Fix outliers whose auth_events are in a different room are correctly rejected (#2791)

Fixes `outliers whose auth_events are in a different room are correctly
rejected`, by validating that auth events are all from the same room and
not using rejected events for event auth.

roomserver/internal/input/input_events_test.go

@@ -0,0 +1,63 @@
+package input
+
+import (
+	"testing"
+
+	"github.com/matrix-org/gomatrixserverlib"
+
+	"github.com/matrix-org/dendrite/test"
+)
+
+func Test_EventAuth(t *testing.T) {
+	alice := test.NewUser(t)
+	bob := test.NewUser(t)
+
+	// create two rooms, so we can craft "illegal" auth events
+	room1 := test.NewRoom(t, alice)
+	room2 := test.NewRoom(t, alice, test.RoomPreset(test.PresetPublicChat))
+
+	authEventIDs := make([]string, 0, 4)
+	authEvents := []*gomatrixserverlib.Event{}
+
+	// Add the legal auth events from room2
+	for _, x := range room2.Events() {
+		if x.Type() == gomatrixserverlib.MRoomCreate {
+			authEventIDs = append(authEventIDs, x.EventID())
+			authEvents = append(authEvents, x.Event)
+		}
+		if x.Type() == gomatrixserverlib.MRoomPowerLevels {
+			authEventIDs = append(authEventIDs, x.EventID())
+			authEvents = append(authEvents, x.Event)
+		}
+		if x.Type() == gomatrixserverlib.MRoomJoinRules {
+			authEventIDs = append(authEventIDs, x.EventID())
+			authEvents = append(authEvents, x.Event)
+		}
+	}
+
+	// Add the illegal auth event from room1 (rooms are different)
+	for _, x := range room1.Events() {
+		if x.Type() == gomatrixserverlib.MRoomMember {
+			authEventIDs = append(authEventIDs, x.EventID())
+			authEvents = append(authEvents, x.Event)
+		}
+	}
+
+	// Craft the illegal join event, with auth events from different rooms
+	ev := room2.CreateEvent(t, bob, "m.room.member", map[string]interface{}{
+		"membership": "join",
+	}, test.WithStateKey(bob.ID), test.WithAuthIDs(authEventIDs))
+
+	// Add the auth events to the allower
+	allower := gomatrixserverlib.NewAuthEvents(nil)
+	for _, a := range authEvents {
+		if err := allower.AddEvent(a); err != nil {
+			t.Fatalf("allower.AddEvent failed: %v", err)
+		}
+	}
+
+	// Finally check that the event is NOT allowed
+	if err := gomatrixserverlib.Allowed(ev.Event, &allower); err == nil {
+		t.Fatalf("event should not be allowed, but it was")
+	}
+}