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

[Manwaring, Derek]

On 2024-11-08 at 10:36, Elena Reshetova wrote:
> On 2024-11-06 at 17:04, Derek Manwaring wrote:
> > 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.

Thanks, that makes sense. I'm a little hyperfocused on guest->host which
is the opposite direction of what is generally used for the CoCo threat
model. I think what both cases care about though is guest->guest. For
me, guest->host matters because it's a route for guest->guest (at least
in the non-CoCo world). There's some good discussion on this in David's
series on attack vector controls [1].

Like you mention, beyond direction it matters which CoCo countermeasures
are at play and how far they go during transient execution. That part is
not clear to me for the host->guest direction involving the direct map,
but agree any countermeasures like direct map removal should be
evaluated based on a better understanding of what those existing
countermeasures already cover and what attack is intended to be
mitigated.

Derek


[1] https://lore.kernel.org/lkml/LV3PR12MB92658EA6CCF18F90DAAA168394532@LV3PR12MB9265.namprd12.prod.outlook.com/