[PATCH] hwmon: (max6620) Add locking to avoid TOCTOU

[Gui-Dong Han]

The function max6620_read checks shared data (tach and target) for zero
before passing it to max6620_fan_tach_to_rpm, which uses 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.

This change also aligns the locking behavior with the hwmon_fan_alarm
case, which already uses the update lock.

Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=5z8B-g@xxxxxxxxxxxxxx/
Fixes: e8ac01e5db32 ("hwmon: Add Maxim MAX6620 hardware monitoring driver")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Gui-Dong Han <hanguidong02@xxxxxxxxx>
---
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/max6620.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/hwmon/max6620.c b/drivers/hwmon/max6620.c
index 13201fb755c9..0dce2f5cb61b 100644
--- a/drivers/hwmon/max6620.c
+++ b/drivers/hwmon/max6620.c
@@ -290,20 +290,24 @@ max6620_read(struct device *dev, enum hwmon_sensor_types type, u32 attr,
 			*val = max6620_fan_div_from_reg(data->fandyn[channel]);
 			break;
 		case hwmon_fan_input:
+			mutex_lock(&data->update_lock);
 			if (data->tach[channel] == 0) {
 				*val = 0;
 			} else {
 				div = max6620_fan_div_from_reg(data->fandyn[channel]);
 				*val = max6620_fan_tach_to_rpm(div, data->tach[channel]);
 			}
+			mutex_unlock(&data->update_lock);
 			break;
 		case hwmon_fan_target:
+			mutex_lock(&data->update_lock);
 			if (data->target[channel] == 0) {
 				*val = 0;
 			} else {
 				div = max6620_fan_div_from_reg(data->fandyn[channel]);
 				*val = max6620_fan_tach_to_rpm(div, data->target[channel]);
 			}
+			mutex_unlock(&data->update_lock);
 			break;
 		default:
 			return -EOPNOTSUPP;
-- 
2.43.0