KVM: nVMX: Release eVMCS when enlightened VMENTRY was disabled

In theory, L1 can try to disable enlightened VMENTRY in VP assist page and
try to issue VMLAUNCH/VMRESUME. While nested_vmx_handle_enlightened_vmptrld()
properly handles this as 'EVMPTRLD_DISABLED', previously mapped eVMCS
remains mapped and thus all evmptr_is_valid() checks will still pass and
nested_vmx_run() will proceed when it shouldn't.

Release eVMCS immediately when we detect that enlightened vmentry was
disabled by L1.
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210526132026.270394-4-vkuznets@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -1979,8 +1979,10 @@ static enum nested_evmptrld_status nested_vmx_handle_enlightened_vmptrld(
 	if (likely(!vmx->nested.enlightened_vmcs_enabled))
 		return EVMPTRLD_DISABLED;
 
-	if (!nested_enlightened_vmentry(vcpu, &evmcs_gpa))
+	if (!nested_enlightened_vmentry(vcpu, &evmcs_gpa)) {
+		nested_release_evmcs(vcpu);
 		return EVMPTRLD_DISABLED;
+	}
 
 	if (unlikely(evmcs_gpa != vmx->nested.hv_evmcs_vmptr)) {
 		vmx->nested.current_vmptr = -1ull;