Hi Eric,
On 4/11/22, Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
On Fri, Apr 08, 2022 at 08:21:35PM +0200, Jason A. Donenfeld wrote:
By my first guess, we have ktime_get_boottime_ns(), jiffies, and
sched_clock(). It seems like sched_clock() has already done a lot of
work in being always available with some incrementing value, falling
back to jiffies as necessary. So this series goes with that as a
fallback, for when the architecture doesn't define random_get_entropy in
its own way and when there's no working cycle counter.
Won't this interact badly with how try_to_generate_entropy() (a.k.a. the
"Linus
Jitter Dance") detects the presence of an appropriate timer currently?
stack.cycles = random_get_entropy();
/* Slow counter - or none. Don't even bother */
if (stack.cycles == random_get_entropy())
return;
So if random_get_entropy() always returns 0, then try_to_generate_entropy()
won't run. However, if random_get_entropy() is even just a low-precision
timer,
then try_to_generate_entropy() will have a chance of running, since the
timer
might change between the two calls to random_get_entropy(). And if
try_to_generate_entropy() does run, then it credits 1 bit of entropy for
every
iteration, regardless of the timer's precision.
This is an existing problem, but this patchset will make it worse, as it
changes
a lot of cases from "no timer" to "low precision timer".
Perhaps try_to_generate_entropy() should check the timer at least 3 times
and
verify that it changed each time?
What you've identified is actually already the case for platforms
where the cycle counter is already just slow (and there are a few such
platforms; my odroid C2 even exhibits this). As you identified, the
cycle counter might already be too slow, yet we get [un]lucky and
reach this code right on the cusp or a change.
So the problem isn't new here, per say, for this patchset. But indeed
perhaps we should consider adjusting the heuristics for that a bit in
a separate patch. Your check three times idea seems like a good
starting point, if you want to send a patch and we can poke at it.
Jason
