Re: [PATCH] KVM: SVM: Do not terminate SEV-ES guests on GHCB validation failure

[Sean Christopherson <seanjc@google.com>]

On Thu, May 20, 2021, Sean Christopherson wrote:
> On Thu, May 20, 2021, Sean Christopherson wrote:
> > On Mon, May 17, 2021, Tom Lendacky wrote:
> > > On 5/14/21 6:06 PM, Peter Gonda wrote:
> > > > On Fri, May 14, 2021 at 1:22 PM Tom Lendacky <thomas.lendacky@amd.com> wrote:
> > > >>
> > > >> Currently, an SEV-ES guest is terminated if the validation of the VMGEXIT
> > > >> exit code and parameters fail. Since the VMGEXIT instruction can be issued
> > > >> from userspace, even though userspace (likely) can't update the GHCB,
> > > >> don't allow userspace to be able to kill the guest.
> > > >>
> > > >> Return a #GP request through the GHCB when validation fails, rather than
> > > >> terminating the guest.
> > > > 
> > > > Is this a gap in the spec? I don't see anything that details what
> > > > should happen if the correct fields for NAE are not set in the first
> > > > couple paragraphs of section 4 'GHCB Protocol'.
> > > 
> > > No, I don't think the spec needs to spell out everything like this. The
> > > hypervisor is free to determine its course of action in this case.
> > 
> > The hypervisor can decide whether to inject/return an error or kill the guest,
> > but what errors can be returned and how they're returned absolutely needs to be
> > ABI between guest and host, and to make the ABI vendor agnostic the GHCB spec
> > is the logical place to define said ABI.
> > 
> > For example, "injecting" #GP if the guest botched the GHCB on #VMGEXIT(CPUID) is
> > completely nonsensical.  As is, a Linux guest appears to blindly forward the #GP,
> > which means if something does go awry KVM has just made debugging the guest that
> > much harder, e.g. imagine the confusion that will ensue if the end result is a
> > SIGBUS to userspace on CPUID.
> > 
> > There needs to be an explicit error code for "you gave me bad data", otherwise
> > we're signing ourselves up for future pain.
> 
> More concretely, I think the best course of action is to define a new return code
> in SW_EXITINFO1[31:0], e.g. '2', with additional information in SW_EXITINFO2.
> 
> In theory, an old-but-sane guest will interpret the unexpected return code as
> fatal to whatever triggered the #VMGEXIT, e.g. SIGBUS to userspace.  Unfortunately
> Linux isn't sane because sev_es_ghcb_hv_call() assumes any non-'1' result means
> success, but that's trivial to fix and IMO should be fixed irrespective of where
> this goes.

One last thing (hopefully): Erdem pointed out that if the GCHB GPA (or any
derferenced pointers within the GHCB) is invalid or is set to a private GPA
(mostly in the context of SNP) then the VMM will likely have no choice but to
kill the guest in response to #VMGEXIT.

It's probably a good idea to add a blurb in one of the specs explicitly calling
out that #VMGEXIT can be executed from userspace, and that before returning to
uesrspace the guest kernel must always ensure that the GCHB points at a legal
GPA _and_ all primary fields are marked invalid.