linux-kernel - Re: [PATCH] random: do crng pre-init loading in worker rather than irq

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Message-ID: <Yhc4LwK3biZFIqwQ@owl.dominikbrodowski.net>
Date:   Thu, 24 Feb 2022 08:47:59 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        bigeasy@...utronix.de, Sultan Alsawaf <sultan@...neltoast.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] random: do crng pre-init loading in worker rather than
 irq

Am Wed, Feb 23, 2022 at 07:55:11PM +0100 schrieb Jason A. Donenfeld:
> Taking spinlocks from IRQ context is problematic for PREEMPT_RT. That
> is, in part, why we take trylocks instead. But apparently this still
> trips up various lock dependency analyzers. That seems like a bug in the
> analyzers that should be fixed, rather than having to change things
> here.
> 
> But maybe there's another reason to change things up: by deferring the
> crng pre-init loading to the worker, we can use the cryptographic hash
> function rather than xor, which is perhaps a meaningful difference when
> considering this data has only been through the relatively weak
> fast_mix() function.
> 
> The biggest downside of this approach is that the pre-init loading is
> now deferred until later, which means things that need random numbers
> after interrupts are enabled, but before workqueues are running -- or
> before this particular worker manages to run -- are going to get into
> trouble. Hopefully in the real world, this window is rather small,
> especially since this code won't run until 64 interrupts had occurred.
> 
> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Cc: Sultan Alsawaf <sultan@...neltoast.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Theodore Ts'o <tytso@....edu>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  drivers/char/random.c | 62 ++++++++++++-------------------------------
>  1 file changed, 17 insertions(+), 45 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 536237a0f073..9fb06fc298d3 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1298,7 +1278,12 @@ static void mix_interrupt_randomness(struct work_struct *work)
>  	local_irq_enable();
>  
>  	mix_pool_bytes(pool, sizeof(pool));
> -	credit_entropy_bits(1);
> +
> +	if (unlikely(crng_init == 0))
> +		crng_pre_init_inject(pool, sizeof(pool), true);
> +	else
> +		credit_entropy_bits(1);
> +
>  	memzero_explicit(pool, sizeof(pool));
>  }

Might it make sense to call crng_pre_init_inject() before mix_pool_bytes?
Otherwise, all looks fine:

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

Thanks
	Dominik