linux-kernel - Re: rowhammer protection [was Re: Getting interrupt every million cache misses]

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Date:   Fri, 28 Oct 2016 11:47:07 +0200
From:   Vegard Nossum <vegard.nossum@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, Pavel Machek <pavel@....cz>,
        Kees Cook <keescook@...omium.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: rowhammer protection [was Re: Getting interrupt every million
 cache misses]

On 28 October 2016 at 11:35, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Vegard Nossum <vegard.nossum@...il.com> wrote:
>
>> Would it make sense to sample the counter on context switch, do some
>> accounting on a per-task cache miss counter, and slow down just the
>> single task(s) with a too high cache miss rate? That way there's no
>> global slowdown (which I assume would be the case here). The task's
>> slice of CPU would have to be taken into account because otherwise you
>> could have multiple cooperating tasks that each escape the limit but
>> taken together go above it.
>
> Attackers could work this around by splitting the rowhammer workload between
> multiple threads/processes.
>
> I.e. the problem is that the risk may come from any 'unprivileged user-space
> code', where the rowhammer workload might be spread over multiple threads,
> processes or even users.

That's why I emphasised the number of misses per CPU slice rather than
just the total number of misses. I assumed there must be at least one
task continuously hammering memory for a successful attack, in which
case it should be observable with as little as 1 slice of CPU (however
long that is), no?


Vegard