hwmon: (vt8231) Convert macros to functions to avoid TOCTOU

The macro FAN_FROM_REG evaluates its arguments multiple times. When used
with shared driver data, this leads to Time-of-Check to Time-of-Use
(TOCTOU) race conditions, potentially causing divide-by-zero errors.

Convert the macro to a static function to ensure arguments are evaluated
only once.

Additionally, in fan_div_store, move the reading of the old register
value and the calculation of the minimum limit inside the update lock.
This ensures that the read-modify-write sequence operates on consistent
data, preventing race conditions during fan divider updates.

Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=5z8B-g@mail.gmail.com/
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Link: https://lore.kernel.org/r/20251124165900.4713-1-hanguidong02@gmail.com
[groeck: Dropped unnecessary line split]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

diff --git a/drivers/hwmon/vt8231.c b/drivers/hwmon/vt8231.c
index 3bf27c21845ba505333fc3ecafbef842fc62587b..5757a0979f3f3a12df47a17256fbc54beabf8193 100644 (file)
--- a/drivers/hwmon/vt8231.c
+++ b/drivers/hwmon/vt8231.c
@@ -138,7 +138,12 @@ static inline u8 FAN_TO_REG(long rpm, int div)
        return clamp_val(1310720 / (rpm * div), 1, 255);
 }
 
-#define FAN_FROM_REG(val, div) ((val) == 0 ? 0 : 1310720 / ((val) * (div)))
+static int fan_from_reg(int val, int div)
+{
+       if (val == 0)
+               return 0;
+       return 1310720 / (val * div);
+}
 
 struct vt8231_data {
        unsigned short addr;
@@ -561,7 +566,7 @@ static ssize_t fan_show(struct device *dev, struct device_attribute *attr,
        struct sensor_device_attribute *sensor_attr = to_sensor_dev_attr(attr);
        int nr = sensor_attr->index;
        struct vt8231_data *data = vt8231_update_device(dev);
-       return sprintf(buf, "%d\n", FAN_FROM_REG(data->fan[nr],
+       return sprintf(buf, "%d\n", fan_from_reg(data->fan[nr],
                                DIV_FROM_REG(data->fan_div[nr])));
 }
 
@@ -571,7 +576,7 @@ static ssize_t fan_min_show(struct device *dev, struct device_attribute *attr,
        struct sensor_device_attribute *sensor_attr = to_sensor_dev_attr(attr);
        int nr = sensor_attr->index;
        struct vt8231_data *data = vt8231_update_device(dev);
-       return sprintf(buf, "%d\n", FAN_FROM_REG(data->fan_min[nr],
+       return sprintf(buf, "%d\n", fan_from_reg(data->fan_min[nr],
                        DIV_FROM_REG(data->fan_div[nr])));
 }
 
@@ -613,9 +618,8 @@ static ssize_t fan_div_store(struct device *dev,
        struct sensor_device_attribute *sensor_attr = to_sensor_dev_attr(attr);
        unsigned long val;
        int nr = sensor_attr->index;
-       int old = vt8231_read_value(data, VT8231_REG_FANDIV);
-       long min = FAN_FROM_REG(data->fan_min[nr],
-                                DIV_FROM_REG(data->fan_div[nr]));
+       int old;
+       long min;
        int err;
 
        err = kstrtoul(buf, 10, &val);
@@ -623,6 +627,8 @@ static ssize_t fan_div_store(struct device *dev,
                return err;
 
        mutex_lock(&data->update_lock);
+       old = vt8231_read_value(data, VT8231_REG_FANDIV);
+       min = fan_from_reg(data->fan_min[nr], DIV_FROM_REG(data->fan_div[nr]));
        switch (val) {
        case 1:
                data->fan_div[nr] = 0;