linux-kernel - Re: [PATCH v3] random: split initialization into early step and later step

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Message-ID: <202209270728.5FE3CB0@keescook>
Date:   Tue, 27 Sep 2022 07:28:25 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v3] random: split initialization into early step and
 later step

On Tue, Sep 27, 2022 at 01:09:57PM +0200, Jason A. Donenfeld 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. 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.
> 
> One subtle side effect of this change is that on systems with no RDRAND,
> RDTSC is now only queried by random_init() once, committing the moment
> of the function call, instead of multiple times as before. This is
> intentional, as the multiple RDTSCs in a loop before weren't
> accomplishing very much, with jitter being better provided by
> try_to_generate_entropy(). Plus, filling blocks with RDTSC is still
> being done in extract_entropy(), which is necessarily called before
> random bytes are served anyway.
> 
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

Thanks for the updates!

-- 
Kees Cook