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

[Gui-Dong Han]

On Sat, Nov 29, 2025 at 12:34 AM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> 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.

Hi Guenter,

Thanks for the information. I missed the new hwmon subsystem locking
implementation earlier as it wasn't present in v6.17.9. I have since
studied the code in v6.18-rc, and it looks like an excellent
improvement. I will focus exclusively on drivers not using
devm_hwmon_device_register_with_info() going forward.

In our previous discussion, you also suggested adding a note to
submitting-patches.rst about "avoiding calculations in macros" to
explicitly explain the risk of race conditions. Is this something you
would still like to see added? If so, I would be happy to prepare a
patch.

Best regards,
Gui-Dong Han