[PATCH] hwmon: (max16065) Use READ/WRITE_ONCE to avoid compiler optimization induced race

[Gui-Dong Han]

Simply copying shared data to a local variable cannot prevent data
races. The compiler is allowed to optimize away the local copy and
re-read the shared memory, causing a Time-of-Check Time-of-Use (TOCTOU)
issue if the data changes between the check and the usage.

To enforce the use of the local variable, use READ_ONCE() when reading
the shared data and WRITE_ONCE() when updating it. Apply these macros to
the three identified locations (curr_sense, adc, and fault) where local
variables are used for error validation, ensuring the value remains
consistent.

Reported-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/all/6fe17868327207e8b850cf9f88b7dc58b2021f73.camel@xxxxxxxxxxxxxxx/
Fixes: f5bae2642e3d ("hwmon: Driver for MAX16065 System Manager and compatibles")
Fixes: b8d5acdcf525 ("hwmon: (max16065) Use local variable to avoid TOCTOU")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gui-Dong Han <hanguidong02@xxxxxxxxx>
---
 drivers/hwmon/max16065.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/drivers/hwmon/max16065.c b/drivers/hwmon/max16065.c
index 4c9e7892a73c..43fbb9b26b10 100644
--- a/drivers/hwmon/max16065.c
+++ b/drivers/hwmon/max16065.c
@@ -151,27 +151,27 @@ static struct max16065_data *max16065_update_device(struct device *dev)
 		int i;
 
 		for (i = 0; i < data->num_adc; i++)
-			data->adc[i]
-			  = max16065_read_adc(client, MAX16065_ADC(i));
+			WRITE_ONCE(data->adc[i],
+				   max16065_read_adc(client, MAX16065_ADC(i)));
 
 		if (data->have_current) {
-			data->adc[MAX16065_NUM_ADC]
-			  = max16065_read_adc(client, MAX16065_CSP_ADC);
-			data->curr_sense
-			  = i2c_smbus_read_byte_data(client,
-						     MAX16065_CURR_SENSE);
+			WRITE_ONCE(data->adc[MAX16065_NUM_ADC],
+				   max16065_read_adc(client, MAX16065_CSP_ADC));
+			WRITE_ONCE(data->curr_sense,
+				   i2c_smbus_read_byte_data(client, MAX16065_CURR_SENSE));
 		}
 
 		for (i = 0; i < 2; i++)
-			data->fault[i]
-			  = i2c_smbus_read_byte_data(client, MAX16065_FAULT(i));
+			WRITE_ONCE(data->fault[i],
+				   i2c_smbus_read_byte_data(client, MAX16065_FAULT(i)));
 
 		/*
 		 * MAX16067 and MAX16068 have separate undervoltage and
 		 * overvoltage alarm bits. Squash them together.
 		 */
 		if (data->chip == max16067 || data->chip == max16068)
-			data->fault[0] |= data->fault[1];
+			WRITE_ONCE(data->fault[0],
+				   data->fault[0] | data->fault[1]);
 
 		data->last_updated = jiffies;
 		data->valid = true;
@@ -185,7 +185,7 @@ static ssize_t max16065_alarm_show(struct device *dev,
 {
 	struct sensor_device_attribute_2 *attr2 = to_sensor_dev_attr_2(da);
 	struct max16065_data *data = max16065_update_device(dev);
-	int val = data->fault[attr2->nr];
+	int val = READ_ONCE(data->fault[attr2->nr]);
 
 	if (val < 0)
 		return val;
@@ -203,7 +203,7 @@ static ssize_t max16065_input_show(struct device *dev,
 {
 	struct sensor_device_attribute *attr = to_sensor_dev_attr(da);
 	struct max16065_data *data = max16065_update_device(dev);
-	int adc = data->adc[attr->index];
+	int adc = READ_ONCE(data->adc[attr->index]);
 
 	if (unlikely(adc < 0))
 		return adc;
@@ -216,7 +216,7 @@ static ssize_t max16065_current_show(struct device *dev,
 				     struct device_attribute *da, char *buf)
 {
 	struct max16065_data *data = max16065_update_device(dev);
-	int curr_sense = data->curr_sense;
+	int curr_sense = READ_ONCE(data->curr_sense);
 
 	if (unlikely(curr_sense < 0))
 		return curr_sense;
-- 
2.43.0