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

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Date:   Tue, 17 Jan 2017 22:49:07 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     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 Jason,

I've been taking a look at your patch, and i think it's... problematic.

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.

That's not very random.   :-(

						- Ted