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

[Guenter Roeck]

On Fri, Nov 28, 2025 at 08:43:51PM +0800, Gui-Dong Han wrote:
> 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.

This patch is not necessary. The driver registers with the hwmon subsystem
using devm_hwmon_device_register_with_info(). That means the hwmon subsystem
handles the necessary locking. On top of that, removing the existing driver
internal locking code is queued for v6.19.

Thanks,
Guenter