[PATCH] hwmon: (hp-wmi-sensors) Fix use-after-free in fungible_show()

[Muhammad Bilal]

nsensor->current_state is dynamically replaced as the sensor's state
changes. update_numeric_sensor_from_wobj() does this by freeing the
old string and installing a new one:

	if (strcmp(trimmed, nsensor->current_state)) {
		new_string = hp_wmi_strdup(dev, trimmed);
		if (new_string) {
			devm_kfree(dev, nsensor->current_state);
			nsensor->current_state = new_string;
		}
	}

This function is only ever called from hp_wmi_update_info() while
state->lock is held, so the free-and-replace itself is properly
serialized against concurrent updates.

fungible_show(), however, reads the same pointer after the lock has
already been dropped:

	err = hp_wmi_update_info(state, info);
	if (err)
		return err;

	switch (prop) {
	...
	case HP_WMI_PROPERTY_CURRENT_STATE:
		seq_printf(seqf, "%s\n", nsensor->current_state);
		break;

hp_wmi_update_info() takes state->lock internally and releases it
before returning, so by the time fungible_show() dereferences
nsensor->current_state in seq_printf(), no lock is held. Two
processes reading a sensor's current_state debugfs entry at
overlapping times (or one reading it while another read of the same
sensor triggers a refresh) can race: one thread's seq_printf() can
be part-way through printing the string at the moment another
thread's call into update_numeric_sensor_from_wobj() frees it with
devm_kfree() and installs a new pointer, causing a use-after-free
read.

Take state->lock around the read in fungible_show() as well, so it
can never run concurrently with the free-and-replace in
update_numeric_sensor_from_wobj().

Fixes: 23902f98f8d4 ("hwmon: add HP WMI Sensors driver")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Muhammad Bilal <meatuni001@xxxxxxxxx>
---
 drivers/hwmon/hp-wmi-sensors.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/hwmon/hp-wmi-sensors.c b/drivers/hwmon/hp-wmi-sensors.c
index 03c684b..55aee16 100644
--- a/drivers/hwmon/hp-wmi-sensors.c
+++ b/drivers/hwmon/hp-wmi-sensors.c
@@ -1247,7 +1247,9 @@ static int fungible_show(struct seq_file *seqf, enum hp_wmi_property prop)
 		break;
 
 	case HP_WMI_PROPERTY_CURRENT_STATE:
+		mutex_lock(&state->lock);
 		seq_printf(seqf, "%s\n", nsensor->current_state);
+		mutex_unlock(&state->lock);
 		break;
 
 	case HP_WMI_PROPERTY_UNIT_MODIFIER:
-- 
2.43.0