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Date:   Thu, 27 Feb 2020 23:09:30 -0500
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org
Subject: Re: [PATCH v2] random: always use batched entropy for
 get_random_u{32,64}
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 <20200221201037.30231-1-Jason@zx2c4.com>
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On Fri, Feb 21, 2020 at 09:10:37PM +0100, Jason A. Donenfeld wrote:
> It turns out that RDRAND is pretty slow. Comparing these two
> constructions:
> 
>   for (i = 0; i < CHACHA_BLOCK_SIZE; i += sizeof(ret))
>     arch_get_random_long(&ret);
> 
> and
> 
>   long buf[CHACHA_BLOCK_SIZE / sizeof(long)];
>   extract_crng((u8 *)buf);
> 
> it amortizes out to 352 cycles per long for the top one and 107 cycles
> per long for the bottom one, on Coffee Lake Refresh, Intel Core i9-9880H.
> 
> And importantly, the top one has the drawback of not benefiting from the
> real rng, whereas the bottom one has all the nice benefits of using our
> own chacha rng. As get_random_u{32,64} gets used in more places (perhaps
> beyond what it was originally intended for when it was introduced as
> get_random_{int,long} back in the md5 monstrosity era), it seems like it
> might be a good thing to strengthen its posture a tiny bit. Doing this
> should only be stronger and not any weaker because that pool is already
> initialized with a bunch of rdrand data (when available). This way, we
> get the benefits of the hardware rng as well as our own rng.
> 
> Another benefit of this is that we no longer hit pitfalls of the recent
> stream of AMD bugs in RDRAND. One often used code pattern for various
> things is:
> 
>   do {
>   	val = get_random_u32();
>   } while (hash_table_contains_key(val));
> 
> That recent AMD bug rendered that pattern useless, whereas we're really
> very certain that chacha20 output will give pretty distributed numbers,
> no matter what.
> 
> So, this simplification seems better both from a security perspective
> and from a performance perspective.
> 
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Thanks, applied.

						- Ted