On Thu, Feb 24, 2022 at 4:11 PM Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> wrote: > > Am Thu, Feb 24, 2022 at 10:49:12AM +0100 schrieb Jason A. Donenfeld: > > On 2/24/22, Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> > > >> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > > >> Cc: Sultan Alsawaf <sultan@xxxxxxxxxxxxxxx> > > >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > >> Cc: Theodore Ts'o <tytso@xxxxxxx> > > >> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> > > >> --- > > >> 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.
