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Date:   Sat, 11 May 2019 17:12:41 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        David Laight <David.Laight@aculab.com>,
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        Peter Zijlstra <peterz@infradead.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
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        "jpoimboe@redhat.com" <jpoimboe@redhat.com>,
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        "Perla, Enrico" <enrico.perla@intel.com>,
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        "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
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On Sat, May 11, 2019 at 03:45:19PM -0700, Andy Lutomirski wrote:
> ISTM maybe a better first step would be to make get_random_bytes() be
> much faster? :)

I'm not opposed to that, but I want to make sure we don't break it for
"real" crypto uses...

I still think just using something very simply like rdtsc would be
good enough. This isn't meant to be a perfect defense: it's meant to
disrupt the ability to trivially predict (usually another thread's)
stack offset. And any sufficiently well-positioned local attacker can
defeat this no matter what the entropy source, given how small the number
of bits actually ends up being, assuming they can just keep launching
whatever they're trying to attack. (They can just hold still and try the
same offset until the randomness aligns: but that comes back to us also
needing a brute-force exec deterance, which is a separate subject...)

The entropy source bikeshedding doesn't seem helpful given how few bits
we're dealing with.

-- 
Kees Cook