KVM: vVMX: signal failure for nested VMEntry if emulation_required

Fail a nested VMEntry with EXIT_REASON_INVALID_STATE if L2 guest state
is invalid, i.e. vmcs12 contained invalid guest state, and unrestricted
guest is disabled in L0 (and by extension disabled in L1).

WARN_ON_ONCE in handle_invalid_guest_state() if we're attempting to
emulate L2, i.e. nested_run_pending is true, to aid debug in the
(hopefully unlikely) scenario that we somehow skip the nested VMEntry
consistency check, e.g. due to a L0 bug.

Note: KVM relies on hardware to detect the scenario where unrestricted
guest is enabled in L0 but disabled in L1 and vmcs12 contains invalid
guest state, i.e. checking emulation_required in prepare_vmcs02 is
required only to handle the case were unrestricted guest is disabled
in L0 since L0 never actually attempts VMLAUNCH/VMRESUME with vmcs02.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -6865,6 +6865,13 @@ static int handle_invalid_guest_state(struct kvm_vcpu *vcpu)
 	bool intr_window_requested;
 	unsigned count = 130;
 
+	/*
+	 * We should never reach the point where we are emulating L2
+	 * due to invalid guest state as that means we incorrectly
+	 * allowed a nested VMEntry with an invalid vmcs12.
+	 */
+	WARN_ON_ONCE(vmx->emulation_required && vmx->nested.nested_run_pending);
+
 	cpu_exec_ctrl = vmcs_read32(CPU_BASED_VM_EXEC_CONTROL);
 	intr_window_requested = cpu_exec_ctrl & CPU_BASED_VIRTUAL_INTR_PENDING;
 
@@ -10990,6 +10997,14 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
 		vmx->nested.dirty_vmcs12 = false;
 	}
 
+	/*
+	 * Guest state is invalid and unrestricted guest is disabled,
+	 * which means L1 attempted VMEntry to L2 with invalid state.
+	 * Fail the VMEntry.
+	 */
+	if (vmx->emulation_required)
+		return 1;
+
 	/* Shadow page tables on either EPT or shadow page tables. */
 	if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3, nested_cpu_has_ept(vmcs12),
 				entry_failure_code))