KVM: x86: Check for nested events if there is an injectable interrupt

With commit b6b8a1451fc40412c57d1 that introduced
vmx_check_nested_events, checks for injectable interrupts happen
at different points in time for L1 and L2 that could potentially
cause a race. The regression occurs because KVM_REQ_EVENT is always
set when nested_run_pending is set even if there's no pending interrupt.
Consequently, there could be a small window when check_nested_events
returns without exiting to L1, but an interrupt comes through soon
after and it incorrectly, gets injected to L2 by inject_pending_event
Fix this by adding a call to check for nested events too when a check
for injectable interrupt returns true

Signed-off-by: Bandan Das <bsd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -5887,6 +5887,18 @@ static int inject_pending_event(struct kvm_vcpu *vcpu, bool req_int_win)
 			kvm_x86_ops->set_nmi(vcpu);
 		}
 	} else if (kvm_cpu_has_injectable_intr(vcpu)) {
+		/*
+		 * Because interrupts can be injected asynchronously, we are
+		 * calling check_nested_events again here to avoid a race condition.
+		 * See https://lkml.org/lkml/2014/7/2/60 for discussion about this
+		 * proposal and current concerns.  Perhaps we should be setting
+		 * KVM_REQ_EVENT only on certain events and not unconditionally?
+		 */
+		if (is_guest_mode(vcpu) && kvm_x86_ops->check_nested_events) {
+			r = kvm_x86_ops->check_nested_events(vcpu, req_int_win);
+			if (r != 0)
+				return r;
+		}
 		if (kvm_x86_ops->interrupt_allowed(vcpu)) {
 			kvm_queue_interrupt(vcpu, kvm_cpu_get_interrupt(vcpu),
 					    false);