linux-kernel - Re: [PATCH v2 1/2] random: split initialization into early step and later step

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Message-ID: <CAHmME9oGkjAxvoBvWMBRSjFmKLzOdzfcQAB4q3P869BsySSfNg@mail.gmail.com>
Date:   Tue, 27 Sep 2022 10:28:11 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux@...inikbrodowski.net
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kasan-dev@...glegroups.com, Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] random: split initialization into early step and
 later step

On Tue, Sep 27, 2022 at 8:35 AM Dominik Brodowski
<linux@...inikbrodowski.net> wrote:
> >  #if defined(LATENT_ENTROPY_PLUGIN)
> >       static const u8 compiletime_seed[BLAKE2S_BLOCK_SIZE] __initconst __latent_entropy;
> > @@ -803,34 +798,46 @@ int __init random_init(const char *command_line)
> >                       i += longs;
> >                       continue;
> >               }
> > -             entropy[0] = random_get_entropy();
> > -             _mix_pool_bytes(entropy, sizeof(*entropy));
> >               arch_bits -= sizeof(*entropy) * 8;
> >               ++i;
> >       }
>
>
> Previously, random_get_entropy() was mixed into the pool ARRAY_SIZE(entropy)
> times.
>
> > +/*
> > + * This is called a little bit after the prior function, and now there is
> > + * access to timestamps counters. Interrupts are not yet enabled.
> > + */
> > +void __init random_init(void)
> > +{
> > +     unsigned long entropy = random_get_entropy();
> > +     ktime_t now = ktime_get_real();
> > +
> > +     _mix_pool_bytes(utsname(), sizeof(*(utsname())));
>
> But now, it's only mixed into the pool once. Is this change on purpose?

Yea, it is. I don't think it's really doing much of use. Before we did
it because it was convenient -- because we simply could. But in
reality mostly what we care about is capturing when it gets to that
point in the execution. For jitter, the actual jitter function
(try_to_generate_entropy()) is better here.

However, before feeling too sad about it, remember that
extract_entropy() is still filling a block with rdtsc when rdrand
fails, the same way as this function was. So it's still in there
anyway.

Jason