Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

Borislav Petkov
2021-11-12T20:55:48Z

On Fri, Nov 12, 2021 at 08:37:59PM +0000, Sean Christopherson wrote:
> Let userspace decide what is mapped shared and what is mapped private.

With "userspace", you mean the *host* userspace?

> The kernel and KVM provide the APIs/infrastructure to do the actual
> conversions in a thread-safe fashion and also to enforce the current
> state, but userspace is the control plane.
>
> It would require non-trivial changes in userspace if there are multiple processes
> accessing guest memory, e.g. Peter's networking daemon example, but it _is_ fully
> solvable. The exit to userspace means all three components (guest, kernel,
> and userspace) have full knowledge of what is shared and what is private. There
> is zero ambiguity:
>
> - if userspace accesses guest private memory, it gets SIGSEGV or whatever.

That SIGSEGV is generated by the host kernel, I presume, after it checks
whether the memory belongs to the guest?

> - if kernel accesses guest private memory, it does BUG/panic/oops[*]

If *it* is the host kernel, then you probably shouldn't do that -
otherwise you just killed the host kernel on which all those guests are
running.

> - if guest accesses memory with the incorrect C/SHARED-bit, it gets killed.

Yah, that's the easy one.

> This is the direction KVM TDX support is headed, though it's obviously still a WIP.
>
> And ideally, to avoid implicit conversions at any level, hardware vendors' ABIs
> define that:
>
> a) All convertible memory, i.e. RAM, starts as private.
> b) Conversions between private and shared must be done via explicit hypercall.

I like the explicit nature of this but devil's in the detail and I'm no
virt guy...

> Without (b), userspace and thus KVM have to treat guest accesses to the incorrect
> type as implicit conversions.
>
> [*] Sadly, fully preventing kernel access to guest private is not possible with
> TDX, especially if the direct map is left intact. But maybe in the future
> TDX will signal a fault instead of poisoning memory and leaving a #MC mine.

Yah, the #MC thing sounds like someone didn't think things through. ;-\

Thx.

--
Regards/Gruss,
Boris.