linux-kernel - Re: [PATCH] random: allow writes to /dev/urandom to influence fast init

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Message-ID: <YjzMPymC3uXQUTrq@gmail.com>
Date:   Thu, 24 Mar 2022 19:53:35 +0000
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Theodore Ts'o <tytso@....edu>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] random: allow writes to /dev/urandom to influence fast
 init

On Tue, Mar 22, 2022 at 01:14:36PM -0600, Jason A. Donenfeld wrote:
> For as far back as I can tell, writing to /dev/urandom or /dev/random
> will put entropy into the pool, but won't immediately use it, and won't
> credit it either.

Did you check kernels v4.7 and earlier?  It looks like this actually changed in
v4.8 when the ChaCha20 CRNG was introduced.  v4.7 would mix the data written to
/dev/{u,}random into {non,}blocking_pool, which would immediately be reflected
in reads from /dev/{u,}random, sys_getrandom(), and get_random_bytes().  Writes
to /dev/{u,}random didn't affect the input_pool, which was separate.

Was the change in behavior in v4.8 a regression?

- Eric