[PATCH] hwmon: (emc2103) Add locking to avoid TOCTOU

[PATCH] hwmon: (emc2103) Add locking to avoid TOCTOU

[Gui-Dong Han]

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=5z8B-g@mail.gmail.com/
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
---
Based on the discussion in the link, I will submit a series of patches to
address TOCTOU issues in the hwmon subsystem by converting macros to
functions or adjusting locking where appropriate.
---
 drivers/hwmon/emc2103.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/hwmon/emc2103.c b/drivers/hwmon/emc2103.c
index 60eddc7b0270..9b8e925af030 100644
--- a/drivers/hwmon/emc2103.c
+++ b/drivers/hwmon/emc2103.c
@@ -277,8 +277,10 @@ fan1_input_show(struct device *dev, struct device_attribute *da, char *buf)
 {
 	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);
 }
 
@@ -363,10 +365,12 @@ fan1_target_show(struct device *dev, struct device_attribute *da, char *buf)
 	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);
 }
-- 
2.34.1

Re: [PATCH] hwmon: (emc2103) Add locking to avoid TOCTOU

[Guenter Roeck]

On Tue, Nov 25, 2025 at 12:55:08AM +0800, Gui-Dong Han wrote:
> 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=5z8B-g@mail.gmail.com/
> Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
> ---

Applied.

Thanks,
Guenter