Handle guest access [1/2?] (#2872)

Needs https://github.com/matrix-org/sytest/pull/1315, as otherwise the
membership events aren't persisted yet when hitting `/state` after
kicking guest users.

Makes the following tests pass:
```
Guest users denied access over federation if guest access prohibited
Guest users are kicked from guest_access rooms on revocation of guest_access
Guest users are kicked from guest_access rooms on revocation of guest_access over federation
```

Todo (in a follow up PR):
- Restrict access to CS API Endpoints as per
https://spec.matrix.org/v1.4/client-server-api/#client-behaviour-14

Co-authored-by: kegsay <kegan@matrix.org>

20 files changed, 606 insertions, 44 deletions

diff --git a/clientapi/clientapi.go b/clientapi/clientapi.go
index 62ffa615..2d17e092 100644
--- a/clientapi/clientapi.go
+++ b/clientapi/clientapi.go
@@ -15,6 +15,8 @@
 package clientapi
 
 import (
+	"github.com/matrix-org/gomatrixserverlib"
+
 	appserviceAPI "github.com/matrix-org/dendrite/appservice/api"
 	"github.com/matrix-org/dendrite/clientapi/api"
 	"github.com/matrix-org/dendrite/clientapi/producers"
@@ -26,7 +28,6 @@ import (
 	"github.com/matrix-org/dendrite/setup/base"
 	"github.com/matrix-org/dendrite/setup/jetstream"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
-	"github.com/matrix-org/gomatrixserverlib"
 )
 
