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

[Al Viro]

On Thu, Jan 04, 2018 at 05:50:12AM +0000, Al Viro wrote:
> 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?

Sorry, s/files->fdt/files->fdt->fd/.  Still the same question - what information
would that extract and how would attacker use that?