thermal/debugfs: Fix two locking issues with thermal zone debug

With the current thermal zone locking arrangement in the debugfs code,
user space can open the "mitigations" file for a thermal zone before
the zone's debugfs pointer is set which will result in a NULL pointer
dereference in tze_seq_start().

Moreover, thermal_debug_tz_remove() is not called under the thermal
zone lock, so it can run in parallel with the other functions accessing
the thermal zone's struct thermal_debugfs object.  Then, it may clear
tz->debugfs after one of those functions has checked it and the
struct thermal_debugfs object may be freed prematurely.

To address the first problem, pass a pointer to the thermal zone's
struct thermal_debugfs object to debugfs_create_file() in
thermal_debug_tz_add() and make tze_seq_start(), tze_seq_next(),
tze_seq_stop(), and tze_seq_show() retrieve it from s->private
instead of a pointer to the thermal zone object.  This will ensure
that tz_debugfs will be valid across the "mitigations" file accesses
until thermal_debugfs_remove_id() called by thermal_debug_tz_remove()
removes that file.

To address the second problem, use tz->lock in thermal_debug_tz_remove()
around the tz->debugfs value check (in case the same thermal zone is
removed at the same time in two different threads) and its reset to NULL.

Fixes: 7ef01f22 ("thermal/debugfs: Add thermal debugfs information for mitigation episodes")
Cc :6.8+ <stable@vger.kernel.org> # 6.8+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>

drivers/thermal/thermal_debugfs.c

@@ -139,11 +139,13 @@ struct tz_episode {
  * we keep track of the current position in the history array.
  *
  * @tz_episodes: a list of thermal mitigation episodes
+ * @tz: thermal zone this object belongs to
  * @trips_crossed: an array of trip points crossed by id
  * @nr_trips: the number of trip points currently being crossed
  */
 struct tz_debugfs {
 	struct list_head tz_episodes;
+	struct thermal_zone_device *tz;
 	int *trips_crossed;
 	int nr_trips;
 };
@@ -716,8 +718,7 @@ void thermal_debug_update_temp(struct thermal_zone_device *tz)
 
 static void *tze_seq_start(struct seq_file *s, loff_t *pos)
 {
-	struct thermal_zone_device *tz = s->private;
-	struct thermal_debugfs *thermal_dbg = tz->debugfs;
+	struct thermal_debugfs *thermal_dbg = s->private;
 	struct tz_debugfs *tz_dbg = &thermal_dbg->tz_dbg;
 
 	mutex_lock(&thermal_dbg->lock);
@@ -727,8 +728,7 @@ static void *tze_seq_start(struct seq_file *s, loff_t *pos)
 
 static void *tze_seq_next(struct seq_file *s, void *v, loff_t *pos)
 {
-	struct thermal_zone_device *tz = s->private;
-	struct thermal_debugfs *thermal_dbg = tz->debugfs;
+	struct thermal_debugfs *thermal_dbg = s->private;
 	struct tz_debugfs *tz_dbg = &thermal_dbg->tz_dbg;
 
 	return seq_list_next(v, &tz_dbg->tz_episodes, pos);
@@ -736,15 +736,15 @@ static void *tze_seq_next(struct seq_file *s, void *v, loff_t *pos)
 
 static void tze_seq_stop(struct seq_file *s, void *v)
 {
-	struct thermal_zone_device *tz = s->private;
-	struct thermal_debugfs *thermal_dbg = tz->debugfs;
+	struct thermal_debugfs *thermal_dbg = s->private;
 
 	mutex_unlock(&thermal_dbg->lock);
 }
 
 static int tze_seq_show(struct seq_file *s, void *v)
 {
-	struct thermal_zone_device *tz = s->private;
+	struct thermal_debugfs *thermal_dbg = s->private;
+	struct thermal_zone_device *tz = thermal_dbg->tz_dbg.tz;
 	struct thermal_trip *trip;
 	struct tz_episode *tze;
 	const char *type;
@@ -810,6 +810,8 @@ void thermal_debug_tz_add(struct thermal_zone_device *tz)
 
 	tz_dbg = &thermal_dbg->tz_dbg;
 
+	tz_dbg->tz = tz;
+
 	tz_dbg->trips_crossed = kzalloc(sizeof(int) * tz->num_trips, GFP_KERNEL);
 	if (!tz_dbg->trips_crossed) {
 		thermal_debugfs_remove_id(thermal_dbg);
@@ -818,20 +820,30 @@ void thermal_debug_tz_add(struct thermal_zone_device *tz)
 
 	INIT_LIST_HEAD(&tz_dbg->tz_episodes);
 
-	debugfs_create_file("mitigations", 0400, thermal_dbg->d_top, tz, &tze_fops);
+	debugfs_create_file("mitigations", 0400, thermal_dbg->d_top,
+			    thermal_dbg, &tze_fops);
 
 	tz->debugfs = thermal_dbg;
 }
 
 void thermal_debug_tz_remove(struct thermal_zone_device *tz)
 {
-	struct thermal_debugfs *thermal_dbg = tz->debugfs;
+	struct thermal_debugfs *thermal_dbg;
 	struct tz_episode *tze, *tmp;
 	struct tz_debugfs *tz_dbg;
 	int *trips_crossed;
 
-	if (!thermal_dbg)
+	mutex_lock(&tz->lock);
+
+	thermal_dbg = tz->debugfs;
+	if (!thermal_dbg) {
+		mutex_unlock(&tz->lock);
 		return;
+	}
+
+	tz->debugfs = NULL;
+
+	mutex_unlock(&tz->lock);
 
 	tz_dbg = &thermal_dbg->tz_dbg;
 
@@ -844,8 +856,6 @@ void thermal_debug_tz_remove(struct thermal_zone_device *tz)
 		kfree(tze);
 	}
 
-	tz->debugfs = NULL;
-
 	mutex_unlock(&thermal_dbg->lock);
 
 	thermal_debugfs_remove_id(thermal_dbg);