linux-kernel - Re: [PATCH v2 2/2] gpiolib: protect the GPIO device against being dropped while in use by user-space

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Date:   Mon, 28 Nov 2022 22:36:09 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Kent Gibson <warthog618@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 2/2] gpiolib: protect the GPIO device against being
 dropped while in use by user-space

On Mon, Nov 28, 2022 at 8:21 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Mon, Nov 28, 2022 at 06:52:14PM +0100, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > While any of the GPIO cdev syscalls is in progress, the kernel can call
> > gpiochip_remove() (for instance, when a USB GPIO expander is disconnected)
> > which will set gdev->chip to NULL after which any subsequent access will
> > cause a crash.
> >
> > To avoid that: use an RW-semaphore in which the syscalls take it for
> > reading (so that we don't needlessly prohibit the user-space from calling
> > syscalls simultaneously) while gpiochip_remove() takes it for writing so
> > that it can only happen once all syscalls return.
>
> Missed also Dependency (the previous change).
>
> > Fixes: d7c51b47ac11 ("gpio: userspace ABI for reading/writing GPIO lines")
> > Fixes: 3c0d9c635ae2 ("gpiolib: cdev: support GPIO_V2_GET_LINE_IOCTL and GPIO_V2_LINE_GET_VALUES_IOCTL")
> > Fixes: aad955842d1c ("gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL")
> > Fixes: a54756cb24ea ("gpiolib: cdev: support GPIO_V2_LINE_SET_CONFIG_IOCTL")
> > Fixes: 7b8e00d98168 ("gpiolib: cdev: support GPIO_V2_LINE_SET_VALUES_IOCTL")
>
> ...
>
> > +     down_read(&gdev->sem);
> >
> > -     if (!gdev->chip)
> > +     if (!gdev->chip) {
> > +             up_read(&gdev->sem);
> >               return -ENODEV;
> > +     }
>
> Wouldn't be easier to wrap existing functions (with their renaming) into a new
> ones with semaphore?
>

You're probably right, it would be much cleaner. I'll see about it.

Bart