[MODERATED] Additional sampling fun

On Thu, Feb 20, 2020 at 09:14:20AM +0100, speck for Greg KH wrote:
> Then we need to stop using RDRAND internally for our "give me a random
> number api" which has spread to more and more parts of the kernel.
> 
> Here's a patch that does so:
> 	https://lore.kernel.org/lkml/[email protected]/
> which I'm going to advise get merged now and backported to the stable
> branches.

So one of our guys - Nicolai Stange - was looking at this
wrt backporting it to trees and there's another problem in
add_interrupt_randomness() which could potentially turn out
to be nasty.

We asked him to write it up for speck@ (he's not subscribed) so that we
can discuss it here first. Here is the deal in his own words:

"In the context of the get_random_long() patch posted at [1], I noticed
that there's also a RDSEED insn issued from the interrupt path, which
perhaps might have undesired effects performance-wise.

More specifically, add_interrupt_randomness() would issue one RDSEED
either once a second or every 64 interrupts, whichever comes first:

  void add_interrupt_randomness(int irq, int irq_flags)
  {
    fast_mix(fast_pool); /* increments fast_pool->count */
    ...
    if ((fast_pool->count < 64) &&
         !time_after(now, fast_pool->last + HZ))
      return;
    ...
    fast_pool->last = now;
    if (arch_get_random_seed_long(&seed)) {}
    ...
  }

So while this certainly won't matter much on average, I'm still
wondering whether or not this RDSEED could potentially cause IRQ
latency spikes relevant e.g. to -RT and/or under high IRQ load?

FWIW, the commit introducing the arch_get_random_seed_long() invocation
to add_interrupt_randomness() was commit 83664a6928a4 ("random: Use
arch_get_random_seed*() at init time and once a second").

I can only guess, but I think the motivation for mixing
arch_get_random_seed_long() from the interrupt path was probably to
sync that with the IRQ rate. That is, to make sure that the entropy
mixed from RDSEED doesn't dominate the interrupt entropy source.

[1] https://lkml.kernel.org/r/[email protected]
"

Thx.

-- 
Regards/Gruss,
    Boris.

SUSE Software Solutions Germany GmbH, GF: Felix Imendörffer, HRB 36809, AG Nürnberg
--