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

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Date:   Tue, 16 Apr 2019 12:45:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>
CC:     Ingo Molnar <mingo@...nel.org>, "tytso@....edu" <tytso@....edu>,
        "Daniel Borkmann" <daniel@...earbox.net>,
        "luto@...nel.org" <luto@...nel.org>,
        "luto@...capital.net" <luto@...capital.net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Subject: RE: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

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.

	David

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