Re: [PATCH 10/22] gpiolib: cdev: fix minor race in GET_LINEINFO_WATCH

[Kent Gibson <warthog618@xxxxxxxxx>]

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@xxxxxxxxx> 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@xxxxxxxxx> 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.