From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: EuroSec'11 Presentation Date: Mon, 11 Apr 2011 12:22:19 -0500 Message-ID: <4DA338CB.5050708@codemonkey.ws> References: <20110410.232340.01368317.k.suzaki@aist.go.jp> <4DA1C390.10003@redhat.com> <20110411.001930.73371943.k.suzaki@aist.go.jp> <4DA31DDF.7060605@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Kuniyasu Suzaki , avi@redhat.com, kvm@vger.kernel.org To: Stefan Hajnoczi Return-path: Received: from mail-gy0-f174.google.com ([209.85.160.174]:60604 "EHLO mail-gy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753394Ab1DKRWW (ORCPT ); Mon, 11 Apr 2011 13:22:22 -0400 Received: by gyd10 with SMTP id 10so2232208gyd.19 for ; Mon, 11 Apr 2011 10:22:21 -0700 (PDT) In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 04/11/2011 11:25 AM, Stefan Hajnoczi wrote: > On Mon, Apr 11, 2011 at 4:27 PM, Anthony Liguori wrote: >> >> It's a deviation of a previously demonstrated attack where memory access >> timing is used to guess memory content. This has been demonstrated in the >> past to be a viable technique to reduce the keyspace of things like ssh keys >> which makes attack a bit easier. > How can you reduce the key space by determining whether the guest has > arbitrary 4 KB data in physical memory? I'm not sure that you can. But the way the cache timing attack worked is that by doing a cache timing analysis in another process that's sharing the cache with a process doing key generation, you can make predictions about the paths taken by the key generation code which let's you narrow down the key space. Of course, even this is extremely hard to exploit because you need to happen to be sharing a cache with something that's doing ssh key generation, you have to know when it starts and when it ends, and you have to know exactly what version of ssh is running. And even then, it's just reduces the time needed to brute force. It still takes a long time. I think knowing whether a 4kb page is shared by some other guest in the system is so little information that I don't see what you could practically do with it that can't already be done via a cache timing attack. Regards, Anthony Liguori > Stefan