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Date:	Sun, 16 Jul 2006 03:54:27 +0200
From:	Pavel Machek <>
Cc:, Alan Cox <>,,
	Lee Revell <>,
	"Randy.Dunlap" <>,
	Andrew Morton <>,,,
Subject: Re: [2.6 patch] let CONFIG_SECCOMP default to n

> On Fri, Jul 14, 2006 at 10:55:28PM -0400, wrote:
> > In fact, the best you can do here is to reduce the effective bandwidth
> > the signal can have, as Shannon showed quite clearly.
> Yes.
> > And even 20 years ago, the guys who did the original DoD Orange Book
> > requirements understood this - they didn't make a requirement that covert
> > channels (both timing and other) be totally closed down, they only made
> > a requirement that for higher security configurations the bandwidth of
> > the channel be reduced below a specified level...
> Why I think it's trivial to guarantee the closure of the seccomp side
> channel timing attack even on a very fast internet by simply
> introducing the random delay, is that below a certain sampling
> frequency you won't be able to extract data from the latencies of the
> cache. The max length of the random noise has to be >= of what it
> takes to refill the whole cache. Then you won't know if it was a
> cache

You won't know for sure... but. Let t be time takes to reload the
cache. Let your random noise be in <0, t> interval. According to you,
that would be okay. IT IS NOT.

If the original delay was long, and your generator returned t,
attacker sees 2*t. He can be _sure_ delay was long now.

If the delay was short, and your generator returns 0, attacker sees 0,
and _knows_ delay was short. (Chance that generator produces 0 or t is
small, but non zero).

Even if you do random noise in <0, 2*t) interval, I'll be able to
gather some statistics.

Thanks, Sharp!
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