Fix potential leak of per-room profiles when the user dir is rebuilt. (#10981)

There are two steps to rebuilding the user directory:

1. a scan over rooms, followed by
2. a scan over local users.

The former reads avatars and display names from the `room_memberships`
table and therefore contains potentially private avatars and
display names. The latter reads from the the `profiles` table which only
contains public data; moreover it will overwrite any private profiles
that the rooms scan may have written to the user directory. This means
that the rebuild could leak private user while the rebuild was in
progress, only to later cover up the leaks once the rebuild had completed.

This change skips over local users when writing user_directory rows
when scanning rooms. Doing so means that it'll take longer for a rebuild
to make local users searchable, which is unfortunate. I think a future
PR can improve this by swapping the order of the two steps above. (And
indeed there's more to do here, e.g. copying from `profiles` without
going via Python.)

Small tidy-ups while I'm here:

* Remove duplicated code from test_initial. This was meant to be pulled into `purge_and_rebuild_user_dir`.
* Move `is_public` before updating sharing tables. No functional change; it's still before the first read of `is_public`.
* Don't bother creating a set from dict keys. Slightly nicer and makes the code simpler.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

3 files changed, 99 insertions, 29 deletions

diff --git a/changelog.d/10981.bugfix b/changelog.d/10981.bugfix
new file mode 100644
index 0000000000..d7bf660348
--- /dev/null
+++ b/changelog.d/10981.bugfix
@@ -0,0 +1 @@
+Fix a bug that could leak local users' per-room nicknames and avatars when the user directory is rebuilt.
\ No newline at end of file
diff --git a/synapse/storage/databases/main/user_directory.py b/synapse/storage/databases/main/user_directory.py
index 5f538947ec..5c713a732e 100644
--- a/synapse/storage/databases/main/user_directory.py
+++ b/synapse/storage/databases/main/user_directory.py
@@ -228,10 +228,6 @@ class UserDirectoryBackgroundUpdateStore(StateDeltasStore):
             is_in_room = await self.is_host_joined(room_id, self.server_name)
 
             if is_in_room:
-                is_public = await self.is_room_world_readable_or_publicly_joinable(
-                    room_id
-                )
-
                 users_with_profile = await self.get_users_in_room_with_profiles(room_id)
                 # Throw away users excluded from the directory.
                 users_with_profile = {
@@ -241,22 +237,33 @@ class UserDirectoryBackgroundUpdateStore(StateDeltasStore):
                     or await self.should_include_local_user_in_dir(user_id)
                 }
 
-                # Update each user in the user directory.
+                # Upsert a user_directory record for each remote user we see.
                 for user_id, profile in users_with_profile.items():
+                    # Local users are processed separately in
+                    # `_populate_user_directory_users`; there we can read from
+                    # the `profiles` table to ensure we don't leak their per-room
+                    # profiles. It also means we write local users to this table
+                    # exactly once, rather than once for every room they're in.
+                    if self.hs.is_mine_id(user_id):
+                        continue
+                    # TODO `users_with_profile` above reads from the `user_directory`
+                    #   table, meaning that `profile` is bespoke to this room.
+                    #   and this leaks remote users' per-room profiles to the user directory.
                     await self.update_profile_in_user_dir(
                         user_id, profile.display_name, profile.avatar_url
                     )
 
-                to_insert = set()
-
+                # Now update the room sharing tables to include this room.
+                is_public = await self.is_room_world_readable_or_publicly_joinable(
+                    room_id
+                )
                 if is_public:
-                    for user_id in users_with_profile:
-                        to_insert.add(user_id)
-
-                    if to_insert:
-                        await self.add_users_in_public_rooms(room_id, to_insert)
-                        to_insert.clear()
+                    if users_with_profile:
+                        await self.add_users_in_public_rooms(
+                            room_id, users_with_profile.keys()
+                        )
                 else:
+                    to_insert = set()
                     for user_id in users_with_profile:
                         # We want the set of pairs (L, M) where L and M are
                         # in `users_with_profile` and L is local.
diff --git a/tests/storage/test_user_directory.py b/tests/storage/test_user_directory.py
index 6884ca9b7a..fddfb8db28 100644
--- a/tests/storage/test_user_directory.py
+++ b/tests/storage/test_user_directory.py
@@ -11,17 +11,19 @@
 # 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.
-from typing import Dict, List, Set, Tuple
+from typing import Any, Dict, List, Set, Tuple
+from unittest import mock
 from unittest.mock import Mock, patch
 
