[PATCH v3 6/7] KVM: SEV: Don't advertise VM types that are disabled by firmware

[Sean Christopherson <seanjc@google.com>]

From: Tycho Andersen <tycho@kernel.org>

As called out in a footnote for a recent SNP vulnerability[1], it is
possible for a specific flavor of SEV+ to be disabled by the firmware even
when the flavor is fully supported by the CPU and platform:

  Applying mitigation CVE-2025-48514 will result in disabling SEV-ES when
  SEV-SNP is enabled.

Restrict KVM's set of supported VM types based on the VM types that are
fully supported by firmware to avoid over-reporting what KVM can actually
support.  Like KVM's handling of ASID space exhaustion, don't modify KVM's
CPUID capabilities, as the CPU/platform still supports the underlying
technology and clearing e.g. SEV_ES while advertising SEV_SNP would confuse
KVM and userspace.

Link: https://www.amd.com/en/resources/product-security/bulletin/amd-sb-3023.html [1]
Link: https://lore.kernel.org/all/aZyLIWtffvEnmtYh@google.com
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
[sean: rewrite changelog to provide details on why/how this can happen]
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 0971cf652b0b..ab386aa0c284 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3202,6 +3202,7 @@ void __init sev_hardware_setup(void)
 		vm_types |= BIT(KVM_X86_SEV_ES_VM);
 	if (sev_snp_supported)
 		vm_types |= BIT(KVM_X86_SNP_VM);
+	vm_types &= sev_firmware_supported_vm_types();
 
 	kvm_caps.supported_vm_types |= vm_types;
 
-- 
2.54.0.rc1.513.gad8abe7a5a-goog