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SubjectRe: for stable -- random: use chacha20 for get_random_int/long
On Fri, Apr 07, 2017 at 05:27:40AM +0200, Jason A. Donenfeld wrote:
> Given that the below commit isn't very big and adds a nice security
> property (in addition to performance), it might be worthwhile to
> backport this to 4.9 stable. It's not a candidate for 4.4, since that
> kernel doesn't use chacha for the rng at all.
>
> As this is in random.c, it's Ted's and Greg's judgement call.
>
> commit f5b98461cb8167ba362ad9f74c41d126b7becea7
> Author: Jason A. Donenfeld <Jason@zx2c4.com>
> Date: Fri Jan 6 19:32:01 2017 +0100
>
> random: use chacha20 for get_random_int/long
>
> Now that our crng uses chacha20, we can rely on its speedy
> characteristics for replacing MD5, while simultaneously achieving a
> higher security guarantee. Before the idea was to use these functions if
> you wanted random integers that aren't stupidly insecure but aren't
> necessarily secure either, a vague gray zone, that hopefully was "good
> enough" for its users. With chacha20, we can strengthen this claim,
> since either we're using an rdrand-like instruction, or we're using the
> same crng as /dev/urandom. And it's faster than what was before.
>
> We could have chosen to replace this with a SipHash-derived function,
> which might be slightly faster, but at the cost of having yet another
> RNG construction in the kernel. By moving to chacha20, we have a single
> RNG to analyze and verify, and we also already get good performance
> improvements on all platforms.
>
> Implementation-wise, rather than use a generic buffer for both
> get_random_int/long and memcpy based on the size needs, we use a
> specific buffer for 32-bit reads and for 64-bit reads. This way, we're
> guaranteed to always have aligned accesses on all platforms. While
> slightly more verbose in C, the assembly this generates is a lot
> simpler than otherwise.
>
> Finally, on 32-bit platforms where longs and ints are the same size,
> we simply alias get_random_int to get_random_long.
>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> Suggested-by: Theodore Ts'o <tytso@mit.edu>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Seems reasonable to me, now queued up, thanks!

greg k-h

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