 // AddPublicRoutes sets up and registers HTTP handlers for the ClientAPI component.
diff --git a/clientapi/routing/joinroom.go b/clientapi/routing/joinroom.go
index c50e552b..e371d921 100644
--- a/clientapi/routing/joinroom.go
+++ b/clientapi/routing/joinroom.go
@@ -37,6 +37,7 @@ func JoinRoomByIDOrAlias(
 	joinReq := roomserverAPI.PerformJoinRequest{
 		RoomIDOrAlias: roomIDOrAlias,
 		UserID:        device.UserID,
+		IsGuest:       device.AccountType == api.AccountTypeGuest,
 		Content:       map[string]interface{}{},
 	}
 	joinRes := roomserverAPI.PerformJoinResponse{}
@@ -84,7 +85,14 @@ func JoinRoomByIDOrAlias(
 		if err := rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes); err != nil {
 			done <- jsonerror.InternalAPIError(req.Context(), err)
 		} else if joinRes.Error != nil {
-			done <- joinRes.Error.JSONResponse()
+			if joinRes.Error.Code == roomserverAPI.PerformErrorNotAllowed && device.AccountType == api.AccountTypeGuest {
+				done <- util.JSONResponse{
+					Code: http.StatusForbidden,
+					JSON: jsonerror.GuestAccessForbidden(joinRes.Error.Msg),
+				}
+			} else {
+				done <- joinRes.Error.JSONResponse()
+			}
 		} else {
 			done <- util.JSONResponse{
 				Code: http.StatusOK,
diff --git a/clientapi/routing/joinroom_test.go b/clientapi/routing/joinroom_test.go
new file mode 100644
index 00000000..9e8208e6
--- /dev/null
+++ b/clientapi/routing/joinroom_test.go
@@ -0,0 +1,158 @@
+package routing
+
+import (
+	"bytes"
+	"context"
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/matrix-org/gomatrixserverlib"
+
+	"github.com/matrix-org/dendrite/appservice"
+	"github.com/matrix-org/dendrite/keyserver"
+	"github.com/matrix-org/dendrite/roomserver"
+	"github.com/matrix-org/dendrite/test"
+	"github.com/matrix-org/dendrite/test/testrig"
+	"github.com/matrix-org/dendrite/userapi"
+	uapi "github.com/matrix-org/dendrite/userapi/api"
+)
+
+func TestJoinRoomByIDOrAlias(t *testing.T) {
+	alice := test.NewUser(t)
+	bob := test.NewUser(t)
+	charlie := test.NewUser(t, test.WithAccountType(uapi.AccountTypeGuest))
+
+	ctx := context.Background()
+	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
+		base, baseClose := testrig.CreateBaseDendrite(t, dbType)
+		defer baseClose()
+
+		rsAPI := roomserver.NewInternalAPI(base)
+		keyAPI := keyserver.NewInternalAPI(base, &base.Cfg.KeyServer, nil, rsAPI)
+		userAPI := userapi.NewInternalAPI(base, &base.Cfg.UserAPI, nil, keyAPI, rsAPI, nil)
+		asAPI := appservice.NewInternalAPI(base, userAPI, rsAPI)
+		rsAPI.SetFederationAPI(nil, nil) // creates the rs.Inputer etc
+
+		// Create the users in the userapi
+		for _, u := range []*test.User{alice, bob, charlie} {
+			localpart, serverName, _ := gomatrixserverlib.SplitID('@', u.ID)
+			userRes := &uapi.PerformAccountCreationResponse{}
+			if err := userAPI.PerformAccountCreation(ctx, &uapi.PerformAccountCreationRequest{
+				AccountType: u.AccountType,
+				Localpart:   localpart,
+				ServerName:  serverName,
+				Password:    "someRandomPassword",
+			}, userRes); err != nil {
+				t.Errorf("failed to create account: %s", err)
+			}
+
+		}
+
+		aliceDev := &uapi.Device{UserID: alice.ID}
+		bobDev := &uapi.Device{UserID: bob.ID}
+		charlieDev := &uapi.Device{UserID: charlie.ID, AccountType: uapi.AccountTypeGuest}
+
+		// create a room with disabled guest access and invite Bob
+		resp := createRoom(ctx, createRoomRequest{
+			Name:          "testing",
+			IsDirect:      true,
+			Topic:         "testing",
+			Visibility:    "public",
+			Preset:        presetPublicChat,
+			RoomAliasName: "alias",
+			Invite:        []string{bob.ID},
+			GuestCanJoin:  false,
+		}, aliceDev, &base.Cfg.ClientAPI, userAPI, rsAPI, asAPI, time.Now())
+		crResp, ok := resp.JSON.(createRoomResponse)
+		if !ok {
+			t.Fatalf("response is not a createRoomResponse: %+v", resp)
+		}
+
+		// create a room with guest access enabled and invite Charlie
+		resp = createRoom(ctx, createRoomRequest{
+			Name:         "testing",
+			IsDirect:     true,
+			Topic:        "testing",
+			Visibility:   "public",
+			Preset:       presetPublicChat,
+			Invite:       []string{charlie.ID},
+			GuestCanJoin: true,
+		}, aliceDev, &base.Cfg.ClientAPI, userAPI, rsAPI, asAPI, time.Now())
+		crRespWithGuestAccess, ok := resp.JSON.(createRoomResponse)
+		if !ok {
+			t.Fatalf("response is not a createRoomResponse: %+v", resp)
+		}
+
+		// Dummy request
+		body := &bytes.Buffer{}
+		req, err := http.NewRequest(http.MethodPost, "/?server_name=test", body)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		testCases := []struct {
+			name        string
+			device      *uapi.Device
+			roomID      string
+			wantHTTP200 bool
+		}{
+			{
+				name:        "User can join successfully by alias",
+				device:      bobDev,
+				roomID:      crResp.RoomAlias,
+				wantHTTP200: true,
+			},
+			{
+				name:        "User can join successfully by roomID",
+				device:      bobDev,
+				roomID:      crResp.RoomID,
+				wantHTTP200: true,
+			},
+			{
+				name:   "join is forbidden if user is guest",
+				device: charlieDev,
+				roomID: crResp.RoomID,
+			},
+			{
+				name:   "room does not exist",
+				device: aliceDev,
+				roomID: "!doesnotexist:test",
+			},
+			{
+				name:   "user from different server",
+				device: &uapi.Device{UserID: "@wrong:server"},
+				roomID: crResp.RoomAlias,
+			},
+			{
+				name:   "user doesn't exist locally",
+				device: &uapi.Device{UserID: "@doesnotexist:test"},
+				roomID: crResp.RoomAlias,
+			},
+			{
+				name:   "invalid room ID",
+				device: aliceDev,
+				roomID: "invalidRoomID",
+			},
+			{
+				name:   "roomAlias does not exist",
+				device: aliceDev,
+				roomID: "#doesnotexist:test",
+			},
+			{
+				name:   "room with guest_access event",
+				device: charlieDev,
+				roomID: crRespWithGuestAccess.RoomID,
+			},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				joinResp := JoinRoomByIDOrAlias(req, tc.device, rsAPI, userAPI, tc.roomID)
+				if tc.wantHTTP200 && !joinResp.Is2xx() {
+					t.Fatalf("expected join room to succeed, but didn't: %+v", joinResp)
+				}
+			})
+		}
+	})
+}
diff --git a/roomserver/api/perform.go b/roomserver/api/perform.go
index e70e5ea9..e789b956 100644
--- a/roomserver/api/perform.go
+++ b/roomserver/api/perform.go
@@ -78,6 +78,7 @@ const (
 type PerformJoinRequest struct {
 	RoomIDOrAlias string                         `json:"room_id_or_alias"`
 	UserID        string                         `json:"user_id"`
+	IsGuest       bool                           `json:"is_guest"`
 	Content       map[string]interface{}         `json:"content"`
 	ServerNames   []gomatrixserverlib.ServerName `json:"server_names"`
 	Unsigned      map[string]interface{}         `json:"unsigned"`
diff --git a/roomserver/internal/api.go b/roomserver/internal/api.go
index 1a362660..451b3769 100644
--- a/roomserver/internal/api.go
+++ b/roomserver/internal/api.go
@@ -4,6 +4,10 @@ import (
 	"context"
 
 	"github.com/getsentry/sentry-go"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/nats-io/nats.go"
+	"github.com/sirupsen/logrus"
+
 	asAPI "github.com/matrix-org/dendrite/appservice/api"
 	fsAPI "github.com/matrix-org/dendrite/federationapi/api"
 	"github.com/matrix-org/dendrite/internal/caching"
@@ -19,9 +23,6 @@ import (
 	"github.com/matrix-org/dendrite/setup/jetstream"
 	"github.com/matrix-org/dendrite/setup/process"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
-	"github.com/matrix-org/gomatrixserverlib"
-	"github.com/nats-io/nats.go"
-	"github.com/sirupsen/logrus"
 )
 
 // RoomserverInternalAPI is an implementation of api.RoomserverInternalAPI
@@ -104,6 +105,11 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.RoomserverFederatio
 	r.fsAPI = fsAPI
 	r.KeyRing = keyRing
 
