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

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Message-ID: <d3b5379d-4900-492f-a5bc-49078468df06@roeck-us.net>
Date: Fri, 28 Nov 2025 10:09:47 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Gui-Dong Han <hanguidong02@...il.com>
Cc: linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH] hwmon: (max6620) Add locking to avoid TOCTOU

On 11/28/25 09:59, Gui-Dong Han wrote:
> On Sat, Nov 29, 2025 at 12:34 AM Guenter Roeck <linux@...ck-us.net> 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@mail.gmail.com/
>>> Fixes: e8ac01e5db32 ("hwmon: Add Maxim MAX6620 hardware monitoring driver")
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Gui-Dong Han <hanguidong02@...il.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.
>>
>> 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.
> 

Yes, that would be great. Thanks!

Guenter