RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

From: David Laight
Date: Tue Apr 16 2019 - 08:44:13 EST

From: Peter Zijlstra
> Sent: 16 April 2019 13:08
> So the argument against using TSC directly was that it might be easy to
> guess most of the TSC bits in timing attack. But IIRC there is fairly
> solid evidence that the lowest TSC bits are very hard to guess and might
> in fact be a very good random source.
> So what one could do, is for each invocation mix in the low (2?) bits of
> the TSC into a per-cpu/task PRNG state. By always adding some fresh
> entropy it would become very hard indeed to predict the outcome, even
> for otherwise 'trivial' PRNGs.

You could just feed 8 bits of TSC into a CRC.
Or even xor the entire TSC over a CRC state and then cycle it at least 6 bits.
Probably doesn't matter which CRC - but you may want one that is
cheap in software.
Even a 16bit CRC might be enough.


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