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=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/adm1029.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/hwmon/adm1029.c b/drivers/hwmon/adm1029.c
index 761c13092488..71eea8ae51b9 100644
--- a/drivers/hwmon/adm1029.c
+++ b/drivers/hwmon/adm1029.c
@@ -171,14 +171,17 @@ fan_show(struct device *dev, struct device_attribute *devattr, char *buf)
 	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);
 }
 
-- 
2.43.0

On Wed, Nov 26, 2025 at 07:40:46PM +0800, Gui-Dong Han wrote:
> 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=5z8B-g@mail.gmail.com/
> Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>

Applied.

Thanks,
Guenter