+	identity, err := r.Cfg.Matrix.SigningIdentityFor(r.ServerName)
+	if err != nil {
+		logrus.Panic(err)
+	}
+
 	r.Inputer = &input.Inputer{
 		Cfg:                 &r.Base.Cfg.RoomServer,
 		Base:                r.Base,
@@ -114,7 +120,8 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.RoomserverFederatio
 		JetStream:           r.JetStream,
 		NATSClient:          r.NATSClient,
 		Durable:             nats.Durable(r.Durable),
-		ServerName:          r.Cfg.Matrix.ServerName,
+		ServerName:          r.ServerName,
+		SigningIdentity:     identity,
 		FSAPI:               fsAPI,
 		KeyRing:             keyRing,
 		ACLs:                r.ServerACLs,
@@ -135,7 +142,7 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.RoomserverFederatio
 		Queryer: r.Queryer,
 	}
 	r.Peeker = &perform.Peeker{
-		ServerName: r.Cfg.Matrix.ServerName,
+		ServerName: r.ServerName,
 		Cfg:        r.Cfg,
 		DB:         r.DB,
 		FSAPI:      r.fsAPI,
@@ -146,7 +153,7 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.RoomserverFederatio
 		Inputer: r.Inputer,
 	}
 	r.Unpeeker = &perform.Unpeeker{
-		ServerName: r.Cfg.Matrix.ServerName,
+		ServerName: r.ServerName,
 		Cfg:        r.Cfg,
 		DB:         r.DB,
 		FSAPI:      r.fsAPI,
@@ -193,6 +200,7 @@ func (r *RoomserverInternalAPI) SetFederationAPI(fsAPI fsAPI.RoomserverFederatio
 
 func (r *RoomserverInternalAPI) SetUserAPI(userAPI userapi.RoomserverUserAPI) {
 	r.Leaver.UserAPI = userAPI
+	r.Inputer.UserAPI = userAPI
 }
 
 func (r *RoomserverInternalAPI) SetAppserviceAPI(asAPI asAPI.AppServiceInternalAPI) {
diff --git a/roomserver/internal/input/input.go b/roomserver/internal/input/input.go
index e965691c..94131103 100644
--- a/roomserver/internal/input/input.go
+++ b/roomserver/internal/input/input.go
@@ -23,6 +23,8 @@ import (
 	"sync"
 	"time"
 
+	userapi "github.com/matrix-org/dendrite/userapi/api"
+
 	"github.com/Arceliar/phony"
 	"github.com/getsentry/sentry-go"
 	"github.com/matrix-org/gomatrixserverlib"
@@ -79,6 +81,7 @@ type Inputer struct {
 	JetStream           nats.JetStreamContext
 	Durable             nats.SubOpt
 	ServerName          gomatrixserverlib.ServerName
+	SigningIdentity     *gomatrixserverlib.SigningIdentity
 	FSAPI               fedapi.RoomserverFederationAPI
 	KeyRing             gomatrixserverlib.JSONVerifier
 	ACLs                *acls.ServerACLs
@@ -87,6 +90,7 @@ type Inputer struct {
 	workers             sync.Map // room ID -> *worker
 
 	Queryer *query.Queryer
+	UserAPI userapi.RoomserverUserAPI
 }
 
 // If a room consumer is inactive for a while then we will allow NATS
diff --git a/roomserver/internal/input/input_events.go b/roomserver/internal/input/input_events.go
index 10b8ee27..4179fc1e 100644
--- a/roomserver/internal/input/input_events.go
+++ b/roomserver/internal/input/input_events.go
@@ -19,6 +19,7 @@ package input
 import (
 	"context"
 	"database/sql"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"time"
@@ -31,6 +32,8 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/sirupsen/logrus"
 
+	userAPI "github.com/matrix-org/dendrite/userapi/api"
+
 	fedapi "github.com/matrix-org/dendrite/federationapi/api"
 	"github.com/matrix-org/dendrite/internal"
 	"github.com/matrix-org/dendrite/internal/eventutil"
@@ -440,6 +443,13 @@ func (r *Inputer) processRoomEvent(
 		}
 	}
 
