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

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Message-ID: <CAHmME9pTfwbs9xUJy_jrdPcrhSyVixSXBM==9EuB8v7ufWe9Pw@mail.gmail.com>
Date:   Thu, 24 Feb 2022 16:15:28 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Sebastian Siewior <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

On Thu, Feb 24, 2022 at 4:11 PM Dominik Brodowski
<linux@...inikbrodowski.net> wrote:
>
> Am Thu, Feb 24, 2022 at 10:49:12AM +0100 schrieb Jason A. Donenfeld:
> > On 2/24/22, Dominik Brodowski <linux@...inikbrodowski.net> wrote:
> > > 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?
> >
> > What exactly is the difference you see mattering in the order? I keep
> > chasing my tail trying to think about it.
>
> We had that order beforehand -- and even if it probably doesn't matter, this
> means crng_pre_init_inject() gets called a tiny bit earlier. That means
> there's a chance to progres to crng_init=1 a tiny bit earlier as well.

Alright, I'll send a v2.