Re: [PING][PATCH v3] x86: Use `get_random_u8' for kernel stack offset randomization

[Maciej W. Rozycki]

On Mon, 13 Feb 2023, Thomas Gleixner wrote:

> >> For x86 kernel stack offset randomization uses the RDTSC instruction, 
> >> which according to H. Peter Anvin is not a secure source of entropy:
> >> 
> >> "RDTSC isn't a super fast instruction either, but what is *way* more
> >> significant is that this use of RDTSC is NOT safe: in certain power states
> >> it may very well be that stone number of lower bits of TSC contain no
> >> entropy at all."
> >
> >  Ping for:
> > <https://lore.kernel.org/all/alpine.DEB.2.21.2301302011150.55843@xxxxxxxxxxxxxxxxx/>.
> 
> I'm waiting for you to address Peter Anvins feedback.

 Do you mean this part:

On Tue, 31 Jan 2023, H. Peter Anvin wrote:

> Well, what I said was that masking out the low bits of TSC is not a valid use to
> extract a random(-ish) number this way, because the lower bits may be affected
> by quantization. Something like a circular multiply using a large prime with a
> good 0:1 balance can be used to mitigate that.
> 
> However, the second part is that subsequent RDTSCs will be highly correlated,
> and so a CSPRNG is needed if you are actually trying to get reasonable security
> this way – and, well, we already have one of those.

?  Well, I inferred, perhaps incorrectly, from the second paragraph that 
Peter agrees with my approach (with the CSPRNG being what `get_random_u8' 
and friends get at).

> You also cite him
> w/o providing a link to the conversation, so any context is missing.

 Sorry about that.  I put the change heading for the previous iterations 
in the change log, but I agree actual web links would've been better:
<https://lore.kernel.org/all/alpine.DEB.2.21.2301081919550.65308@xxxxxxxxxxxxxxxxx/>,
<https://lore.kernel.org/all/alpine.DEB.2.21.2301082113350.65308@xxxxxxxxxxxxxxxxx/>.

 Please let me know if you need anything else.  Thank you for your review.

  Maciej