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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Tue, 27 Sep 2022 10:40:51 +0200
Message-ID: <CAHmME9q3w1XHyS5QpyW79xK9xjnZmzyBr-Pk3QOsp=mJ_Loauw@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] random: split initialization into early step and
 later step
To:     Dominik Brodowski <linux@dominikbrodowski.net>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        kasan-dev@googlegroups.com, Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        stable@vger.kernel.org
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On Tue, Sep 27, 2022 at 10:30 AM Dominik Brodowski
<linux@dominikbrodowski.net> wrote:
>
> Am Tue, Sep 27, 2022 at 10:28:11AM +0200 schrieb Jason A. Donenfeld:
> > On Tue, Sep 27, 2022 at 8:35 AM Dominik Brodowski
> > <linux@dominikbrodowski.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.
>
> With that explanation on the record (I think it's important to make such
> subtle changes explicit),
>
>         Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>

I'll augment the commit message to note this too. Thanks for the review.

Jason