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

[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@mail.gmail.com/
Fixes: e8ac01e5db32 ("hwmon: Add Maxim MAX6620 hardware monitoring driver")
Cc: stable@vger.kernel.org
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/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

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@mail.gmail.com/
> Fixes: e8ac01e5db32 ("hwmon: Add Maxim MAX6620 hardware monitoring driver")
> Cc: stable@vger.kernel.org
> 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.

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

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

[Gui-Dong Han]

On Sat, Nov 29, 2025 at 12:34 AM Guenter Roeck <linux@roeck-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@vger.kernel.org
> > 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.
>
> 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

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

[Guenter Roeck]

On 11/28/25 09:59, Gui-Dong Han wrote:
> On Sat, Nov 29, 2025 at 12:34 AM Guenter Roeck <linux@roeck-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@vger.kernel.org
>>> 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.
>>
>> 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

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

[david laight]

On Sat, 29 Nov 2025 01:59:51 +0800
Gui-Dong Han <hanguidong02@gmail.com> wrote:

...
> 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.

The real problem with #defines evaluating their parameters more than
once is just side effects of the expansion.
Even if the current users just pass a simple variable, you never
really know what is going to happen in the future.

There is also a secondary issue of pre-processor output 'bloat'.
This happens when large #define expansions get nested.
With the current headers FIELD_PREP(GENMASK(8, 5), val) expands to
about 18kB [1] (even though it is just (val >> 5) & 15).
I think one of your #defines get passed one of those - and then expands
it several times. As well as the massive line, the compiler may well
generate the code multiple times.
(CSE will normally stop foo->bar[x] being executed multiple times).

[1] Nothing like the 30MB that triple nested min() generated for a while.

	David