Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

[Al Viro]

On Wed, Jan 03, 2018 at 09:44:33PM -0800, Dan Williams wrote:
> On Wed, Jan 3, 2018 at 8:44 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Jan 04, 2018 at 03:10:51AM +0000, Williams, Dan J wrote:
> >
> >> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> >> index 1c65817673db..dbc12007da51 100644
> >> --- a/include/linux/fdtable.h
> >> +++ b/include/linux/fdtable.h
> >> @@ -82,8 +82,10 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i
> >>  {
> >>       struct fdtable *fdt = rcu_dereference_raw(files->fdt);
> >>
> >> -     if (fd < fdt->max_fds)
> >> +     if (fd < fdt->max_fds) {
> >> +             osb();
> >>               return rcu_dereference_raw(fdt->fd[fd]);
> >> +     }
> >>       return NULL;
> >>  }
> >
> > ... and the point of that would be?  Possibly revealing the value of files->fdt?
> > Why would that be a threat, assuming you manage to extract the information in
> > question in the first place?
> 
> No, the concern is that an fd value >= fdt->max_fds may cause the cpu
> to read arbitrary memory addresses relative to files->fdt and
> userspace can observe that it got loaded.

Yes.  And all that might reveal is the value of files->fdt.  Who cares?