linux-kernel - Re: [RFC PATCH v3 0/6] Direct Map Removal for guest

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Message-ID: <37fbfc65-b145-4a22-a48c-1921204d5635@amazon.com>
Date: Fri, 1 Nov 2024 11:31:09 -0700
From: "Manwaring, Derek" <derekmn@...zon.com>
To: <dave.hansen@...el.com>
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Subject: Re: [RFC PATCH v3 0/6] Direct Map Removal for guest_memfd

On 2024-11-01 at 17:20+0000, Dave Hansen wrote:
> On 11/1/24 09:56, Manwaring, Derek wrote:
> > 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