KVM: VMX: set "blocked by NMI" flag if EPT violation happens during IRET from NMI

Set "blocked by NMI" flag if EPT violation happens during IRET from NMI
otherwise NMI can be called recursively causing stack corruption.
Signed-off-by: NGleb Natapov <gleb@redhat.com>

arch/x86/kvm/vmx.c

@@ -5339,6 +5339,15 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
 		return 0;
 	}
 
+	/*
+	 * EPT violation happened while executing iret from NMI,
+	 * "blocked by NMI" bit has to be set before next VM entry.
+	 * There are errata that may cause this bit to not be set:
+	 * AAK134, BY25.
+	 */
+	if (exit_qualification & INTR_INFO_UNBLOCK_NMI)
+		vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO, GUEST_INTR_STATE_NMI);
+
 	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
 	trace_kvm_page_fault(gpa, exit_qualification);