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    SubjectRe: [PATCH] random: do crng pre-init loading in worker rather than irq
    On Thu, Feb 24, 2022 at 4:11 PM Dominik Brodowski
    <linux@dominikbrodowski.net> wrote:
    >
    > Am Thu, Feb 24, 2022 at 10:49:12AM +0100 schrieb Jason A. Donenfeld:
    > > On 2/24/22, Dominik Brodowski <linux@dominikbrodowski.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@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?
    > >
    > > 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.

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