Re: [PATCH v2] random: avoid mis-detecting a slow counter as a cycle counter

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Apr 22, 2022 at 01:40:25AM +0200, Jason A. Donenfeld wrote:
> Hi Eric,
> 
> Thanks. This looks better.
> 
> On Thu, Apr 21, 2022 at 04:31:52PM -0700, Eric Biggers wrote:
> > Therefore, increase the number of counter comparisons from 1 to 3, to
> > greatly reduce the rate of false positive cycle counter detections.
> > +	for (i = 0; i < 3; i++) {
> > +		unsigned long entropy = random_get_entropy();
>  
> Wondering: why do you do 3 comparisons rather than 2? What does 3 get
> you that 2 doesn't already? I thought the only real requirement was that
> in the event where (a)!=(b), (b) is read as meaningfully close as
> possible to when the counter changes.
> 

On CONFIG_PREEMPT kernels this code usually runs with preemption enabled, so I
don't think it's guaranteed that any particular number of comparisons will be
sufficient, since the task could get preempted for a long time between each call
to random_get_entropy().  However, the chance of a false positive should
decrease exponentially, and should be pretty small in the first place, so 3
comparisons seems like a good number.

We could also disable IRQs while checking, if you'd prefer to go that route.  We
would still need 2 comparisons.

- Eric