On Fri, Jan 05, 2018 at 09:24:31PM +0100, Willy Tarreau wrote:
> On Fri, Jan 05, 2018 at 07:58:04PM +0000, Alan Cox wrote:
> > and the techniques
> > to deal with rdtsc disabling are well known and used in other existing
> > attacks.
> 
> Yes i've tested one of them for the spectre poc, but it really did not
> work well, leading to about 1 among 10 bytes only to be valid. In fact
> either you run the counter thread on the other sibling of the same core
> and it significantly perturbates the local activity, or you run it on
> another core, and the time it takes to retrieve the time requires some
> L1+L2 traversal. I'm not saying it doesn't work at all, I'm saying that
> the accuracy is highly degraded and that can turn something 100%
> reproducible into something requiring a long time to run, making the
> attack more noticeable (and possibly letting observed data degrade
> during the period).

So I worked on an improved RDTSC emulation (attached) and it works
reasonably well on the spectre poc found online. Its accuracy is almost
as good as rdtsc on my i7-6700k on two threads running on the same core,
and 8-10 times worse on two distinct cores, but still leads to ~50%
success rate on the PoC. So my conclusion now is that it's indeed
pointless to invest time trying to make RDTSC less accessible/accurate.

Willy