Do checks on aliases for incoming m.room.aliases events (#5128)

Follow-up to #5124

Also added a bunch of checks to make sure everything (both the stuff added on #5124 and this PR) works as intended.

7 files changed, 225 insertions, 8 deletions

diff --git a/changelog.d/5128.bugfix b/changelog.d/5128.bugfix
new file mode 100644
index 0000000000..46df1e9fd5
--- /dev/null
+++ b/changelog.d/5128.bugfix
@@ -0,0 +1 @@
+Add some missing limitations to room alias creation.
diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index 0860b75905..8547a63535 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -20,6 +20,9 @@
 # the "depth" field on events is limited to 2**63 - 1
 MAX_DEPTH = 2**63 - 1
 
+# the maximum length for a room alias is 255 characters
+MAX_ALIAS_LENGTH = 255
+
 
 class Membership(object):
 
diff --git a/synapse/events/snapshot.py b/synapse/events/snapshot.py
index 368b5f6ae4..fa09c132a0 100644
--- a/synapse/events/snapshot.py
+++ b/synapse/events/snapshot.py
@@ -187,7 +187,9 @@ class EventContext(object):
 
         Returns:
             Deferred[dict[(str, str), str]|None]: Returns None if state_group
-            is None, which happens when the associated event is an outlier.
+                is None, which happens when the associated event is an outlier.
+                Maps a (type, state_key) to the event ID of the state event matching
+                this tuple.
         """
 
         if not self._fetching_state_deferred:
@@ -205,7 +207,9 @@ class EventContext(object):
 
         Returns:
             Deferred[dict[(str, str), str]|None]: Returns None if state_group
-            is None, which happens when the associated event is an outlier.
+                is None, which happens when the associated event is an outlier.
+                Maps a (type, state_key) to the event ID of the state event matching
+                this tuple.
         """
 
         if not self._fetching_state_deferred:
diff --git a/synapse/events/validator.py b/synapse/events/validator.py
index 514273c792..711af512b2 100644
--- a/synapse/events/validator.py
+++ b/synapse/events/validator.py
@@ -15,8 +15,8 @@
 
 from six import string_types
 
-from synapse.api.constants import EventTypes, Membership
-from synapse.api.errors import SynapseError
+from synapse.api.constants import MAX_ALIAS_LENGTH, EventTypes, Membership
+from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import EventFormatVersions
 from synapse.types import EventID, RoomID, UserID
 
@@ -56,6 +56,17 @@ class EventValidator(object):
             if not isinstance(getattr(event, s), string_types):
                 raise SynapseError(400, "'%s' not a string type" % (s,))
 
+        if event.type == EventTypes.Aliases:
+            if "aliases" in event.content:
+                for alias in event.content["aliases"]:
+                    if len(alias) > MAX_ALIAS_LENGTH:
+                        raise SynapseError(
+                            400,
+                            ("Can't create aliases longer than"
+                             " %d characters" % (MAX_ALIAS_LENGTH,)),
+                            Codes.INVALID_PARAM,
+                        )
+
     def validate_builder(self, event):
         """Validates that the builder/event has roughly the right format. Only
         checks values that we expect a proto event to have, rather than all the
diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index 50c587aa61..a12f9508d8 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -19,7 +19,7 @@ import string
 
 from twisted.internet import defer
 
-from synapse.api.constants import EventTypes
+from synapse.api.constants import MAX_ALIAS_LENGTH, EventTypes
 from synapse.api.errors import (
     AuthError,
     CodeMessageException,
@@ -36,7 +36,6 @@ logger = logging.getLogger(__name__)
 
 
 class DirectoryHandler(BaseHandler):
-    MAX_ALIAS_LENGTH = 255
 
     def __init__(self, hs):
         super(DirectoryHandler, self).__init__(hs)
@@ -105,10 +104,10 @@ class DirectoryHandler(BaseHandler):
 
         user_id = requester.user.to_string()
 
-        if len(room_alias.to_string()) > self.MAX_ALIAS_LENGTH:
+        if len(room_alias.to_string()) > MAX_ALIAS_LENGTH:
             raise SynapseError(
                 400,
-                "Can't create aliases longer than %s characters" % self.MAX_ALIAS_LENGTH,
+                "Can't create aliases longer than %s characters" % MAX_ALIAS_LENGTH,
                 Codes.INVALID_PARAM,
             )
 
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 224d34ef3a..e5afeadf68 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -228,6 +228,7 @@ class EventCreationHandler(object):
         self.ratelimiter = hs.get_ratelimiter()
         self.notifier = hs.get_notifier()
         self.config = hs.config
+        self.require_membership_for_aliases = hs.config.require_membership_for_aliases
 
         self.send_event_to_master = ReplicationSendEventRestServlet.make_client(hs)
 
@@ -336,6 +337,35 @@ class EventCreationHandler(object):
             prev_events_and_hashes=prev_events_and_hashes,
         )
 