+	// If guest_access changed and is not can_join, kick all guest users.
+	if event.Type() == gomatrixserverlib.MRoomGuestAccess && gjson.GetBytes(event.Content(), "guest_access").Str != "can_join" {
+		if err = r.kickGuests(ctx, event, roomInfo); err != nil {
+			logrus.WithError(err).Error("failed to kick guest users on m.room.guest_access revocation")
+		}
+	}
+
 	// Everything was OK — the latest events updater didn't error and
 	// we've sent output events. Finally, generate a hook call.
 	hooks.Run(hooks.KindNewEventPersisted, headered)
@@ -729,3 +739,98 @@ func (r *Inputer) calculateAndSetState(
 	succeeded = true
 	return nil
 }
+
+// kickGuests kicks guests users from m.room.guest_access rooms, if guest access is now prohibited.
+func (r *Inputer) kickGuests(ctx context.Context, event *gomatrixserverlib.Event, roomInfo *types.RoomInfo) error {
+	membershipNIDs, err := r.DB.GetMembershipEventNIDsForRoom(ctx, roomInfo.RoomNID, true, true)
+	if err != nil {
+		return err
+	}
+
+	memberEvents, err := r.DB.Events(ctx, membershipNIDs)
+	if err != nil {
+		return err
+	}
+
+	inputEvents := make([]api.InputRoomEvent, 0, len(memberEvents))
+	latestReq := &api.QueryLatestEventsAndStateRequest{
+		RoomID: event.RoomID(),
+	}
+	latestRes := &api.QueryLatestEventsAndStateResponse{}
+	if err = r.Queryer.QueryLatestEventsAndState(ctx, latestReq, latestRes); err != nil {
+		return err
+	}
+
+	prevEvents := latestRes.LatestEvents
+	for _, memberEvent := range memberEvents {
+		if memberEvent.StateKey() == nil {
+			continue
+		}
+
+		localpart, senderDomain, err := gomatrixserverlib.SplitID('@', *memberEvent.StateKey())
+		if err != nil {
+			continue
+		}
+
+		accountRes := &userAPI.QueryAccountByLocalpartResponse{}
+		if err = r.UserAPI.QueryAccountByLocalpart(ctx, &userAPI.QueryAccountByLocalpartRequest{
+			Localpart:  localpart,
+			ServerName: senderDomain,
+		}, accountRes); err != nil {
+			return err
+		}
+		if accountRes.Account == nil {
+			continue
+		}
+
+		if accountRes.Account.AccountType != userAPI.AccountTypeGuest {
+			continue
+		}
+
+		var memberContent gomatrixserverlib.MemberContent
+		if err = json.Unmarshal(memberEvent.Content(), &memberContent); err != nil {
+			return err
+		}
+		memberContent.Membership = gomatrixserverlib.Leave
+
+		stateKey := *memberEvent.StateKey()
+		fledglingEvent := &gomatrixserverlib.EventBuilder{
+			RoomID:     event.RoomID(),
+			Type:       gomatrixserverlib.MRoomMember,
+			StateKey:   &stateKey,
+			Sender:     stateKey,
+			PrevEvents: prevEvents,
+		}
+
+		if fledglingEvent.Content, err = json.Marshal(memberContent); err != nil {
+			return err
+		}
+
+		eventsNeeded, err := gomatrixserverlib.StateNeededForEventBuilder(fledglingEvent)
+		if err != nil {
+			return err
+		}
+
+		event, err := eventutil.BuildEvent(ctx, fledglingEvent, r.Cfg.Matrix, r.SigningIdentity, time.Now(), &eventsNeeded, latestRes)
+		if err != nil {
+			return err
+		}
+
+		inputEvents = append(inputEvents, api.InputRoomEvent{
+			Kind:         api.KindNew,
+			Event:        event,
+			Origin:       senderDomain,
+			SendAsServer: string(senderDomain),
+		})
+		prevEvents = []gomatrixserverlib.EventReference{
+			event.EventReference(),
+		}
+	}
+
+	inputReq := &api.InputRoomEventsRequest{
+		InputRoomEvents: inputEvents,
+		Asynchronous:    true, // Needs to be async, as we otherwise create a deadlock
+	}
+	inputRes := &api.InputRoomEventsResponse{}
+	return r.InputRoomEvents(ctx, inputReq, inputRes)
+}
diff --git a/roomserver/internal/perform/perform_join.go b/roomserver/internal/perform/perform_join.go
index 4de008c6..fc7ba940 100644
--- a/roomserver/internal/perform/perform_join.go
+++ b/roomserver/internal/perform/perform_join.go
@@ -16,6 +16,7 @@ package perform
 
 import (
 	"context"
+	"database/sql"
 	"errors"
 	"fmt"
 	"strings"
@@ -270,6 +271,28 @@ func (r *Joiner) performJoinRoomByID(
 		}
 	}
 
+	// If a guest is trying to join a room, check that the room has a m.room.guest_access event
+	if req.IsGuest {
+		var guestAccessEvent *gomatrixserverlib.HeaderedEvent
+		guestAccess := "forbidden"
+		guestAccessEvent, err = r.DB.GetStateEvent(ctx, req.RoomIDOrAlias, gomatrixserverlib.MRoomGuestAccess, "")
+		if (err != nil && !errors.Is(err, sql.ErrNoRows)) || guestAccessEvent == nil {
+			logrus.WithError(err).Warn("unable to get m.room.guest_access event, defaulting to 'forbidden'")
+		}
+		if guestAccessEvent != nil {
+			guestAccess = gjson.GetBytes(guestAccessEvent.Content(), "guest_access").String()
+		}
+
+		// Servers MUST only allow guest users to join rooms if the m.room.guest_access state event
+		// is present on the room and has the guest_access value can_join.
+		if guestAccess != "can_join" {
+			return "", "", &rsAPI.PerformError{
+				Code: rsAPI.PerformErrorNotAllowed,
+				Msg:  "Guest access is forbidden",
+			}
+		}
+	}
+
 	// If we should do a forced federated join then do that.
 	var joinedVia gomatrixserverlib.ServerName
 	if forceFederatedJoin {
diff --git a/roomserver/roomserver_test.go b/roomserver/roomserver_test.go
index 518bb372..595ceb52 100644
--- a/roomserver/roomserver_test.go
+++ b/roomserver/roomserver_test.go
@@ -3,18 +3,23 @@ package roomserver_test
 import (
 	"context"
 	"net/http"
+	"reflect"
 	"testing"
 	"time"
 
 	"github.com/gorilla/mux"
+	"github.com/matrix-org/dendrite/internal/httputil"
+	"github.com/matrix-org/dendrite/setup/base"
+	"github.com/matrix-org/dendrite/userapi"
+
+	userAPI "github.com/matrix-org/dendrite/userapi/api"
+
 	"github.com/matrix-org/gomatrixserverlib"
 
-	"github.com/matrix-org/dendrite/internal/httputil"
 	"github.com/matrix-org/dendrite/roomserver"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/roomserver/inthttp"
 	"github.com/matrix-org/dendrite/roomserver/storage"
-	"github.com/matrix-org/dendrite/setup/base"
 	"github.com/matrix-org/dendrite/test"
 	"github.com/matrix-org/dendrite/test/testrig"
 )
@@ -29,7 +34,28 @@ func mustCreateDatabase(t *testing.T, dbType test.DBType) (*base.BaseDendrite, s
 	return base, db, close
 }
 
