On 2024-11-01 at 17:20+0000, Dave Hansen wrote:

On 11/1/24 09:56, Manwaring, Derek wrote:

quoted

But if other mitigations completely prevent even speculative access
of TD private memory like you're saying, then agree nothing to gain
from direct map removal in the TDX case.

Remember, guest unmapping is done in the VMM.  The VMM is not trusted in
the TDX (or SEV-SNP) model.  If any VMM can harm the protections on
guest memory, then we have a big problem.

That isn't to say big problem can't happen.  Say some crazy attack comes
to light where the VMM can attack TDX if the VMM has mapping for a guest
(or TDX module) memory.  Crazier things have happened, and guest
unmapping _would_ help there, if you trusted the VMM.

Basically, I think guest unmapping only helps system security as a whole
if you must _already_ trust the VMM.

Yeah that makes a lot of sense. I just view the ideal outcome as a
composition of strong, independent defenses. So as a guest you have the
confidentiality and integrity guarantees of the hardware, *and* you have
an up-to-date, good-hygiene (albeit not attested) host kernel just in
case some crazy attack/gap comes up.

From that standpoint I'm still tempted to turn the question around a bit
for the host kernel's perspective. Like if the host kernel should not
(and indeed cannot with TDX controls in place) access guest private
memory, why not remove it from the direct map?

Derek