+        # In an ideal world we wouldn't need the second part of this condition. However,
+        # this behaviour isn't spec'd yet, meaning we should be able to deactivate this
+        # behaviour. Another reason is that this code is also evaluated each time a new
+        # m.room.aliases event is created, which includes hitting a /directory route.
+        # Therefore not including this condition here would render the similar one in
+        # synapse.handlers.directory pointless.
+        if builder.type == EventTypes.Aliases and self.require_membership_for_aliases:
+            # Ideally we'd do the membership check in event_auth.check(), which
+            # describes a spec'd algorithm for authenticating events received over
+            # federation as well as those created locally. As of room v3, aliases events
+            # can be created by users that are not in the room, therefore we have to
+            # tolerate them in event_auth.check().
+            prev_state_ids = yield context.get_prev_state_ids(self.store)
+            prev_event_id = prev_state_ids.get((EventTypes.Member, event.sender))
+            prev_event = yield self.store.get_event(prev_event_id, allow_none=True)
+            if not prev_event or prev_event.membership != Membership.JOIN:
+                logger.warning(
+                    ("Attempt to send `m.room.aliases` in room %s by user %s but"
+                     " membership is %s"),
+                    event.room_id,
+                    event.sender,
+                    prev_event.membership if prev_event else None,
+                )
+
+                raise AuthError(
+                    403,
+                    "You must be in the room to create an alias for it",
+                )
+
         self.validator.validate_new(event)
 
         defer.returnValue((event, context))
diff --git a/tests/rest/client/v1/test_directory.py b/tests/rest/client/v1/test_directory.py
new file mode 100644
index 0000000000..4804000ec7
--- /dev/null
+++ b/tests/rest/client/v1/test_directory.py
@@ -0,0 +1,169 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import json
+
+from synapse.rest.admin import register_servlets
+from synapse.rest.client.v1 import directory, login, room
+from synapse.types import RoomAlias
+from synapse.util.stringutils import random_string
+
+from tests import unittest
+
+
+class DirectoryTestCase(unittest.HomeserverTestCase):
+
+    servlets = [
+        register_servlets,
+        directory.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+    ]
+
+    def make_homeserver(self, reactor, clock):
+        config = self.default_config()
+        config.require_membership_for_aliases = True
+
+        self.hs = self.setup_test_homeserver(config=config)
+
+        return self.hs
+
+    def prepare(self, reactor, clock, homeserver):
+        self.room_owner = self.register_user("room_owner", "test")
+        self.room_owner_tok = self.login("room_owner", "test")
+
+        self.room_id = self.helper.create_room_as(
+            self.room_owner, tok=self.room_owner_tok,
+        )
+
+        self.user = self.register_user("user", "test")
+        self.user_tok = self.login("user", "test")
+
+    def test_state_event_not_in_room(self):
+        self.ensure_user_left_room()
+        self.set_alias_via_state_event(403)
+
+    def test_directory_endpoint_not_in_room(self):
+        self.ensure_user_left_room()
+        self.set_alias_via_directory(403)
+
+    def test_state_event_in_room_too_long(self):
+        self.ensure_user_joined_room()
+        self.set_alias_via_state_event(400, alias_length=256)
+
+    def test_directory_in_room_too_long(self):
+        self.ensure_user_joined_room()
+        self.set_alias_via_directory(400, alias_length=256)
+
+    def test_state_event_in_room(self):
+        self.ensure_user_joined_room()
+        self.set_alias_via_state_event(200)
+
+    def test_directory_in_room(self):
+        self.ensure_user_joined_room()
+        self.set_alias_via_directory(200)
+
+    def test_room_creation_too_long(self):
+        url = "/_matrix/client/r0/createRoom"
+
+        # We use deliberately a localpart under the length threshold so
+        # that we can make sure that the check is done on the whole alias.
+        data = {
+            "room_alias_name": random_string(256 - len(self.hs.hostname)),
+        }
+        request_data = json.dumps(data)
+        request, channel = self.make_request(
+            "POST", url, request_data, access_token=self.user_tok,
+        )
+        self.render(request)
+        self.assertEqual(channel.code, 400, channel.result)
+
+    def test_room_creation(self):
+        url = "/_matrix/client/r0/createRoom"
+
+        # Check with an alias of allowed length. There should already be
+        # a test that ensures it works in test_register.py, but let's be
+        # as cautious as possible here.
+        data = {
+            "room_alias_name": random_string(5),
+        }
+        request_data = json.dumps(data)
+        request, channel = self.make_request(
+            "POST", url, request_data, access_token=self.user_tok,
+        )
+        self.render(request)
+        self.assertEqual(channel.code, 200, channel.result)
+
+    def set_alias_via_state_event(self, expected_code, alias_length=5):
+        url = ("/_matrix/client/r0/rooms/%s/state/m.room.aliases/%s"
+               % (self.room_id, self.hs.hostname))
+
+        data = {
+            "aliases": [
+                self.random_alias(alias_length),
+            ],
+        }
+        request_data = json.dumps(data)
+
+        request, channel = self.make_request(
+            "PUT", url, request_data, access_token=self.user_tok,
+        )
+        self.render(request)
+        self.assertEqual(channel.code, expected_code, channel.result)
+
+    def set_alias_via_directory(self, expected_code, alias_length=5):
+        url = "/_matrix/client/r0/directory/room/%s" % self.random_alias(alias_length)
+        data = {
+            "room_id": self.room_id,
+        }
+        request_data = json.dumps(data)
+
+        request, channel = self.make_request(
+            "PUT", url, request_data, access_token=self.user_tok,
+        )
+        self.render(request)
+        self.assertEqual(channel.code, expected_code, channel.result)
+
+    def random_alias(self, length):
+        return RoomAlias(
+            random_string(length),
+            self.hs.hostname,
+        ).to_string()
+
+    def ensure_user_left_room(self):
+        self.ensure_membership("leave")
+
+    def ensure_user_joined_room(self):
+        self.ensure_membership("join")
+
+    def ensure_membership(self, membership):
+        try:
+            if membership == "leave":
+                self.helper.leave(
+                    room=self.room_id,
+                    user=self.user,
+                    tok=self.user_tok,
+                )
+            if membership == "join":
+                self.helper.join(
+                    room=self.room_id,
+                    user=self.user,
+                    tok=self.user_tok,
+                )
+        except AssertionError:
+            # We don't care whether the leave request didn't return a 200 (e.g.
+            # if the user isn't already in the room), because we only want to
+            # make sure the user isn't in the room.
+            pass