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

[Peter Gonda]

On Fri, Nov 12, 2021 at 1:55 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> 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.

I agree, it seems better to terminate the single guest with an issue.
Rather than killing the host (and therefore all guests). So I'd
suggest even in this case we do the 'convert to shared' approach or
just outright terminate the guest.

Are there already examples in KVM of a KVM bug in servicing a VM's
request results in a BUG/panic/oops? That seems not ideal ever.

>
> >   - 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...

This seems like a reasonable approach that can help with the issue of
terminating the entity behaving poorly. Could this feature be an
improvement that comes later? This improvement could be gated behind a
per VM KVM CAP, a kvm module param, or insert other solution here, to
not blind side userspace with this change?

>
> > 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. ;-\

Yes #MC in TDX seems much harder to deal with than the #PF for SNP.
I'd propose TDX keeps the host kernel safe bug not have that solution
block SNP. As suggested above I like the idea but it seems like it can
come as a future improvement to SNP support.

>
> Thx.
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette