KVM: nVMX: check for invalid hdr.vmx.flags

hdr.vmx.flags is meant for future extensions to the ABI, rejecting
invalid flags is necessary to avoid broken half-loads of the
nVMX state.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -6079,6 +6079,9 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
 	    ~(KVM_STATE_NESTED_SMM_GUEST_MODE | KVM_STATE_NESTED_SMM_VMXON))
 		return -EINVAL;
 
+	if (kvm_state->hdr.vmx.flags & ~KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE)
+		return -EINVAL;
+
 	/*
 	 * SMM temporarily disables VMX, so we cannot be in guest mode,
 	 * nor can VMLAUNCH/VMRESUME be pending.  Outside SMM, SMM flags

tools/testing/selftests/kvm/x86_64/vmx_set_nested_state_test.c

@@ -146,6 +146,11 @@ void test_vmx_nested_state(struct kvm_vm *vm)
 	state->hdr.vmx.smm.flags = 1;
 	test_nested_state_expect_einval(vm, state);
 
+	/* Invalid flags are rejected. */
+	set_default_vmx_state(state, state_sz);
+	state->hdr.vmx.flags = ~0;
+	test_nested_state_expect_einval(vm, state);
+
 	/* It is invalid to have vmxon_pa == -1ull and vmcs_pa != -1ull. */
 	set_default_vmx_state(state, state_sz);
 	state->hdr.vmx.vmxon_pa = -1ull;
@@ -206,6 +211,14 @@ void test_vmx_nested_state(struct kvm_vm *vm)
 	state->flags = 0;
 	test_nested_state(vm, state);
 
+	/* Invalid flags are rejected, even if no VMCS loaded. */
+	set_default_vmx_state(state, state_sz);
+	state->size = sizeof(*state);
+	state->flags = 0;
+	state->hdr.vmx.vmcs12_pa = -1;
+	state->hdr.vmx.flags = ~0;
+	test_nested_state_expect_einval(vm, state);
+
 	/* vmxon_pa cannot be the same address as vmcs_pa. */
 	set_default_vmx_state(state, state_sz);
 	state->hdr.vmx.vmxon_pa = 0;