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From:   Michael Brooks <m@sweetwater.ai>
Date:   Thu, 31 Mar 2022 09:02:27 -0700
Message-ID: <CAOnCY6Sv1goLQMJkvjsVb+TGdowUk6_vQ4ELQ_GkHu8Wjs3PAA@mail.gmail.com>
Subject: Re: [PATCH v2] random: mix build-time latent entropy into pool at init
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        "Theodore Ts'o" <tytso@mit.edu>
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mix_pool_bytes() has numerous problems, as discussed in prior emails.
Do we still want to be putting so much effort into a development dead
end?

-Michael

On Thu, Mar 31, 2022 at 8:28 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Prior, the "input_pool_data" array needed no real initialization, and so
> it was easy to mark it with __latent_entropy to populate it during
> compile-time. In switching to using a hash function, this required us to
> specifically initialize it to some specific state, which means we
> dropped the __latent_entropy attribute. An unfortunate side effect was
> this meant the pool was no longer seeded using compile-time random data.
> In order to bring this back, we declare an array in rand_initialize()
> with __latent_entropy and call mix_pool_bytes() on that at init, which
> accomplishes the same thing as before. We make this __initconst, so that
> it doesn't take up space at runtime after init.
>
> Fixes: 6e8ec2552c7d ("random: use computational hash for entropy extraction")
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> Changes v1->v2:
> - Use __initconst.
>
>  drivers/char/random.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 40107f8b9e9e..1d8242969751 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -975,6 +975,11 @@ int __init rand_initialize(void)
>         bool arch_init = true;
>         unsigned long rv;
>
> +#if defined(LATENT_ENTROPY_PLUGIN)
> +       static const u8 compiletime_seed[BLAKE2S_BLOCK_SIZE] __initconst __latent_entropy;
> +       _mix_pool_bytes(compiletime_seed, sizeof(compiletime_seed));
> +#endif
> +
>         for (i = 0; i < BLAKE2S_BLOCK_SIZE; i += sizeof(rv)) {
>                 if (!arch_get_random_seed_long_early(&rv) &&
>                     !arch_get_random_long_early(&rv)) {
> --
> 2.35.1
>