[PATCH v2] drm/panthor: Fix UAF race between device unplug and FW event processing

[Ketil Johnsen <ketil.johnsen@arm.com>]

The function panthor_fw_unplug() will free the FW memory sections.
The problem is that there could still be pending FW events which are yet
not handled at this point. process_fw_events_work() can in this case try
to access said freed memory.

This fix introduces a destroyed state for the panthor_scheduler object,
and we check for this before processing FW events.

Signed-off-by: Ketil Johnsen <ketil.johnsen@arm.com>
Fixes: de85488138247 ("drm/panthor: Add the scheduler logical block")
---
v2:
- Followed Boris's advice and handle the race purely within the
  scheduler block (by adding a destroyed state)
---
 drivers/gpu/drm/panthor/panthor_sched.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
index 0cc9055f4ee52..4996f987b8183 100644
--- a/drivers/gpu/drm/panthor/panthor_sched.c
+++ b/drivers/gpu/drm/panthor/panthor_sched.c
@@ -315,6 +315,13 @@ struct panthor_scheduler {
 		 */
 		struct list_head stopped_groups;
 	} reset;
+
+	/**
+	 * @destroyed: Scheduler object is (being) destroyed
+	 *
+	 * Normal scheduler operations should no longer take place.
+	 */
+	bool destroyed;
 };
 
 /**
@@ -1765,7 +1772,10 @@ static void process_fw_events_work(struct work_struct *work)
 	u32 events = atomic_xchg(&sched->fw_events, 0);
 	struct panthor_device *ptdev = sched->ptdev;
 
-	mutex_lock(&sched->lock);
+	guard(mutex)(&sched->lock);
+
+	if (sched->destroyed)
+		return;
 
 	if (events & JOB_INT_GLOBAL_IF) {
 		sched_process_global_irq_locked(ptdev);
@@ -1778,8 +1788,6 @@ static void process_fw_events_work(struct work_struct *work)
 		sched_process_csg_irq_locked(ptdev, csg_id);
 		events &= ~BIT(csg_id);
 	}
-
-	mutex_unlock(&sched->lock);
 }
 
 /**
@@ -3882,6 +3890,7 @@ void panthor_sched_unplug(struct panthor_device *ptdev)
 	cancel_delayed_work_sync(&sched->tick_work);
 
 	mutex_lock(&sched->lock);
+	sched->destroyed = true;
 	if (sched->pm.has_ref) {
 		pm_runtime_put(ptdev->base.dev);
 		sched->pm.has_ref = false;
-- 
2.47.2