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SubjectRe: [PATCH 10/22] gpiolib: cdev: fix minor race in GET_LINEINFO_WATCH
On Thu, Jun 25, 2020 at 12:23:49PM +0300, Andy Shevchenko wrote:
> On Thu, Jun 25, 2020 at 12:13 PM Kent Gibson <warthog618@gmail.com> wrote:
> > On Thu, Jun 25, 2020 at 11:44:21AM +0300, Andy Shevchenko wrote:
> > > On Thu, Jun 25, 2020 at 1:58 AM Kent Gibson <warthog618@gmail.com> wrote:
> > > > On Wed, Jun 24, 2020 at 11:57:14PM +0800, Kent Gibson wrote:
>
> ...
>
> > > > Perhaps you are referring to the case where the copy_to_user fails?
> > >
> > > Yes.
> > >
> > > > To be honest I considered that to be so unlikely that I ignored it.
> > > > Is there a relevant failure mode that I'm missing?
> > >
> > > The traditional question for such cases is "what can possibly go wrong?"
> > > I wouldn't underestimate the probability of failure.
> > >
> >
> > The worst case is the watch is enabled and the userspace gets an
> > EFAULT so it thinks it failed. If userspace retries then they get
> > EBUSY, so userspace accounting gets muddled.
> >
> > We can clear the watch bit if the copy_to_user fails - before
> > returning the EFAULT. Would that be satisfactory?
>
> Perhaps. I didn't check that scenario.
>

To be clear I'm suggesting this:

gpio_desc_to_lineinfo(desc, &lineinfo);

- if (copy_to_user(ip, &lineinfo, sizeof(lineinfo)))
+ if (copy_to_user(ip, &lineinfo, sizeof(lineinfo))) {
+ clear_bit(lineinfo.offset, gcdev->watched_lines);
return -EFAULT;
+ }

That undoes the set, returning the watch state to what it was before the
call.

Cheers,
Kent.

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