KVM: x86: fix deadlock for KVM_XEN_EVTCHN_RESET

While KVM_XEN_EVTCHN_RESET is usually called with no vCPUs running,
if that happened it could cause a deadlock.  This is due to
kvm_xen_eventfd_reset() doing a synchronize_srcu() inside
a kvm->lock critical section.

To avoid this, first collect all the evtchnfd objects in an
array and free all of them once the kvm->lock critical section
is over and th SRCU grace period has expired.

Reported-by: Michal Luczaj <mhal@rbox.co>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/xen.c

@@ -1942,18 +1942,42 @@ static int kvm_xen_eventfd_deassign(struct kvm *kvm, u32 port)
 
 static int kvm_xen_eventfd_reset(struct kvm *kvm)
 {
-	struct evtchnfd *evtchnfd;
+	struct evtchnfd *evtchnfd, **all_evtchnfds;
 	int i;
+	int n = 0;
 
 	mutex_lock(&kvm->lock);
+
+	/*
+	 * Because synchronize_srcu() cannot be called inside the
+	 * critical section, first collect all the evtchnfd objects
+	 * in an array as they are removed from evtchn_ports.
+	 */
+	idr_for_each_entry(&kvm->arch.xen.evtchn_ports, evtchnfd, i)
+		n++;
+
+	all_evtchnfds = kmalloc_array(n, sizeof(struct evtchnfd *), GFP_KERNEL);
+	if (!all_evtchnfds) {
+		mutex_unlock(&kvm->lock);
+		return -ENOMEM;
+	}
+
+	n = 0;
 	idr_for_each_entry(&kvm->arch.xen.evtchn_ports, evtchnfd, i) {
+		all_evtchnfds[n++] = evtchnfd;
 		idr_remove(&kvm->arch.xen.evtchn_ports, evtchnfd->send_port);
-		synchronize_srcu(&kvm->srcu);
+	}
+	mutex_unlock(&kvm->lock);
+
+	synchronize_srcu(&kvm->srcu);
+
+	while (n--) {
+		evtchnfd = all_evtchnfds[n];
 		if (!evtchnfd->deliver.port.port)
 			eventfd_ctx_put(evtchnfd->deliver.eventfd.ctx);
 		kfree(evtchnfd);
 	}
-	mutex_unlock(&kvm->lock);
+	kfree(all_evtchnfds);
 
 	return 0;
 }

tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c

@@ -962,6 +962,12 @@ int main(int argc, char *argv[])
 	}
 
  done:
+	struct kvm_xen_hvm_attr evt_reset = {
+		.type = KVM_XEN_ATTR_TYPE_EVTCHN,
+		.u.evtchn.flags = KVM_XEN_EVTCHN_RESET,
+	};
+	vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &evt_reset);
+
 	alarm(0);
 	clock_gettime(CLOCK_REALTIME, &max_ts);