Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall

[Andy Lutomirski]

On Thu, May 9, 2019 at 1:43 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Reshetova, Elena <elena.reshetova@xxxxxxxxx> wrote:
>
> > > I find it ridiculous that even with 4K blocked get_random_bytes(),
> > > which gives us 32k bits, which with 5 bits should amortize the RNG
> > > call to something like "once per 6553 calls", we still see 17%
> > > overhead? It's either a measurement artifact, or something doesn't
> > > compute.
> >
> > If you check what happens underneath of get_random_bytes(), there is a
> > fair amount of stuff that is going on, including reseeding CRNG if
> > reseeding interval has passed (see _extract_crng()). It also even
> > attempts to stir in more entropy from rdrand if avalaible:
> >
> > I will look into this whole construction slowly now to investigate. I
> > did't optimize anything yet also (I take 8 bits at the time for
> > offset), but these small optimization won't make performance impact
> > from 17% --> 2%, so pointless for now, need a more radical shift.
>
> So assuming that the 17% overhead primarily comes from get_random_bytes()
> (does it? I don't know), that's incredibly slow for something like the
> system call entry path, even if it's batched.
>

ISTM maybe a better first step would be to make get_random_bytes() be
much faster? :)

--Andy