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Subject: Re: [PATCH v2] gpio: Restrict usage of gc irq members before
 initialization
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <brgl@bgdev.pl>, krisman@collabora.com,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Collabora Kernel ML <kernel@collabora.com>
References: <20220315103813.84407-1-shreeya.patel@collabora.com>
 <CAHp75VdoN+iSu1GLnxWW9BtL-p9uF1sfAw3ZxkFWNpoo44+bZg@mail.gmail.com>
From:   Shreeya Patel <shreeya.patel@collabora.com>
Message-ID: <b22cea07-aa2e-f5b5-d5aa-071e1084f00b@collabora.com>
Date:   Tue, 15 Mar 2022 18:02:05 +0530
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On 15/03/22 4:30 pm, Andy Shevchenko wrote:
> On Tue, Mar 15, 2022 at 12:38 PM Shreeya Patel
> <shreeya.patel@collabora.com> wrote:
>
> Thanks for the update, my comments below.
>
>> gc irq members are exposed before they could be completely
> gc --> GPIO chip
>
>
>> initialized and this leads to race conditions.
> Any example here. like ~3-4 lines of the Oops in question?
>
>> One such issue was observed for the gc->irq.domain variable which
>> was accessed through the I2C interface in gpiochip_to_irq() before
>> it could be initialized by gpiochip_add_irqchip(). This resulted in
>> Kernel NULL pointer dereference.
>>
>> To avoid such scenarios, restrict usage of gc irq members before
> gc --> GPIO chip
>
>> they are completely initialized.
> ...
>
>> +       /*
>> +        * Using barrier() here to prevent compiler from reordering
>> +        * gc->irq.gc_irq_initialized before initialization of above
>> +        * gc irq members.
>> +        */
>> +       barrier();
>> +
>> +       gc->irq.gc_irq_initialized = true;
> There are too many duplications. Why not simply call it 'initialized'?
>
>> -       if (gc->to_irq) {
>> +       if (gc->to_irq && gc->irq.gc_irq_initialized) {
> Why can't this check be added into gpiochip_to_irq() ?
>
>      if (!gc->irq.initialized)
>          return -ENXIO;
>
> ...


Because we don't want to return -ENXIO in case of race condition.

It should return -EPROBE_DEFER similar to how we are doing when gc->to_irq
is NULL.

So in this case when both gc->to_irq = NULL and gc->irq.initialized = FALSE,
we will be returning -EPROBE_DEFER. This will make sure that devices 
like touchscreen
do not become fatal due to returning -ENXIO.

>
>> +       bool gc_irq_initialized;
> Can you move it closer to .init_hw so it will be weakly grouped by
> logic similarities?
> Also see above.

Thanks for your comments, I'll make the necessary changes and send a v3.


Shreeya Patel