2022-05-03 21:25:13

by Jason A. Donenfeld

[permalink] [raw]
Subject: [PATCH] random: mix hwgenerator randomness before sleeping

The add_hwgenerator_randomness() function is called in a loop from a
kthread by the hwgenerator core. It's supposed to sleep when there's
nothing to do, and wake up periodically for more entropy. Right now it
receives entropy, sleeps, and then mixes it in. This commit reverses the
order, so that it always mixes in entropy sooner and sleeps after. This
way the entropy is more fresh.

Cc: Dominik Brodowski <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>
---
drivers/char/random.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 845f610b6611..d4eae4b167b4 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1154,6 +1154,9 @@ void rand_initialize_disk(struct gendisk *disk)
void add_hwgenerator_randomness(const void *buffer, size_t count,
size_t entropy)
{
+ mix_pool_bytes(buffer, count);
+ credit_entropy_bits(entropy);
+
/*
* Throttle writing if we're above the trickle threshold.
* We'll be woken up again once below POOL_MIN_BITS, when
@@ -1164,8 +1167,6 @@ void add_hwgenerator_randomness(const void *buffer, size_t count,
!system_wq || kthread_should_stop() ||
input_pool.entropy_count < POOL_MIN_BITS,
CRNG_RESEED_INTERVAL);
- mix_pool_bytes(buffer, count);
- credit_entropy_bits(entropy);
}
EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);

--
2.35.1


2022-05-04 01:17:20

by Jason A. Donenfeld

[permalink] [raw]
Subject: Re: [PATCH] random: mix hwgenerator randomness before sleeping

Hi Dominik,

On Tue, May 3, 2022 at 9:56 PM Dominik Brodowski
<[email protected]> wrote:
>
> Am Tue, May 03, 2022 at 09:51:41PM +0200 schrieb Jason A. Donenfeld:
> > The add_hwgenerator_randomness() function is called in a loop from a
> > kthread by the hwgenerator core. It's supposed to sleep when there's
> > nothing to do, and wake up periodically for more entropy. Right now it
> > receives entropy, sleeps, and then mixes it in. This commit reverses the
> > order, so that it always mixes in entropy sooner and sleeps after. This
> > way the entropy is more fresh.
>
> ... however, the hwgenerator may take quite some time to accumulate entropy
> after wakeup. So now we might have a delay between a wakeup ("we need more
> entropy!") and that entropy becoming available. Beforehand, the thread only
> went to sleep when there is no current need for "fresh" entropy.

Huh, interesting consideration. I didn't think about that. You wrote,
"hwgenerator may take quite some time to accumulate entropy" -- any
idea how long in the worst case? A second? A minute?

Jason