linux-kernel - Re: [PATCH] random: split initialization into early arch step and later non-arch step

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Message-ID: <CAHmME9oJEMwO1NJ0m-J7yWwGrh8su51TB+pC8pGFYn2uE1xB5Q@mail.gmail.com>
Date:   Mon, 26 Sep 2022 18:29:32 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        stable@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [PATCH] random: split initialization into early arch step and
 later non-arch step

On Mon, Sep 26, 2022 at 6:22 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Mon, 26 Sep 2022 18:03:32 +0200 "Jason A. Donenfeld" <Jason@...c4.com> wrote:
>
> > The full RNG initialization relies on some timestamps, made possible
> > with general functions like time_init() and timekeeping_init(). However,
> > these are only available rather late in initialization. Meanwhile, other
> > things, such as memory allocator functions, make use of the RNG much
> > earlier.
> >
> > So split RNG initialization into two phases. We can give arch randomness
> > very early on, and then later, after timekeeping and such are available,
> > initialize the rest.
> >
> > This ensures that, for example, slabs are properly randomized if RDRAND
> > is available. Another positive consequence is that on systems with
> > RDRAND, running with CONFIG_WARN_ALL_UNSEEDED_RANDOM=y results in no
> > warnings at all.
>
> Please give a full description of the user-visible runtime effects of
> this shortcoming.

Sure. I'll expand that paragraph to read:

This ensures that, for example, slabs are properly randomized if RDRAND
is available. Without this, CONFIG_SLAB_FREELIST_RANDOM=y loses a degree
of its security, because its random seed is potentially deterministic,
since it hasn't yet incorporated RDRAND. It also makes it possible to
use a better seed in kfence, which currently relies on only the cycle
counter.
Another positive consequence is that on systems with RDRAND, running
with CONFIG_WARN_ALL_UNSEEDED_RANDOM=y results in no warnings at all.