-func Test_SharedUsers(t *testing.T) {
+func TestUsers(t *testing.T) {
+	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
+		base, close := testrig.CreateBaseDendrite(t, dbType)
+		defer close()
+		rsAPI := roomserver.NewInternalAPI(base)
+		// SetFederationAPI starts the room event input consumer
+		rsAPI.SetFederationAPI(nil, nil)
+
+		t.Run("shared users", func(t *testing.T) {
+			testSharedUsers(t, rsAPI)
+		})
+
+		t.Run("kick users", func(t *testing.T) {
+			usrAPI := userapi.NewInternalAPI(base, &base.Cfg.UserAPI, nil, nil, rsAPI, nil)
+			rsAPI.SetUserAPI(usrAPI)
+			testKickUsers(t, rsAPI, usrAPI)
+		})
+	})
+
+}
+
+func testSharedUsers(t *testing.T, rsAPI api.RoomserverInternalAPI) {
 	alice := test.NewUser(t)
 	bob := test.NewUser(t)
 	room := test.NewRoom(t, alice, test.RoomPreset(test.PresetTrustedPrivateChat))
@@ -43,36 +69,93 @@ func Test_SharedUsers(t *testing.T) {
 	}, test.WithStateKey(bob.ID))
 
 	ctx := context.Background()
-	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
-		base, _, close := mustCreateDatabase(t, dbType)
-		defer close()
 
-		rsAPI := roomserver.NewInternalAPI(base)
-		// SetFederationAPI starts the room event input consumer
-		rsAPI.SetFederationAPI(nil, nil)
-		// Create the room
-		if err := api.SendEvents(ctx, rsAPI, api.KindNew, room.Events(), "test", "test", "test", nil, false); err != nil {
-			t.Fatalf("failed to send events: %v", err)
-		}
+	// Create the room
+	if err := api.SendEvents(ctx, rsAPI, api.KindNew, room.Events(), "test", "test", "test", nil, false); err != nil {
+		t.Errorf("failed to send events: %v", err)
+	}
 
-		// Query the shared users for Alice, there should only be Bob.
-		// This is used by the SyncAPI keychange consumer.
-		res := &api.QuerySharedUsersResponse{}
-		if err := rsAPI.QuerySharedUsers(ctx, &api.QuerySharedUsersRequest{UserID: alice.ID}, res); err != nil {
-			t.Fatalf("unable to query known users: %v", err)
-		}
-		if _, ok := res.UserIDsToCount[bob.ID]; !ok {
-			t.Fatalf("expected to find %s in shared users, but didn't: %+v", bob.ID, res.UserIDsToCount)
+	// Query the shared users for Alice, there should only be Bob.
+	// This is used by the SyncAPI keychange consumer.
+	res := &api.QuerySharedUsersResponse{}
+	if err := rsAPI.QuerySharedUsers(ctx, &api.QuerySharedUsersRequest{UserID: alice.ID}, res); err != nil {
+		t.Errorf("unable to query known users: %v", err)
+	}
+	if _, ok := res.UserIDsToCount[bob.ID]; !ok {
+		t.Errorf("expected to find %s in shared users, but didn't: %+v", bob.ID, res.UserIDsToCount)
+	}
+	// Also verify that we get the expected result when specifying OtherUserIDs.
+	// This is used by the SyncAPI when getting device list changes.
+	if err := rsAPI.QuerySharedUsers(ctx, &api.QuerySharedUsersRequest{UserID: alice.ID, OtherUserIDs: []string{bob.ID}}, res); err != nil {
+		t.Errorf("unable to query known users: %v", err)
+	}
+	if _, ok := res.UserIDsToCount[bob.ID]; !ok {
+		t.Errorf("expected to find %s in shared users, but didn't: %+v", bob.ID, res.UserIDsToCount)
+	}
+}
+
+func testKickUsers(t *testing.T, rsAPI api.RoomserverInternalAPI, usrAPI userAPI.UserInternalAPI) {
+	// Create users and room; Bob is going to be the guest and kicked on revocation of guest access
+	alice := test.NewUser(t, test.WithAccountType(userAPI.AccountTypeUser))
+	bob := test.NewUser(t, test.WithAccountType(userAPI.AccountTypeGuest))
+
+	room := test.NewRoom(t, alice, test.RoomPreset(test.PresetPublicChat), test.GuestsCanJoin(true))
+
+	// Join with the guest user
+	room.CreateAndInsert(t, bob, gomatrixserverlib.MRoomMember, map[string]interface{}{
+		"membership": "join",
+	}, test.WithStateKey(bob.ID))
+
+	ctx := context.Background()
+
+	// Create the users in the userapi, so the RSAPI can query the account type later
+	for _, u := range []*test.User{alice, bob} {
+		localpart, serverName, _ := gomatrixserverlib.SplitID('@', u.ID)
+		userRes := &userAPI.PerformAccountCreationResponse{}
+		if err := usrAPI.PerformAccountCreation(ctx, &userAPI.PerformAccountCreationRequest{
+			AccountType: u.AccountType,
+			Localpart:   localpart,
+			ServerName:  serverName,
+			Password:    "someRandomPassword",
+		}, userRes); err != nil {
+			t.Errorf("failed to create account: %s", err)
 		}
-		// Also verify that we get the expected result when specifying OtherUserIDs.
-		// This is used by the SyncAPI when getting device list changes.
-		if err := rsAPI.QuerySharedUsers(ctx, &api.QuerySharedUsersRequest{UserID: alice.ID, OtherUserIDs: []string{bob.ID}}, res); err != nil {
-			t.Fatalf("unable to query known users: %v", err)
+	}
+
+	// Create the room in the database
+	if err := api.SendEvents(ctx, rsAPI, api.KindNew, room.Events(), "test", "test", "test", nil, false); err != nil {
+		t.Errorf("failed to send events: %v", err)
+	}
+
+	// Get the membership events BEFORE revoking guest access
+	membershipRes := &api.QueryMembershipsForRoomResponse{}
+	if err := rsAPI.QueryMembershipsForRoom(ctx, &api.QueryMembershipsForRoomRequest{LocalOnly: true, JoinedOnly: true, RoomID: room.ID}, membershipRes); err != nil {
+		t.Errorf("failed to query membership for room: %s", err)
+	}
+
+	// revoke guest access
+	revokeEvent := room.CreateAndInsert(t, alice, gomatrixserverlib.MRoomGuestAccess, map[string]string{"guest_access": "forbidden"}, test.WithStateKey(""))
+	if err := api.SendEvents(ctx, rsAPI, api.KindNew, []*gomatrixserverlib.HeaderedEvent{revokeEvent}, "test", "test", "test", nil, false); err != nil {
+		t.Errorf("failed to send events: %v", err)
+	}
+
+	// TODO: Even though we are sending the events sync, the "kickUsers" function is sending the events async, so we need
+	//		 to loop and wait for the events to be processed by the roomserver.
+	for i := 0; i <= 20; i++ {
+		// Get the membership events AFTER revoking guest access
+		membershipRes2 := &api.QueryMembershipsForRoomResponse{}
+		if err := rsAPI.QueryMembershipsForRoom(ctx, &api.QueryMembershipsForRoomRequest{LocalOnly: true, JoinedOnly: true, RoomID: room.ID}, membershipRes2); err != nil {
+			t.Errorf("failed to query membership for room: %s", err)
 		}
-		if _, ok := res.UserIDsToCount[bob.ID]; !ok {
-			t.Fatalf("expected to find %s in shared users, but didn't: %+v", bob.ID, res.UserIDsToCount)
+
+		// The membership events should NOT match, as Bob (guest user) should now be kicked from the room
+		if !reflect.DeepEqual(membershipRes, membershipRes2) {
+			return
 		}
-	})
+		time.Sleep(time.Millisecond * 10)
+	}
+
+	t.Errorf("memberships didn't change in time")
 }
 
