Re: [PATCH] gpiolib: fix possible use after free on label

[Muchun Song]

Hi Linus,

Thanks for your review.

Linus Walleij <linus.walleij@xxxxxxxxxx> ä2018å10æ31æåä äå6:32åéï
>
> Hi Muchun,
>
> thanks for your patch!
>
> On Wed, Oct 24, 2018 at 3:41 PM Muchun Song <smuchun@xxxxxxxxx> wrote:
>
> > gpiod_request_commit() copies the pointer to the label
> > passed as an argument only to be used later. But there's a
> > chance the caller could immediately free the passed string
> > (e.g., local variable). This could trigger a use after free
> > when we use gpio label(e.g., gpiochip_unlock_as_irq(),
> > gpiochip_is_requested()).
> >
> > To be on the safe side: duplicate the string with
> > kstrdup_const() so that if an unaware user passes an address
> > to a stack-allocated buffer, we won't get the arbitrary label.
> >
> > Signed-off-by: Muchun Song <smuchun@xxxxxxxxx>
>
> I am a bit worried about the memory consumption for this,
> but I guess typically this should not be much.

Yeah, I think so. In most cases, we pass the label, which is
in .rodata section.

>
> I am a little bit lost in const-correctness here, and I do
> understand that the label could point to something allocated on
> the stack, but it seems like an awkward way of shooting
> oneself in the foot really. Allocate something and then
> pass it as a const char *? That is something we could pretty
> much detect with a cocinelle script I think?

Some user may have more than one gpio to request
and may program the following code to request one gpioï

int gpio_request_one(int gpio)
{
        char name[8];

        snprintf(name, sizeof(name), "GPIO_%d", gpio);

        return gpio_request(gpio, name);
}

In this case, it could trigger a use after free when we use gpio label.
But the user may not realize it. With this patch applied, we can get
the right label.

>
> Anyways: if you want to proceed with this approach, also
> make sure to fix gpiod_set_consumer_name() to free previous
> label and make a new strdup when called.
>
> Yours,
> Linus Walleij

Sorry, I forgot to fix gpiod_set_consumer_name().
I will send you a patch of v2 later. Thanks.

Yours,
Muchun Song