[PATCH] thermal: gov_power_allocator: Fix NULL pointer dereference in update_tz()

[Sumeet Pawnikar <sumeet4linux@gmail.com>]

power_allocator_update_tz() unconditionally derives the trip descriptor of
params->trip_max as below,
  const struct thermal_trip_desc *td = trip_to_trip_desc(params->trip_max);
and then walks td->thermal_instances. However, params->trip_max is allowed
to be NULL and in that case the list walk dereferences a bogus pointer
derived from NULL which oops the kernel.

get_governor_trips() picks trip_switch_on and trip_max out of the trip
table of the zone. When the zone has neither a passive nor an active trip
point, last_active is NULL and params->trip_max is left NULL. This is an
explicitly supported configuration, as documented in the function
get_governor_trips() as below,
  If there are no passive or active trip points, then the governor won't
  do anything. In fact, its throttle function won't be called at all.

Return early from power_allocator_update_tz() when params->trip_max is NULL
and only compute the trip descriptor after that check. There is nothing to
update in that case anyway, with no trip_max and there are no thermal
instances for the governor to account for, num_actors stays zero and
total_weight is irrelevant.

Fixes: 912e97c67cc3 ("thermal: gov_power_allocator: Move memory allocation out of throttle()")

Signed-off-by: Sumeet Pawnikar <sumeet4linux@gmail.com>
---
 drivers/thermal/gov_power_allocator.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/thermal/gov_power_allocator.c b/drivers/thermal/gov_power_allocator.c
index 37f2e22a999e..b5c254187628 100644
--- a/drivers/thermal/gov_power_allocator.c
+++ b/drivers/thermal/gov_power_allocator.c
@@ -660,10 +660,15 @@ static void power_allocator_update_tz(struct thermal_zone_device *tz,
 				      enum thermal_notify_event reason)
 {
 	struct power_allocator_params *params = tz->governor_data;
-	const struct thermal_trip_desc *td = trip_to_trip_desc(params->trip_max);
+	const struct thermal_trip_desc *td;
 	struct thermal_instance *instance;
 	int num_actors = 0;
 
+	if (!params->trip_max)
+		return;
+
+	td = trip_to_trip_desc(params->trip_max);
+
 	switch (reason) {
 	case THERMAL_TZ_BIND_CDEV:
 	case THERMAL_TZ_UNBIND_CDEV:
-- 
2.43.0