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

[Dan Williams]

On Wed, Jan 3, 2018 at 9:55 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> 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?

The question is if userspace can ex-filtrate any data from the kernel
that would otherwise be blocked by a bounds check should the kernel
close that hole?

For these patches I do not think the bar should be "can I prove an
information leak is exploitable" it should be "can I prove that a leak
is not exploitable", especially when possibly combined with other leak
sites.