 func Test_QueryLeftUsers(t *testing.T) {
diff --git a/setup/config/config_global.go b/setup/config/config_global.go
index 511951fe..804eb1a2 100644
--- a/setup/config/config_global.go
+++ b/setup/config/config_global.go
@@ -174,7 +174,7 @@ func (c *Global) SigningIdentityFor(serverName gomatrixserverlib.ServerName) (*g
 			return id, nil
 		}
 	}
-	return nil, fmt.Errorf("no signing identity %q", serverName)
+	return nil, fmt.Errorf("no signing identity for %q", serverName)
 }
 
 func (c *Global) SigningIdentities() []*gomatrixserverlib.SigningIdentity {
diff --git a/setup/config/config_test.go b/setup/config/config_test.go
index ee7e7389..3408bf46 100644
--- a/setup/config/config_test.go
+++ b/setup/config/config_test.go
@@ -16,8 +16,10 @@ package config
 
 import (
 	"fmt"
+	"reflect"
 	"testing"
 
+	"github.com/matrix-org/gomatrixserverlib"
 	"gopkg.in/yaml.v2"
 )
 
@@ -290,3 +292,55 @@ func TestUnmarshalDataUnit(t *testing.T) {
 		}
 	}
 }
+
+func Test_SigningIdentityFor(t *testing.T) {
+	tests := []struct {
+		name         string
+		virtualHosts []*VirtualHost
+		serverName   gomatrixserverlib.ServerName
+		want         *gomatrixserverlib.SigningIdentity
+		wantErr      bool
+	}{
+		{
+			name:    "no virtual hosts defined",
+			wantErr: true,
+		},
+		{
+			name:       "no identity found",
+			serverName: gomatrixserverlib.ServerName("doesnotexist"),
+			wantErr:    true,
+		},
+		{
+			name:       "found identity",
+			serverName: gomatrixserverlib.ServerName("main"),
+			want:       &gomatrixserverlib.SigningIdentity{ServerName: "main"},
+		},
+		{
+			name:       "identity found on virtual hosts",
+			serverName: gomatrixserverlib.ServerName("vh2"),
+			virtualHosts: []*VirtualHost{
+				{SigningIdentity: gomatrixserverlib.SigningIdentity{ServerName: "vh1"}},
+				{SigningIdentity: gomatrixserverlib.SigningIdentity{ServerName: "vh2"}},
+			},
+			want: &gomatrixserverlib.SigningIdentity{ServerName: "vh2"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := &Global{
+				VirtualHosts: tt.virtualHosts,
+				SigningIdentity: gomatrixserverlib.SigningIdentity{
+					ServerName: "main",
+				},
+			}
+			got, err := c.SigningIdentityFor(tt.serverName)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("SigningIdentityFor() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("SigningIdentityFor() got = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
diff --git a/sytest-blacklist b/sytest-blacklist
index c35b03bd..99cfbabc 100644
--- a/sytest-blacklist
+++ b/sytest-blacklist
@@ -48,4 +48,7 @@ If a device list update goes missing, the server resyncs on the next one
 Leaves are present in non-gapped incremental syncs
 
