Re: x86/random: Speculation to the rescue

From: Pavel Machek <pavel@ucw.cz>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	"Ahmed S. Darwish" <darwish.07@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Theodore Ts'o <tytso@mit.edu>,
	Nicholas Mc Guire <hofrat@opentech.at>,
	the arch/x86 maintainers <x86@kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Kees Cook <keescook@chromium.org>
Subject: Re: x86/random: Speculation to the rescue
Date: Sun, 6 Oct 2019 13:41:29 +0200	[thread overview]
Message-ID: <20191006114129.GD24605@amd> (raw)
In-Reply-To: <CAHk-=wgjC01UaoV35PZvGPnrQ812SRGPoV7Xp63BBFxAsJjvrg@mail.gmail.com>

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Hi!

On Sat 2019-09-28 16:53:52, Linus Torvalds wrote:
> On Sat, Sep 28, 2019 at 3:24 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > Nicholas presented the idea to (ab)use speculative execution for random
> > number generation years ago at the Real-Time Linux Workshop:
> 
> What you describe is just a particularly simple version of the jitter
> entropy. Not very reliable.
> 
> But hey, here's a made-up patch. It basically does jitter entropy, but
> it uses a more complex load than the fibonacci LFSR folding: it calls
> "schedule()" in a loop, and it sets up a timer to fire.
> 
> And then it mixes in the TSC in that loop.
> 
> And to be fairly conservative, it then credits one bit of entropy for
> every timer tick. Not because the timer itself would be all that
> unpredictable, but because the interaction between the timer and the
> loop is going to be pretty damn unpredictable.
> 
> Ok, I'm handwaving. But I do claim it really is fairly conservative to
> think that a cycle counter would give one bit of entropy when you time
> over a timer actually happening. The way that loop is written, we do
> guarantee that we'll mix in the TSC value both before and after the
> timer actually happened. We never look at the difference of TSC
> values, because the mixing makes that uninteresting, but the code does
> start out with verifying that "yes, the TSC really is changing rapidly
> enough to be meaningful".
> 
> So if we want to do jitter entropy, I'd much rather do something like
> this that actually has a known fairly complex load with timers and
> scheduling.

> +/*
> + * If we have an actual cycle counter, see if we can
> + * generate enough entropy with timing noise
> + */
> +static void try_to_generate_entropy(void)
> +{
> +	struct {
> +		unsigned long now;
> +		struct timer_list timer;
> +	} stack;

Should we have some kind of notifier chain, so that we could utilize
better random sources (spinning rust) if we had them?

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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