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SubjectRe: [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@dominikbrodowski.net>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Sultan Alsawaf <sultan@kerneltoast.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.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@dominikbrodowski.net>

Thanks
Dominik

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