[hwmon PATCH v2 7/8] hwmon: (adt7470) Fix divide-by-zero TOCTOU crash in fan speed read

If the fan data becomes 0 between the FAN_DATA_VALID() check and the
FAN_PERIOD_TO_RPM() conversion, it will result in a divide-by-zero crash
due to a race with a concurrent update of the cached fan value.

Fix a TOCTOU issue by reading fan data once.

Reported-by: [email protected]
Closes: https://lore.kernel.org/r/[email protected]/
Fixes: fc958a61ff6d ("hwmon: (adt7470) Convert to devm_hwmon_device_register_with_info API")
Signed-off-by: Luiz Angelo Daros de Luca <[email protected]>
---
 drivers/hwmon/adt7470.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/drivers/hwmon/adt7470.c b/drivers/hwmon/adt7470.c
index 1fbca4869b7b..772d2a409bb5 100644
--- a/drivers/hwmon/adt7470.c
+++ b/drivers/hwmon/adt7470.c
@@ -660,36 +660,33 @@ static ssize_t alarm_mask_store(struct device *dev,
 static int adt7470_fan_read(struct device *dev, u32 attr, int channel, long *val)
 {
 	struct adt7470_data *data = adt7470_update_device(dev);
+	u16 fan_data;
 
 	if (IS_ERR(data))
 		return PTR_ERR(data);
 
 	switch (attr) {
 	case hwmon_fan_input:
-		if (FAN_DATA_VALID(data->fan[channel]))
-			*val = FAN_PERIOD_TO_RPM(data->fan[channel]);
-		else
-			*val = 0;
+		fan_data = READ_ONCE(data->fan[channel]);
 		break;
 	case hwmon_fan_min:
-		if (FAN_DATA_VALID(data->fan_min[channel]))
-			*val = FAN_PERIOD_TO_RPM(data->fan_min[channel]);
-		else
-			*val = 0;
+		fan_data = READ_ONCE(data->fan_min[channel]);
 		break;
 	case hwmon_fan_max:
-		if (FAN_DATA_VALID(data->fan_max[channel]))
-			*val = FAN_PERIOD_TO_RPM(data->fan_max[channel]);
-		else
-			*val = 0;
+		fan_data = READ_ONCE(data->fan_max[channel]);
 		break;
 	case hwmon_fan_alarm:
 		*val = !!(data->alarm & FAN_ALARM_BIT(channel));
-		break;
+		return 0;
 	default:
 		return -EOPNOTSUPP;
 	}
 
+	if (FAN_DATA_VALID(fan_data))
+		*val = FAN_PERIOD_TO_RPM(fan_data);
+	else
+		*val = 0;
+
 	return 0;
 }
 

-- 
2.55.0