linux-kernel - Re: [PATCH -next] thermal/drivers/thermal_hwmon: Fix a kernel NULL pointer dereference

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Date:   Wed, 29 Mar 2023 16:38:55 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>, linux-pm@...r.kernel.org,
        rafael.j.wysocki@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] thermal/drivers/thermal_hwmon: Fix a kernel NULL
 pointer dereference

On Wed, Mar 29, 2023 at 4:16 PM Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
>
> On 29/03/2023 14:06, Rafael J. Wysocki wrote:
> > On Wed, Mar 29, 2023 at 11:57 AM Daniel Lezcano
> > <daniel.lezcano@...aro.org> wrote:
> >>
> >> On 29/03/2023 11:00, Zhang Rui wrote:
> >>> When the hwmon device node of a thermal zone device is not found,
> >>> using hwmon->device causes a kernel NULL pointer dereference.
> >>>
> >>> Reported-by: Preble Adam C <adam.c.preble@...el.com>
> >>> Signed-off-by: Zhang Rui <rui.zhang@...el.com>
> >>> ---
> >>> Fixes: dec07d399cc8 ("thermal: Don't use 'device' internal thermal zone structure field")
> >>> dec07d399cc8 is a commit in the linux-next branch of linux-pm repo.
> >>> I'm not sure if the Fix tag applies to such commit or not.
> >>
> >> Actually it reverts the work done to encapsulate the thermal zone device
> >> structure.
> >
> > So maybe instead of the wholesale switch to using "driver-specific"
> > device pointers for printing messages, something like
> > thermal_zone_debug/info/warn/error() taking a thermal zone pointer as
> > the first argument can be defined?
> >
> > At least this particular bug could be avoided this way.
>
> Actually we previously said the thermal_hwmon can be considered as part
> of the thermal core code, so we can keep using tz->device.
>
> I'll drop this change from the series.

But it's there in my thermal branch already.

Do you want to revert the thermal_hwmon.c part of commit dec07d399cc8?

> On the other side, adding more thermal_zone_debug/info.. gives
> opportunities to external components of the core thermal framework to
> write thermal zone device related message. I'm not sure that is a good
> thing, each writer should stay in its namespace, no ?

IMV whoever is allowed to use a thermal zone pointer should also be
allowed to print messages related to its use, especially debug ones.

"Encapsulation" means that the members of a thermal zone device object
should not be accessed directly by its users other than the core, not
that it cannot be used as a message tag.