")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reported-by: Chris Mason <clm@meta.com>
Closes: https://lore.kernel.org/all/20260113140234.677117-1-clm@meta.com/


Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Breno Leitao <leitao@debian.org>
Cc: Chris Mason <clm@meta.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent cb7d761b

mm/kfence/core.c

+12 −5

Original line number	Diff line number	Diff line
		@@ -823,6 +823,9 @@ static struct notifier_block kfence_check_canary_notifier = {
		static struct delayed_work kfence_timer;

		#ifdef CONFIG_KFENCE_STATIC_KEYS
		/* Wait queue to wake up allocation-gate timer task. */
		static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);

		static int kfence_reboot_callback(struct notifier_block *nb,
		unsigned long action, void *data)
		{
		@@ -832,7 +835,12 @@ static int kfence_reboot_callback(struct notifier_block *nb,
		*/
		WRITE_ONCE(kfence_enabled, false);
		/* Cancel any pending timer work */
		cancel_delayed_work_sync(&kfence_timer);
		cancel_delayed_work(&kfence_timer);
		/*
		* Wake up any blocked toggle_allocation_gate() so it can complete
		* early while the system is still able to handle IPIs.
		*/
		wake_up(&allocation_wait);

		return NOTIFY_OK;
		}
		@@ -842,9 +850,6 @@ static struct notifier_block kfence_reboot_notifier = {
		.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);

		static void wake_up_kfence_timer(struct irq_work *work)
		{
		wake_up(&allocation_wait);
		@@ -873,7 +878,9 @@ static void toggle_allocation_gate(struct work_struct *work)
		/* Enable static key, and await allocation to happen. */
		static_branch_enable(&kfence_allocation_key);

		wait_event_idle(allocation_wait, atomic_read(&kfence_allocation_gate) > 0);
		wait_event_idle(allocation_wait,
		atomic_read(&kfence_allocation_gate) > 0 \|\|
		!READ_ONCE(kfence_enabled));

		/* Disable static key and reset timer. */
		static_branch_disable(&kfence_allocation_key);