 # Below test was passing for the wrong reason, failing correctly since #2858
-New federated private chats get full presence information (SYN-115)
\ No newline at end of file
+New federated private chats get full presence information (SYN-115)
+
+# We don't have any state to calculate m.room.guest_access when accepting invites
+Guest users can accept invites to private rooms over federation
\ No newline at end of file
diff --git a/sytest-whitelist b/sytest-whitelist
index 49ffb8fe..215889a4 100644
--- a/sytest-whitelist
+++ b/sytest-whitelist
@@ -763,4 +763,7 @@ AS and main public room lists are separate
 local user has tags copied to the new room
 remote user has tags copied to the new room
 /upgrade moves remote aliases to the new room
-Local and remote users' homeservers remove a room from their public directory on upgrade
\ No newline at end of file
+Local and remote users' homeservers remove a room from their public directory on upgrade
+Guest users denied access over federation if guest access prohibited
+Guest users are kicked from guest_access rooms on revocation of guest_access
+Guest users are kicked from guest_access rooms on revocation of guest_access over federation
\ No newline at end of file
diff --git a/test/room.go b/test/room.go
index 4328bf84..685876cb 100644
--- a/test/room.go
+++ b/test/room.go
@@ -38,11 +38,12 @@ var (
 )
 
 type Room struct {
-	ID         string
-	Version    gomatrixserverlib.RoomVersion
-	preset     Preset
-	visibility gomatrixserverlib.HistoryVisibility
-	creator    *User
+	ID           string
+	Version      gomatrixserverlib.RoomVersion
+	preset       Preset
+	guestCanJoin bool
+	visibility   gomatrixserverlib.HistoryVisibility
+	creator      *User
 
 	authEvents   gomatrixserverlib.AuthEvents
 	currentState map[string]*gomatrixserverlib.HeaderedEvent
@@ -120,6 +121,11 @@ func (r *Room) insertCreateEvents(t *testing.T) {
 	r.CreateAndInsert(t, r.creator, gomatrixserverlib.MRoomPowerLevels, plContent, WithStateKey(""))
 	r.CreateAndInsert(t, r.creator, gomatrixserverlib.MRoomJoinRules, joinRule, WithStateKey(""))
 	r.CreateAndInsert(t, r.creator, gomatrixserverlib.MRoomHistoryVisibility, hisVis, WithStateKey(""))
+	if r.guestCanJoin {
+		r.CreateAndInsert(t, r.creator, gomatrixserverlib.MRoomGuestAccess, map[string]string{
+			"guest_access": "can_join",
+		}, WithStateKey(""))
+	}
 }
 
 // Create an event in this room but do not insert it. Does not modify the room in any way (depth, fwd extremities, etc) so is thread-safe.
@@ -268,3 +274,9 @@ func RoomVersion(ver gomatrixserverlib.RoomVersion) roomModifier {
 		r.Version = ver
 	}
 }
+
+func GuestsCanJoin(canJoin bool) roomModifier {
+	return func(t *testing.T, r *Room) {
+		r.guestCanJoin = canJoin
+	}
+}
diff --git a/userapi/api/api.go b/userapi/api/api.go
index d3f5aefc..4ea2e91c 100644
--- a/userapi/api/api.go
+++ b/userapi/api/api.go
@@ -50,6 +50,7 @@ type KeyserverUserAPI interface {
 
 type RoomserverUserAPI interface {
 	QueryAccountData(ctx context.Context, req *QueryAccountDataRequest, res *QueryAccountDataResponse) error
+	QueryAccountByLocalpart(ctx context.Context, req *QueryAccountByLocalpartRequest, res *QueryAccountByLocalpartResponse) (err error)
 }
 
 // api functions required by the media api
@@ -671,3 +672,12 @@ type PerformSaveThreePIDAssociationRequest struct {
 	ServerName gomatrixserverlib.ServerName
 	Medium     string
 }
+
+type QueryAccountByLocalpartRequest struct {
+	Localpart  string
+	ServerName gomatrixserverlib.ServerName
+}
+
+type QueryAccountByLocalpartResponse struct {
+	Account *Account
+}
diff --git a/userapi/api/api_trace.go b/userapi/api/api_trace.go
index ce661770..d10b5767 100644
--- a/userapi/api/api_trace.go
+++ b/userapi/api/api_trace.go
@@ -204,6 +204,12 @@ func (t *UserInternalAPITrace) PerformSaveThreePIDAssociation(ctx context.Contex
 	return err
 }
 
+func (t *UserInternalAPITrace) QueryAccountByLocalpart(ctx context.Context, req *QueryAccountByLocalpartRequest, res *QueryAccountByLocalpartResponse) error {
+	err := t.Impl.QueryAccountByLocalpart(ctx, req, res)
+	util.GetLogger(ctx).Infof("QueryAccountByLocalpart req=%+v res=%+v", js(req), js(res))
+	return err
+}
+
 func js(thing interface{}) string {
 	b, err := json.Marshal(thing)
 	if err != nil {
diff --git a/userapi/internal/api.go b/userapi/internal/api.go
index 3f256457..0bb480da 100644
--- a/userapi/internal/api.go
+++ b/userapi/internal/api.go
@@ -548,6 +548,11 @@ func (a *UserInternalAPI) QueryAccessToken(ctx context.Context, req *api.QueryAc
 	return nil
 }
 
