hwmon: (adm1029) Add locking to avoid TOCTOU

The function fan_show checks shared data for zero or invalid values
before using it as a divisor. These accesses are currently lockless. If
the data changes to zero between the check and the division, it causes a
divide-by-zero error.

Explicitly acquire the update lock around these checks and calculations
to ensure the data remains stable, preventing Time-of-Check to
Time-of-Use (TOCTOU) race conditions.

Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=[e-mail]/
Signed-off-by: Gui-Dong Han <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>

drivers/hwmon/adm1029.c

···
 	struct adm1029_data *data = adm1029_update_device(dev);
 	u16 val;
 
+	mutex_lock(&data->update_lock);
 	if (data->fan[attr->index] == 0 ||
 	    (data->fan_div[attr->index] & 0xC0) == 0 ||
 	    data->fan[attr->index] == 255) {
+		mutex_unlock(&data->update_lock);
 		return sprintf(buf, "0\n");
 	}
 
 	val = 1880 * 120 / DIV_FROM_REG(data->fan_div[attr->index])
 	    / data->fan[attr->index];
+	mutex_unlock(&data->update_lock);
 	return sprintf(buf, "%d\n", val);
 }