kvm: nVMX: Restrict VMX capability MSR changes

Disallow changes to the VMX capability MSRs while the vCPU is in VMX
operation. Although this does break the existing API, it helps to
avoid some potentially tricky situations for which there is no
architected behavior.
Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -3538,6 +3538,13 @@ static int vmx_set_vmx_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
+	/*
+	 * Don't allow changes to the VMX capability MSRs while the vCPU
+	 * is in VMX operation.
+	 */
+	if (vmx->nested.vmxon)
+		return -EBUSY;
+
 	switch (msr_index) {
 	case MSR_IA32_VMX_BASIC:
 		return vmx_restore_vmx_basic(vmx, data);