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Date:   Thu, 2 May 2019 18:43:25 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     David Laight <David.Laight@aculab.com>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        Theodore Ts'o <tytso@mit.edu>,
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        Daniel Borkmann <daniel@iogearbox.net>,
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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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* Andy Lutomirski <luto@kernel.org> wrote:

> > 8 gigabits/sec sounds good throughput in principle, if there's no
> > scalability pathologies with that.
> 
> The latency is horrible.

Latency would be amortized via batching anyway, so 8 gigabits/sec 
suggests something on the order of magnitude of 4 bits per cycle, right? 
With 64 bits extraction at a time that would be 16 cycles per 64-bit 
word, which isn't too bad, is it?

But you are right that get_random_bytes() is probably faster, and also 
more generic.

> > It would also be nice to know whether RDRAND does buffering 
> > *internally*,
> 
> Not in a useful way :(

Too bad ...

> > Any non-CPU source of randomness for system calls and plans to add 
> > several extra function calls to every x86 system call is crazy talk I 
> > believe...
> 
> I think that, in practice, the only real downside to enabling this 
> thing will be the jitter in syscall times. Although we could decide 
> that the benefit is a bit dubious and the whole thing isn't worth it. 
> But it will definitely be optional.

Making it "optional" is not really a technical argument in any way 
though, either distros enable it in which case it's a de-facto default 
setting, or they don't, in which case it de-facto almost doesn't exist.

Thanks,

	Ingo