Re: [PATCH v10 03/50] KVM: SEV: Do not intercept accesses to MSR_IA32_XSS for SEV-ES guests

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/16/23 15:27, Michael Roth wrote:
> Address this by disabling intercepts of MSR_IA32_XSS for SEV-ES guests
> if the host/guest configuration allows it. If the host/guest
> configuration doesn't allow for MSR_IA32_XSS, leave it intercepted so
> that it can be caught by the existing checks in
> kvm_{set,get}_msr_common() if the guest still attempts to access it.

This is wrong, because it allows the guest to do untrapped writes to
MSR_IA32_XSS and therefore (via XRSTORS) to MSRs that the host might not
save or restore.

If the processor cannot let the host validate writes to MSR_IA32_XSS,
KVM simply cannot expose XSAVES to SEV-ES (and SEV-SNP) guests.

Because SVM doesn't provide a way to disable just XSAVES in the guest,
all that KVM can do is keep on trapping MSR_IA32_XSS (which the guest
shouldn't read or write to).  In other words the crash on accesses to
MSR_IA32_XSS is not a bug but a feature (of the hypervisor, that
wants/needs to protect itself just as much as the guest wants to).

The bug is that there is no API to tell userspace "do not enable this
and that CPUID for SEV guests", there is only the extremely limited
KVM_GET_SUPPORTED_CPUID system ioctl.

For now, all we can do is document our wishes, with which userspace had
better comply.  Please send a patch to QEMU that makes it obey.

Paolo

--------------------------- 8< -----------------------
 From 303e66472ddf54c2a945588b133d34eaab291257 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Wed, 13 Dec 2023 07:45:08 -0500
Subject: [PATCH] Documentation: KVM: suggest disabling XSAVES on SEV-ES guests

When intercepts are enabled for MSR_IA32_XSS, the host will swap in/out
the guest-defined values while context-switching to/from guest mode.
However, in the case of SEV-ES, vcpu->arch.guest_state_protected is set,
so the guest-defined value is effectively ignored when switching to
guest mode with the understanding that the VMSA will handle swapping
in/out this register state.

However, SVM is still configured to intercept these accesses for SEV-ES
guests, so the values in the initial MSR_IA32_XSS are effectively
read-only, and a guest will experience undefined behavior if it actually
tries to write to this MSR. Fortunately, only CET/shadowstack makes use
of this register on SEV-ES-capable systems currently, which isn't yet
widely used, but this may become more of an issue in the future.

Additionally, enabling intercepts of MSR_IA32_XSS results in #VC
exceptions in the guest in certain paths that can lead to unexpected #VC
nesting levels. One example is SEV-SNP guests when handling #VC
exceptions for CPUID instructions involving leaf 0xD, subleaf 0x1, since
they will access MSR_IA32_XSS as part of servicing the CPUID #VC, then
generate another #VC when accessing MSR_IA32_XSS, which can lead to
guest crashes if an NMI occurs at that point in time. Running perf on a
guest while it is issuing such a sequence is one example where these can
be problematic.

Unfortunately, there is not really a way to fix this issue; allowing
unfiltered access to MSR_IA32_XSS also lets the guest write (via
XRSTORS) MSRs that the host might not be ready to save or restore.
Because SVM doesn't provide a way to disable just XSAVES in the guest,
all that KVM can do to protect itself is keep on trapping MSR_IA32_XSS.
Userspace has to comply and not enable XSAVES in CPUID, so that the
guest has no business accessing MSR_IA32_XSS at all.

Unfortunately^2, there is no API to tell userspace "do not enable this
and that CPUID for SEV guests", there is only the extremely limited
KVM_GET_SUPPORTED_CPUID system ioctl.  So all we can do for now is
document it.

Reported-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/Documentation/virt/kvm/x86/errata.rst b/Documentation/virt/kvm/x86/errata.rst
index 49a05f24747b..0c91916c0164 100644
--- a/Documentation/virt/kvm/x86/errata.rst
+++ b/Documentation/virt/kvm/x86/errata.rst
@@ -33,6 +33,15 @@ Note however that any software (e.g ``WIN87EM.DLL``) expecting these features
  to be present likely predates these CPUID feature bits, and therefore
  doesn't know to check for them anyway.
  
+Encrypted guests
+~~~~~~~~~~~~~~~~
+
+For SEV-ES guests, it is impossible for KVM to validate writes for MSRs that
+are part of the VMSA.  In the case of MSR_IA32_XSS, however, KVM needs to
+validate writes to the MSR in order to prevent the guest from using XRSTORS
+to overwrite host MSRs.  Therefore, the XSAVES feature should never be exposed
+to SEV-ES guests.
+
  Nested virtualization features
  ------------------------------