linux-kernel - Re: [PATCH 1/2] random: use chacha20 for get_random

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Date:   Fri, 20 Jan 2017 06:27:19 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     "Theodore Ts'o" <tytso@....edu>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 1/2] random: use chacha20 for get_random_int/long

Hi Ted,

On Wed, Jan 18, 2017 at 4:49 AM, Theodore Ts'o <tytso@....edu> wrote:
> You're using a union for the entropy_u64 and entropy_u32 arrays, and
> the position field is used to indexed into those two arrays.
>
> So if the first caller calls get_random_u64 with position=0, it will
> get the first 64 bits out of the batched entropy, and increment the
> position to 1.  If the second caller calls get_random_u32 with
> position=1... it will get the second of the 32-bit chunks in the
> batched entropy array --- which will include the 64 bits returned to
> the first caller.

This is most certainly not the case. Read closely.

I use a different variable for each function. get_random_u32 only
touches batched_entropy_u32, and get_random_u64 only touches
bached_entropy_u64. Under no circumstances will one function use the
other's batched entropy.

I use a union so that only one struct must be declared. If you think
that's unclear, I can declare two separate structs instead. Let me
know which strategy you prefer -- what I have now, or splitting it
into two structs.

Jason