Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

[Jason A. Donenfeld]

Hey Ted,

On Wed, Dec 14, 2016 at 8:12 PM, Jason A. Donenfeld  wrote:
> I think this opens up a big window for optimizing it even
> further.

I optimized it a bit further and siphash is now the clear winner over chacha:

[1.784801] random benchmark!!
[1.785161] get_random_long # cycles: 415983
[1.785595] get_random_long_chacha # cycles: 242047
[1.785997] get_random_long_siphash # cycles: 137130
[1.787450] get_random_bytes # cycles: 1452985
[1.787947] get_random_int # cycles: 343323
[1.788282] get_random_int_chacha # cycles: 170767
[1.788656] get_random_int_siphash # cycles: 86384
[1.789764] get_random_bytes # cycles: 2279519

And even still, there is more that could be optimized. Therefore, I'll
continue to keep this patch in the series and will CC you on the next
patch set that goes out.

Jason

Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

[Jason A. Donenfeld]

Hi again,

On Wed, Dec 14, 2016 at 5:37 PM, Theodore Ts'o  wrote:
> [3.606139] random benchmark!!
> [3.606276] get_random_int # cycles: 326578
> [3.606317] get_random_int_new # cycles: 95438
> [3.607423] get_random_bytes # cycles: 2653388

Looks to me like my siphash implementation is much faster for
get_random_long, and more or less tied for get_random_int:

[1.729370] random benchmark!!
[1.729710] get_random_long # cycles: 349771
[1.730128] get_random_long_chacha # cycles: 359660
[1.730457] get_random_long_siphash # cycles: 94255
[1.731307] get_random_bytes # cycles: 1354894
[1.731707] get_random_int # cycles: 305640
[1.732095] get_random_int_chacha # cycles: 80726
[1.732425] get_random_int_siphash # cycles: 94265
[1.733278] get_random_bytes # cycles: 1315873

Given the increasing usage of get_random_long for ASLR and related, I
think this makes the siphash approach worth pursuing. The chacha
approach is also not significantly different from the md5 approach in
terms of speed for get_rand_long. Additionally, since siphash is a
PRF, I think this opens up a big window for optimizing it even
further.

Benchmark here:
https://git.zx2c4.com/linux-dev/commit/?h=rng-bench

Jason

Re: [kernel-hardening] Re: [PATCH 4/3] random: use siphash24 instead of md5 for get_random_int/long

[Jason A. Donenfeld]

Hey Ted,

On Wed, Dec 14, 2016 at 5:37 PM, Theodore Ts'o  wrote:
> One somewhat undesirable aspect of the current algorithm is that we
> never change random_int_secret.

Why exactly would this be a problem? So long as the secret is kept
secret, the PRF is secure. If an attacker can read arbitrary kernel
memory, there are much much bigger issues to be concerned about. As
well, the "chaining" variable I introduce ensures that the random
numbers are, per-cpu, related to the uniqueness of timing of
subsequent calls.

> So I've been toying with the
> following, which is 4 times faster than md5.  (I haven't tried
> benchmarking against siphash yet.)
>
> [3.606139] random benchmark!!
> [3.606276] get_random_int # cycles: 326578
> [3.606317] get_random_int_new # cycles: 95438
> [3.607423] get_random_bytes # cycles: 2653388

Cool, I'll benchmark it against the siphash implementation. I like
what you did with batching up lots of chacha output, and doling it out
bit by bit. I suspect this will be quite fast, because with chacha20
you get an entire block.

> P.S.  It's interesting to note that siphash24 and chacha20 are both
> add-rotate-xor based algorithms.

Quite! Lots of nice shiny things are turning out be be ARX -- ChaCha,
BLAKE2, Siphash, NORX. The simplicity is really appealing.

Jason