linux-kernel - Re: [PATCH v2 6/9] random: absorb fast pool into input pool after fast load

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Message-ID: <YgN7TvD5rs/5i1dQ@owl.dominikbrodowski.net>
Date:   Wed, 9 Feb 2022 09:29:02 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        tytso@....edu, ebiggers@...nel.org,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v2 6/9] random: absorb fast pool into input pool after
 fast load

Am Wed, Feb 09, 2022 at 02:19:16AM +0100 schrieb Jason A. Donenfeld:
> During crng_init == 0, we never credit entropy in add_interrupt_
> randomness(), but instead dump it directly into the base_crng. That's
> fine, except for the fact that we then wind up throwing away that
> entropy later when we switch to extracting from the input pool and
> overwriting the base_crng key. The two other early init sites --
> add_hwgenerator_randomness()'s use crng_fast_load() and add_device_
> randomness()'s use of crng_slow_load() -- always additionally give their
> inputs to the input pool. But not add_interrupt_randomness().

Hm, up to this patch there is no base_crng key. So maybe change the ordering
of the patches?

> This commit fixes that shortcoming by calling mix_pool_bytes() after
> crng_fast_load() in add_interrupt_randomness(). That's partially
> verboten on PREEMPT_RT, where it implies taking spinlock_t from an IRQ
> handler. But this also only happens during early boot and then never
> again after that. Should this turn into a larger problem later for some
> esoteric reason, we can always use `if (IS_ENABLED(PREEMPT_RT))` or
> something similar to that. But for now, this should do.
> 
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> Suggested-by: Eric Biggers <ebiggers@...gle.com>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  drivers/char/random.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 785a4545c9d7..20374bce9a07 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -864,6 +864,13 @@ void add_interrupt_randomness(int irq)
>  		    crng_fast_load((u8 *)fast_pool->pool, sizeof(fast_pool->pool)) > 0) {
>  			fast_pool->count = 0;
>  			fast_pool->last = now;
> +
> +			/* Technically this call means that we're using a spinlock_t
> +			 * in the IRQ handler, which isn't terrific for PREEMPT_RT.
> +			 * However, this only happens during very early boot, and then

Whether it's only during "very early" boot depends on how fast we progress
to crng_init = 2. So maybe just "during boot"?

Otherwise, all fine, so

	Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>

Thanks,
	Dominik