linux-kernel - Re: [PATCH v5 2/2] gpiolib: protect the GPIO device against being dropped while in use by user-space

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Message-ID: <CAMRc=McBPpLx4Gac-uR1S9-kQBE=AVLbFfWSrTWN6RNXwvaTRw@mail.gmail.com>
Date:   Thu, 1 Dec 2022 13:12:41 +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 v5 2/2] gpiolib: protect the GPIO device against being
 dropped while in use by user-space

On Thu, Dec 1, 2022 at 1:01 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Thu, Dec 01, 2022 at 09:33:35AM +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.
> >
> > 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")
> > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > Reviewed-by: Kent Gibson <warthog618@...il.com>
> > Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > ---
> >  drivers/gpio/gpiolib-cdev.c | 166 +++++++++++++++++++++++++++++++-----
> >  drivers/gpio/gpiolib.c      |   4 +
> >  drivers/gpio/gpiolib.h      |   5 ++
> >  3 files changed, 153 insertions(+), 22 deletions(-)
> >
> > diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> > index 911d91668903..18c5e70ee7de 100644
> > --- a/drivers/gpio/gpiolib-cdev.c
> > +++ b/drivers/gpio/gpiolib-cdev.c
> > @@ -84,6 +84,53 @@ struct linehandle_state {
> >       GPIOHANDLE_REQUEST_OPEN_DRAIN | \
> >       GPIOHANDLE_REQUEST_OPEN_SOURCE)
> >
> > +typedef __poll_t (*poll_fn)(struct file *, struct poll_table_struct *);
> > +typedef long (*ioctl_fn)(struct file *, unsigned int, unsigned long);
> > +typedef ssize_t (*read_fn)(struct file *, char __user *,
> > +                        size_t count, loff_t *);
> > +
> > +static __poll_t call_poll_locked(struct file *file,
> > +                              struct poll_table_struct *wait,
> > +                              struct gpio_device *gdev, poll_fn func)
> > +{
> > +     __poll_t ret;
> > +
> > +     if (!down_read_trylock(&gdev->sem))
>
> > +             return 0;
>
> EPOLLHUP?
>

Or even EPOLLERR | EPOLLHUP.

Sorry in advance for the noise but I really want to get those fixes in
this week, so I'll send a new iteration shortly.

Bart