Re: [PATCH 1/2] x86/random: Retry on RDSEED failure

Theodore Ts'o
2024-02-14T20:11:43Z

On Wed, Feb 14, 2024 at 09:04:34PM +0100, Jason A. Donenfeld wrote:
> AMD people, Intel people: what are the fullest statements we can rely
> on here? Do the following two statements work?
>
> 1) On newer chips, RDRAND never fails.
> 2) On older chips, RDRAND never fails if you try 10 times in a loop,
> unless you consider host->guest attacks, which we're not, because CoCo
> is only a thing on the newer chips.
>
> If those hold true, then the course of action would be to just add a
> WARN_ON(!ok) but keep the loop as-is.

I think we may only want to do the WARN_ON in early boot. Otherwise,
on older chips, if a userspace process executes RDRAND is a tight
loop, it might cause the WARN_ON to trigger, which is considered
undesirable (and is certainly going to be something that could result
in a syzbot complaint).

- Ted