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

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Date:	Tue, 17 Mar 2015 12:16:53 +0100
From:	Pavel Machek <pavel@....cz>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Mark Seaborn <mseaborn@...omium.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>
Subject: rowhammer and pagemap (was Re: [RFC, PATCH] pagemap: do not leak
 physical addresses to non-privileged userspace)



> > 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
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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