KVM: nVMX: Ensure vmcs01 is the loaded VMCS when freeing nested state

Add a WARN in free_nested() to ensure vmcs01 is loaded prior to freeing
vmcs02 and friends, and explicitly switch to vmcs01 if it's not.  KVM is
supposed to keep is_guest_mode() and loaded_vmcs==vmcs02 synchronized,
but bugs happen and freeing vmcs02 while it's in use will escalate a KVM
error to a use-after-free and potentially crash the kernel.

Do the WARN and switch even in the !vmxon case to help detect latent
bugs.  free_nested() is not a hot path, and the check is cheap.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923184452.980-6-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -279,6 +279,9 @@ static void free_nested(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
+	if (WARN_ON_ONCE(vmx->loaded_vmcs != &vmx->vmcs01))
+		vmx_switch_vmcs(vcpu, &vmx->vmcs01);
+
 	if (!vmx->nested.vmxon && !vmx->nested.smm.vmxon)
 		return;