hwmon: (w83791d) Convert macros to functions to avoid TOCTOU

The macro FAN_FROM_REG evaluates its arguments multiple times. When used
in lockless contexts involving 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. This guarantees that arguments
are evaluated only once (pass-by-value), preventing the race
conditions.

Additionally, in store_fan_div, move the calculation of the minimum
limit inside the update lock. This ensures that the read-modify-write
sequence operates on consistent data.

Adhere to the principle of minimal changes by only converting macros
that evaluate arguments multiple times and are used in lockless
contexts.

Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=5z8B-g@mail.gmail.com/
Fixes: 9873964d6eb2 ("[PATCH] HWMON: w83791d: New hardware monitoring driver for the Winbond W83791D")
Cc: stable@vger.kernel.org
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Link: https://lore.kernel.org/r/20251202180105.12842-1-hanguidong02@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

diff --git a/drivers/hwmon/w83791d.c b/drivers/hwmon/w83791d.c
index ace854b370a0542c2df06b6970d2ac0541625a2c..996e36951f9dad4a1f23e3205fe396721f346a34 100644 (file)
--- a/drivers/hwmon/w83791d.c
+++ b/drivers/hwmon/w83791d.c
@@ -218,9 +218,14 @@ static u8 fan_to_reg(long rpm, int div)
        return clamp_val((1350000 + rpm * div / 2) / (rpm * div), 1, 254);
 }
 
-#define FAN_FROM_REG(val, div) ((val) == 0 ? -1 : \
-                               ((val) == 255 ? 0 : \
-                                       1350000 / ((val) * (div))))
+static int fan_from_reg(int val, int div)
+{
+       if (val == 0)
+               return -1;
+       if (val == 255)
+               return 0;
+       return 1350000 / (val * div);
+}
 
 /* for temp1 which is 8-bit resolution, LSB = 1 degree Celsius */
 #define TEMP1_FROM_REG(val)    ((val) * 1000)
@@ -521,7 +526,7 @@ static ssize_t show_##reg(struct device *dev, struct device_attribute *attr, \
        struct w83791d_data *data = w83791d_update_device(dev); \
        int nr = sensor_attr->index; \
        return sprintf(buf, "%d\n", \
-               FAN_FROM_REG(data->reg[nr], DIV_FROM_REG(data->fan_div[nr]))); \
+               fan_from_reg(data->reg[nr], DIV_FROM_REG(data->fan_div[nr]))); \
 }
 
 show_fan_reg(fan);
@@ -585,10 +590,10 @@ static ssize_t store_fan_div(struct device *dev, struct device_attribute *attr,
        if (err)
                return err;
 
+       mutex_lock(&data->update_lock);
        /* Save fan_min */
-       min = FAN_FROM_REG(data->fan_min[nr], DIV_FROM_REG(data->fan_div[nr]));
+       min = fan_from_reg(data->fan_min[nr], DIV_FROM_REG(data->fan_div[nr]));
 
-       mutex_lock(&data->update_lock);
        data->fan_div[nr] = div_to_reg(nr, val);
 
        switch (nr) {