linux-kernel - Re: [PATCH 6/7] thermal: max77620: fix device-node reference imbalance

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Date:   Wed, 31 May 2017 10:28:27 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>
Cc:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Peter Chen <peter.chen@...escale.com>,
        Rob Herring <robh@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Sricharan R <sricharan@...eaurora.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Eduardo Valentin <edubezval@...il.com>,
        linux-pm@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable <stable@...r.kernel.org>,
        Laxman Dewangan <ldewangan@...dia.com>
Subject: Re: [PATCH 6/7] thermal: max77620: fix device-node reference
 imbalance

On Tue, May 30, 2017 at 03:59:10PM -0700, Tyrel Datwyler wrote:
> On 05/30/2017 09:25 AM, Johan Hovold wrote:
> > The thermal child device reuses the parent MFD-device device-tree node
> > when registering a thermal zone, but did not take a reference to the
> > node.
> > 
> > This leads to a reference imbalance, and potential use-after-free, when
> > the node reference is dropped by the platform-bus device destructor
> > (once for the child and later again for the parent).
> > 
> > Fix this by dropping any reference already held to a device-tree node
> > and getting a reference to the parent's node which will be balanced on
> > reprobe or on platform-device release, whichever comes first.
> > 
> > Note that simply clearing the of_node pointer on probe errors and on
> > driver unbind would not allow the use of device-managed resources as
> > specifically thermal_zone_of_sensor_unregister() claims that a valid
> > device-tree node pointer is needed during deregistration (even if it
> > currently does not seem to use it).
> > 
> > Fixes: ec4664b3fd6d ("thermal: max77620: Add thermal driver for reporting junction temp")
> > Cc: stable <stable@...r.kernel.org>     # 4.9
> > Cc: Laxman Dewangan <ldewangan@...dia.com>
> > Signed-off-by: Johan Hovold <johan@...nel.org>
> > ---
> >  drivers/thermal/max77620_thermal.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/thermal/max77620_thermal.c b/drivers/thermal/max77620_thermal.c
> > index e9a1fe342760..71d35f3c9215 100644
> > --- a/drivers/thermal/max77620_thermal.c
> > +++ b/drivers/thermal/max77620_thermal.c
> > @@ -104,8 +104,6 @@ static int max77620_thermal_probe(struct platform_device *pdev)
> >  		return -EINVAL;
> >  	}
> >  
> > -	pdev->dev.of_node = pdev->dev.parent->of_node;
> > -
> >  	mtherm->dev = &pdev->dev;
> >  	mtherm->rmap = dev_get_regmap(pdev->dev.parent, NULL);
> >  	if (!mtherm->rmap) {
> > @@ -113,6 +111,14 @@ static int max77620_thermal_probe(struct platform_device *pdev)
> >  		return -ENODEV;
> >  	}
> >  
> > +	/*
> > +	 * Drop any current reference to a device-tree node and get a
> > +	 * reference to the parent's node which will be balanced on reprobe or
> > +	 * on platform-device release.
> > +	 */
> > +	of_node_put(pdev->dev.of_node);
> > +	pdev->dev.of_node = of_node_get(pdev->dev.parent->of_node);
> > +
> 
> This seems like needless churn. Can't this just be squashed into patch
> #7?

As I just replied to Eduardo, I'm fixing two separate issues here of
which this one might qualify for stable while the other (7/7) does not.

Thanks,
Johan