[MODERATED] Re: [patch 3/8] MDS basics 3

[Andi Kleen <ak@linux.intel.com>]

Thomas,

> Please provide a conclusive technical explanation why a NMI entered from
> user space and returning to that would expose sensible information to an
> attacker in an exploitable way.

Okay so you're not actually implementing full, like you stated earlier,
but a strange undocumented minimal version of lazy.

Lazy is always a trade off. Since we still run with a single address
space the CPU can always do some prefetching or speculative execution
and fetch something nearby that the code doesn't actually access intentionally,
which may end up leaking through some buffer.

In such a case even a NMI could leak.

Of course such a case is unlikely, but in theory it could happen.

The intention of the full option was to allow an option 
for people who cannot accept any risk at all. Of course
that's not the right approach for most users, but for a few
it might be.

If you implement full you should implement it properly
and not leave holes.

If you're not interested in code review please state so and I will
not bother anymore.

-Andi