[1/2] gpio: fix NULL-deref-on-deregistration regression

Fix a NULL-pointer deference when deregistering the gpio character
device that was introduced by the recent stub-driver hack. When the new
"driver" is unbound as part of deregistration, driver core clears the
driver-data pointer which is used to retrieve the struct gpio_device in
its release callback.

Fix this by using container_of() in the release callback as should have
been done all along.

Fixes: 4731210c09f5 ("gpiolib: Bind gpio_device to a driver to enable fw_devlink=on by default")
Cc: Saravana Kannan <saravanak@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: syzbot+d27b4c8adbbff70fbfde@syzkaller.appspotmail.com
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/gpio/gpiolib.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

On Fri, Feb 26, 2021 at 6:55 AM Johan Hovold <johan@kernel.org> wrote:
>
> Fix a NULL-pointer deference when deregistering the gpio character
> device that was introduced by the recent stub-driver hack. When the new
> "driver" is unbound as part of deregistration, driver core clears the
> driver-data pointer which is used to retrieve the struct gpio_device in
> its release callback.
>
> Fix this by using container_of() in the release callback as should have
> been done all along.
>
> Fixes: 4731210c09f5 ("gpiolib: Bind gpio_device to a driver to enable fw_devlink=on by default")
> Cc: Saravana Kannan <saravanak@google.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reported-by: syzbot+d27b4c8adbbff70fbfde@syzkaller.appspotmail.com
> Signed-off-by: Johan Hovold <johan@kernel.org>
> ---
>  drivers/gpio/gpiolib.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index adf55db080d8..e1016bc8cf14 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -474,7 +474,7 @@ EXPORT_SYMBOL_GPL(gpiochip_line_is_valid);
>
>  static void gpiodevice_release(struct device *dev)
>  {
> -       struct gpio_device *gdev = dev_get_drvdata(dev);
> +       struct gpio_device *gdev = container_of(dev, struct gpio_device, dev);

Can you also delete the dev_set_drvdata() in
gpiochip_add_data_with_key() if the drvdata is not used
elsewhere anymore? I skimmed the code and it doesn't look like it, but
I could be wrong.

-Saravana

On Fri, Feb 26, 2021 at 01:54:12PM -0800, Saravana Kannan wrote:
> On Fri, Feb 26, 2021 at 6:55 AM Johan Hovold <johan@kernel.org> wrote:
> >
> > Fix a NULL-pointer deference when deregistering the gpio character
> > device that was introduced by the recent stub-driver hack. When the new
> > "driver" is unbound as part of deregistration, driver core clears the
> > driver-data pointer which is used to retrieve the struct gpio_device in
> > its release callback.
> >
> > Fix this by using container_of() in the release callback as should have
> > been done all along.
> >
> > Fixes: 4731210c09f5 ("gpiolib: Bind gpio_device to a driver to enable fw_devlink=on by default")
> > Cc: Saravana Kannan <saravanak@google.com>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Reported-by: syzbot+d27b4c8adbbff70fbfde@syzkaller.appspotmail.com
> > Signed-off-by: Johan Hovold <johan@kernel.org>
> > ---
> >  drivers/gpio/gpiolib.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> > index adf55db080d8..e1016bc8cf14 100644
> > --- a/drivers/gpio/gpiolib.c
> > +++ b/drivers/gpio/gpiolib.c
> > @@ -474,7 +474,7 @@ EXPORT_SYMBOL_GPL(gpiochip_line_is_valid);
> >
> >  static void gpiodevice_release(struct device *dev)
> >  {
> > -       struct gpio_device *gdev = dev_get_drvdata(dev);
> > +       struct gpio_device *gdev = container_of(dev, struct gpio_device, dev);
> 
> Can you also delete the dev_set_drvdata() in
> gpiochip_add_data_with_key() if the drvdata is not used
> elsewhere anymore? I skimmed the code and it doesn't look like it, but
> I could be wrong.

Yeah, I considered it but didn't want to risk introducing any new
regressions just to clean up a redundant store.

But looking at it again today, I agree that it looks like it isn't used
anywhere else. I'll send a v2. Thanks.

Johan