[RFC PATCH v3 0/6] Direct Map Removal for guest_memfd

[Reshetova, Elena]

> On 2024-11-04 at 08:33+0000, Elena Reshetova wrote:
> > This statement *is* for integrity section. We have a separate TDX guidance
> > on side-channels (including speculative) [3] and some speculative attacks
> > that affect confidentiality (for example spectre v1) are listed as not covered
> > by TDX but remaining SW responsibility (as they are now).
> 
> Thanks for the additional info, Elena. Given that clarification, I
> definitely see direct map removal and TDX as complementary.

Jus to clarify to make sure my comment is not misunderstood.
What I meant is that we cannot generally assume that confidentiality
leaks from CoCo guests to host/VMM via speculative channels
are completely impossible. Spectre V1 is a prime example of such a
possible leak. Dave also elaborated on other potential vectors earlier.

The usefulness of direct map removal for CoCo guests as a concrete
mitigation for certain types of memory attacks must be precisely
evaluated per each attack vector, attack vector direction (host -> guest,
guest ->host, etc) and per each countermeasure that CoCo vendors
provide for each such case. I don't know if there is any existing study
that examines this for major CoCo vendors. I think this is what must
be done for this work in order to have a strong reasoning for its usefulness.

Best Regards,
Elena.