rowhammer and pagemap (was Re: [RFC, PATCH] pagemap: do not leak physical addresses to non-privileged userspace)

[Pavel Machek <pavel@ucw.cz>]

> > Given that, I think it would still be worthwhile to disable /proc/PID/pagemap.
> 
> Having slept on this further, I think that unprivileged pagemap access
> is awful and we should disable it with no option to re-enable.  If we
> absolutely must, we could allow programs to read all zeros or to read
> addresses that are severely scrambled (e.g. ECB-encrypted by a key
> generated once per open of pagemap).

>  - It could easily leak direct-map addresses, and there's a nice paper
> detailing a SMAP bypass using that technique.

Do you have a pointer?

> Can we just try getting rid of it except with global CAP_SYS_ADMIN.
> 
> (Hmm.  Rowhammer attacks targeting SMRAM could be interesting.)

:-).

> >> Can we do anything about that? Disabling cache flushes from userland
> >> should make it no longer exploitable.
> >
> > Unfortunately there's no way to disable userland code's use of
> > CLFLUSH, as far as I know.
> >
> > Maybe Intel or AMD could disable CLFLUSH via a microcode update, but
> > they have not said whether that would be possible.
> 
> The Intel people I asked last week weren't confident.  For one thing,
> I fully expect that rowhammer can be exploited using only reads and
> writes with some clever tricks involving cache associativity.  I don't
> think there are any fully-associative caches, although the cache
> replacement algorithm could make the attacks interesting.

We should definitely get Intel/AMD to disable CLFLUSH, then.

Because if it can be exploited using reads, it is _extremely_
important to know. As it probably means rowhammer can be exploited
using Javascript / Java... and affected machines are unsafe even
without remote users.
									Pavel
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