[PATCH v10 17/18] x86/sev: Prevent SECURE_AVIC_CONTROL MSR interception for Secure AVIC guests

[Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>]

The SECURE_AVIC_CONTROL MSR holds the GPA of the guest APIC backing
page and bitfields to control enablement of Secure AVIC and whether
the guest allows NMIis to be injected by the hypervisor. This MSR is
populated by the guest and can be read by the guest to get the GPA
of the APIC backing page. The MSR can only be accessed in Secure AVIC
mode. Any attempt to access it when not in Secure AVIC mode results
in #GP. So, the hypervisor should not intercept it. A #VC exception
will be generated otherwise. If this occurs and Secure AVIC is enabled,
terminate the guest execution.

Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
---
Changes since v9:
 - Update commit log to explain why SECURE_AVIC_CONTROL MSR
   should not be intercepted by the hypervisor.

 arch/x86/coco/sev/vc-handle.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/coco/sev/vc-handle.c b/arch/x86/coco/sev/vc-handle.c
index c1aa10ce9d54..0fd94b7ce191 100644
--- a/arch/x86/coco/sev/vc-handle.c
+++ b/arch/x86/coco/sev/vc-handle.c
@@ -415,6 +415,15 @@ enum es_result sev_es_ghcb_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt
 		if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
 			return __vc_handle_secure_tsc_msrs(ctxt, write);
 		break;
+	case MSR_AMD64_SAVIC_CONTROL:
+		/*
+		 * AMD64_SAVIC_CONTROL should not be intercepted when
+		 * Secure AVIC is enabled. Terminate the Secure AVIC guest
+		 * if the interception is enabled.
+		 */
+		if (cc_platform_has(CC_ATTR_SNP_SECURE_AVIC))
+			return ES_VMM_ERROR;
+		break;
 	default:
 		break;
 	}
-- 
2.34.1