hwmon: (emc2103) Add locking to avoid TOCTOU

The functions fan1_input_show and fan1_target_show check shared data for
zero 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/emc2103.c

···
 {
 	struct emc2103_data *data = emc2103_update_device(dev);
 	int rpm = 0;
+	mutex_lock(&data->update_lock);
 	if (data->fan_tach != 0)
 		rpm = (FAN_RPM_FACTOR * data->fan_multiplier) / data->fan_tach;
+	mutex_unlock(&data->update_lock);
 	return sprintf(buf, "%d\n", rpm);
 }
 
···
 	struct emc2103_data *data = emc2103_update_device(dev);
 	int rpm = 0;
 
+	mutex_lock(&data->update_lock);
 	/* high byte of 0xff indicates disabled so return 0 */
 	if ((data->fan_target != 0) && ((data->fan_target & 0x1fe0) != 0x1fe0))
 		rpm = (FAN_RPM_FACTOR * data->fan_multiplier)
 			/ data->fan_target;
+	mutex_unlock(&data->update_lock);
 
 	return sprintf(buf, "%d\n", rpm);
 }