Re: [PATCH v1 1/2] gpiolib: Fix line event handling in compatible mode

[Bartosz Golaszewski <brgl@xxxxxxxx>]

śr., 4 gru 2019 o 20:42 Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> napisał(a):
>
> The introduced line even handling ABI in the commit
>
>   61f922db7221 ("gpio: userspace ABI for reading GPIO line events")
>
> missed the fact that 64-bit kernel may serve for 32-bit applications.
> In such case the very first check in the lineevent_read() will fail
> due to alignment differences.
>
> To workaround this we do several things here:
> - put warning comment to UAPI header near to the structure description
> - derive the size of the structure in the compatible mode from its members
> - check for the size of this structure in the ->read() callback
> - return only one event in the compatible mode at a time
>
> Above mitigation will work at least with libgpiod which does one event
> at a time.
>
> Since the bug hasn't been reported earlier we assume that there is close
> to zero actual users of the compatible mode to monitor GPIO events and thus
> we might consider to rework this ABI in the future.
>

How come this only affects the read operation but not the structures
passed as arguments to ioctl() calls?

Bart

> Fixes: 61f922db7221 ("gpio: userspace ABI for reading GPIO line events")
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> ---
>  drivers/gpio/gpiolib.c    | 51 ++++++++++++++++++++++++++++++++++++---
>  include/uapi/linux/gpio.h |  6 +++++
>  2 files changed, 54 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index 7340e4d0e873..134985210619 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -825,17 +825,26 @@ static __poll_t lineevent_poll(struct file *filep,
>         return events;
>  }
>
> -
>  static ssize_t lineevent_read(struct file *filep,
>                               char __user *buf,
>                               size_t count,
>                               loff_t *f_ps)
>  {
>         struct lineevent_state *le = filep->private_data;
> +       struct gpioevent_data event, *e = &event;
> +       /* We need size of each member to avoid endianess issues below */
> +       size_t ts_sz = sizeof(e->timestamp), id_sz = sizeof(e->id), e_sz;
>         unsigned int copied;
>         int ret;
>
> -       if (count < sizeof(struct gpioevent_data))
> +       /*
> +        * In compatible mode, when kernel is 64-bit and user space is 32-bit,
> +        * we may not tell what user wanted here when count is bigger than size
> +        * of one event, so, we just assume that user asks for precisely one
> +        * event.
> +        */
> +       e_sz = in_compat_syscall() ? ts_sz + id_sz : sizeof(*e);
> +       if (count < e_sz)
>                 return -EINVAL;
>
>         do {
> @@ -851,7 +860,43 @@ static ssize_t lineevent_read(struct file *filep,
>
>                 if (mutex_lock_interruptible(&le->read_lock))
>                         return -ERESTARTSYS;
> -               ret = kfifo_to_user(&le->events, buf, count, &copied);
> +               if (in_compat_syscall()) {
> +                       /*
> +                        * First we peek the one event and, if there is
> +                        * no error during copying to user space, skip it
> +                        * later.
> +                        */
> +                       if (kfifo_peek(&le->events, e))
> +                               copied = e_sz;
> +                       else
> +                               copied = 0;
> +
> +                       /* Do not try to copy garbage to the user */
> +                       ret = copied ? 0 : -EFAULT;
> +
> +                       /*
> +                        * Due to endianess concerns we have to copy this
> +                        * member-by-member. Luckily there are no members
> +                        * less than 32-bit.
> +                        */
> +                       if (!ret)
> +                               ret = copy_to_user(buf, &e->timestamp, ts_sz);
> +                       if (!ret)
> +                               ret = copy_to_user(buf + ts_sz, &e->id, id_sz);
> +
> +                       if (ret) {
> +                               /*
> +                                * Either we have got nothing from the FIFO or
> +                                * one of copy_to_user() calls failed.
> +                                */
> +                               ret = -EFAULT;
> +                       } else {
> +                               /* Skip peeked event if no error happened */
> +                               kfifo_skip(&le->events);
> +                       }
> +               } else {
> +                       ret = kfifo_to_user(&le->events, buf, count, &copied);
> +               }
>                 mutex_unlock(&le->read_lock);
>
>                 if (ret)
> diff --git a/include/uapi/linux/gpio.h b/include/uapi/linux/gpio.h
> index 799cf823d493..054756bf6991 100644
> --- a/include/uapi/linux/gpio.h
> +++ b/include/uapi/linux/gpio.h
> @@ -168,6 +168,12 @@ struct gpioevent_request {
>   * struct gpioevent_data - The actual event being pushed to userspace
>   * @timestamp: best estimate of time of event occurrence, in nanoseconds
>   * @id: event identifier
> + *
> + * Warning! This structure has issues in the compatible mode, when
> + * kernel is 64-bit and user space is 32-bit, due to alignment
> + * differences.
> + *
> + * It's not recommended to retrieve more than one event at a time.
>   */
>  struct gpioevent_data {
>         __u64 timestamp;
> --
> 2.24.0
>