+func (a *UserInternalAPI) QueryAccountByLocalpart(ctx context.Context, req *api.QueryAccountByLocalpartRequest, res *api.QueryAccountByLocalpartResponse) (err error) {
+	res.Account, err = a.DB.GetAccountByLocalpart(ctx, req.Localpart, req.ServerName)
+	return
+}
+
 // Return the appservice 'device' or nil if the token is not an appservice. Returns an error if there was a problem
 // creating a 'device'.
 func (a *UserInternalAPI) queryAppServiceToken(ctx context.Context, token, appServiceUserID string) (*api.Device, error) {
diff --git a/userapi/inthttp/client.go b/userapi/inthttp/client.go
index 87ae058c..51b0fe3e 100644
--- a/userapi/inthttp/client.go
+++ b/userapi/inthttp/client.go
@@ -60,6 +60,7 @@ const (
 	QueryAccountByPasswordPath     = "/userapi/queryAccountByPassword"
 	QueryLocalpartForThreePIDPath  = "/userapi/queryLocalpartForThreePID"
 	QueryThreePIDsForLocalpartPath = "/userapi/queryThreePIDsForLocalpart"
+	QueryAccountByLocalpartPath    = "/userapi/queryAccountType"
 )
 
 // NewUserAPIClient creates a UserInternalAPI implemented by talking to a HTTP POST API.
@@ -440,3 +441,14 @@ func (h *httpUserInternalAPI) PerformSaveThreePIDAssociation(
 		h.httpClient, ctx, request, response,
 	)
 }
+
+func (h *httpUserInternalAPI) QueryAccountByLocalpart(
+	ctx context.Context,
+	req *api.QueryAccountByLocalpartRequest,
+	res *api.QueryAccountByLocalpartResponse,
+) error {
+	return httputil.CallInternalRPCAPI(
+		"QueryAccountByLocalpart", h.apiURL+QueryAccountByLocalpartPath,
+		h.httpClient, ctx, req, res,
+	)
+}
diff --git a/userapi/inthttp/server.go b/userapi/inthttp/server.go
index f0579079..b40b507c 100644
--- a/userapi/inthttp/server.go
+++ b/userapi/inthttp/server.go
@@ -189,4 +189,9 @@ func AddRoutes(internalAPIMux *mux.Router, s api.UserInternalAPI, enableMetrics
 		PerformSaveThreePIDAssociationPath,
 		httputil.MakeInternalRPCAPI("UserAPIPerformSaveThreePIDAssociation", enableMetrics, s.PerformSaveThreePIDAssociation),
 	)
+
+	internalAPIMux.Handle(
+		QueryAccountByLocalpartPath,
+		httputil.MakeInternalRPCAPI("AccountByLocalpart", enableMetrics, s.QueryAccountByLocalpart),
+	)
 }
diff --git a/userapi/userapi_test.go b/userapi/userapi_test.go
index 8a19af19..dada56de 100644
--- a/userapi/userapi_test.go
+++ b/userapi/userapi_test.go
@@ -307,3 +307,64 @@ func TestLoginToken(t *testing.T) {
 		})
 	})
 }
+
+func TestQueryAccountByLocalpart(t *testing.T) {
+	alice := test.NewUser(t)
+
+	localpart, userServername, _ := gomatrixserverlib.SplitID('@', alice.ID)
+
+	ctx := context.Background()
+	test.WithAllDatabases(t, func(t *testing.T, dbType test.DBType) {
+		intAPI, db, close := MustMakeInternalAPI(t, apiTestOpts{}, dbType)
+		defer close()
+
+		createdAcc, err := db.CreateAccount(ctx, localpart, userServername, "", "", alice.AccountType)
+		if err != nil {
+			t.Error(err)
+		}
+
+		testCases := func(t *testing.T, internalAPI api.UserInternalAPI) {
+			// Query existing account
+			queryAccResp := &api.QueryAccountByLocalpartResponse{}
+			if err = internalAPI.QueryAccountByLocalpart(ctx, &api.QueryAccountByLocalpartRequest{
+				Localpart:  localpart,
+				ServerName: userServername,
+			}, queryAccResp); err != nil {
+				t.Error(err)
+			}
+			if !reflect.DeepEqual(createdAcc, queryAccResp.Account) {
+				t.Fatalf("created and queried accounts don't match:\n%+v vs.\n%+v", createdAcc, queryAccResp.Account)
+			}
+
+			// Query non-existent account, this should result in an error
+			err = internalAPI.QueryAccountByLocalpart(ctx, &api.QueryAccountByLocalpartRequest{
+				Localpart:  "doesnotexist",
+				ServerName: userServername,
+			}, queryAccResp)
+
+			if err == nil {
+				t.Fatalf("expected an error, but got none: %+v", queryAccResp)
+			}
+		}
+
+		t.Run("Monolith", func(t *testing.T) {
+			testCases(t, intAPI)
+			// also test tracing
+			testCases(t, &api.UserInternalAPITrace{Impl: intAPI})
+		})
+
+		t.Run("HTTP API", func(t *testing.T) {
+			router := mux.NewRouter().PathPrefix(httputil.InternalPathPrefix).Subrouter()
+			userapi.AddInternalRoutes(router, intAPI, false)
+			apiURL, cancel := test.ListenAndServe(t, router, false)
+			defer cancel()
+
+			userHTTPApi, err := inthttp.NewUserAPIClient(apiURL, &http.Client{Timeout: time.Second * 5})
+			if err != nil {
+				t.Fatalf("failed to create HTTP client: %s", err)
+			}
+			testCases(t, userHTTPApi)
+
+		})
+	})
+}