KVM: nVMX: Truncate writes to vmcs.SYSENTER_EIP/ESP for 32-bit vCPU

Explicitly truncate the data written to vmcs.SYSENTER_EIP/ESP on WRMSR
if the virtual CPU doesn't support 64-bit mode.  The SYSENTER address
fields in the VMCS are natural width, i.e. bits 63:32 are dropped if the
CPU doesn't support Intel 64 architectures.  This behavior is visible to
the guest after a VM-Exit/VM-Exit roundtrip, e.g. if the guest sets bits
63:32 in the actual MSR.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200428231025.12766-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -1943,6 +1943,16 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	return 0;
 }
 
+static u64 nested_vmx_truncate_sysenter_addr(struct kvm_vcpu *vcpu,
+						    u64 data)
+{
+#ifdef CONFIG_X86_64
+	if (!guest_cpuid_has(vcpu, X86_FEATURE_LM))
+		return (u32)data;
+#endif
+	return (unsigned long)data;
+}
+
 /*
  * Writes msr value into the appropriate "register".
  * Returns 0 on success, non-0 otherwise.
@@ -1980,13 +1990,17 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		vmcs_write32(GUEST_SYSENTER_CS, data);
 		break;
 	case MSR_IA32_SYSENTER_EIP:
-		if (is_guest_mode(vcpu))
+		if (is_guest_mode(vcpu)) {
+			data = nested_vmx_truncate_sysenter_addr(vcpu, data);
 			get_vmcs12(vcpu)->guest_sysenter_eip = data;
+		}
 		vmcs_writel(GUEST_SYSENTER_EIP, data);
 		break;
 	case MSR_IA32_SYSENTER_ESP:
-		if (is_guest_mode(vcpu))
+		if (is_guest_mode(vcpu)) {
+			data = nested_vmx_truncate_sysenter_addr(vcpu, data);
 			get_vmcs12(vcpu)->guest_sysenter_esp = data;
+		}
 		vmcs_writel(GUEST_SYSENTER_ESP, data);
 		break;
 	case MSR_IA32_DEBUGCTLMSR: