[PATCH v4 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 NMI by
guest vCPUs. This MSR is populated by the guest and the hypervisor
should not intercept it. A #VC exception will be generated otherwise.
If this occurs and Secure AVIC is enabled, terminate guest execution.

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
---
Changes since v3:

 - Changed "fallthrough" to "break" for MSR_AMD64_SECURE_AVIC_CONTROL
   "case" in __vc_handle_msr().

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

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 49cf0f97e372..a2d670ceef2f 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -1398,6 +1398,15 @@ static enum es_result __vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt
 		if (sev_status & MSR_AMD64_SNP_SECURE_TSC)
 			return __vc_handle_secure_tsc_msrs(regs, write);
 		break;
+	case MSR_AMD64_SECURE_AVIC_CONTROL:
+		/*
+		 * AMD64_SECURE_AVIC_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