 from twisted.test.proto_helpers import MemoryReactor
 
-from synapse.api.constants import UserTypes
+from synapse.api.constants import EventTypes, Membership, UserTypes
 from synapse.appservice import ApplicationService
 from synapse.rest import admin
 from synapse.rest.client import login, register, room
 from synapse.server import HomeServer
 from synapse.storage import DataStore
+from synapse.storage.roommember import ProfileInfo
 from synapse.util import Clock
 
 from tests.test_utils.event_injection import inject_member_event
@@ -52,6 +54,11 @@ class GetUserDirectoryTables:
         return r
 
     async def get_users_in_public_rooms(self) -> List[Tuple[str, str]]:
+        """Fetch the entire `users_in_public_rooms` table.
+
+        Returns a list of tuples (user_id, room_id) where room_id is public and
+        contains the user with the given id.
+        """
         r = await self.store.db_pool.simple_select_list(
             "users_in_public_rooms", None, ("user_id", "room_id")
         )
@@ -62,6 +69,13 @@ class GetUserDirectoryTables:
         return retval
 
     async def get_users_who_share_private_rooms(self) -> List[Dict[str, str]]:
+        """Fetch the entire `users_who_share_private_rooms` table.
+
+        Returns a dict containing "user_id", "other_user_id" and "room_id" keys.
+        The dicts can be flattened to Tuples with the `_compress_shared` method.
+        (This seems a little awkward---maybe we could clean this up.)
+        """
+
         return await self.store.db_pool.simple_select_list(
             "users_who_share_private_rooms",
             None,
@@ -69,6 +83,10 @@ class GetUserDirectoryTables:
         )
 
     async def get_users_in_user_directory(self) -> Set[str]:
+        """Fetch the set of users in the `user_directory` table.
+
+        This is useful when checking we've correctly excluded users from the directory.
+        """
         result = await self.store.db_pool.simple_select_list(
             "user_directory",
             None,
@@ -76,6 +94,25 @@ class GetUserDirectoryTables:
         )
         return {row["user_id"] for row in result}
 
+    async def get_profiles_in_user_directory(self) -> Dict[str, ProfileInfo]:
+        """Fetch users and their profiles from the `user_directory` table.
+
+        This is useful when we want to inspect display names and avatars.
+        It's almost the entire contents of the `user_directory` table: the only
+        thing missing is an unused room_id column.
+        """
+        rows = await self.store.db_pool.simple_select_list(
+            "user_directory",
+            None,
+            ("user_id", "display_name", "avatar_url"),
+        )
+        return {
+            row["user_id"]: ProfileInfo(
+                display_name=row["display_name"], avatar_url=row["avatar_url"]
+            )
+            for row in rows
+        }
+
 
 class UserDirectoryInitialPopulationTestcase(HomeserverTestCase):
     """Ensure that rebuilding the directory writes the correct data to the DB.
@@ -201,20 +238,6 @@ class UserDirectoryInitialPopulationTestcase(HomeserverTestCase):
         self.helper.invite(private_room, src=u1, targ=u3, tok=u1_token)
         self.helper.join(private_room, user=u3, tok=u3_token)
 
-        self.get_success(self.store.update_user_directory_stream_pos(None))
-        self.get_success(self.store.delete_all_from_user_dir())
-
-        shares_private = self.get_success(
-            self.user_dir_helper.get_users_who_share_private_rooms()
-        )
-        public_users = self.get_success(
-            self.user_dir_helper.get_users_in_public_rooms()
-        )
-
-        # Nothing updated yet
-        self.assertEqual(shares_private, [])
-        self.assertEqual(public_users, [])
-
         # Do the initial population of the user directory via the background update
         self._purge_and_rebuild_user_dir()
 
@@ -346,6 +369,45 @@ class UserDirectoryInitialPopulationTestcase(HomeserverTestCase):
         # Check the AS user is not in the directory.
         self._check_room_sharing_tables(user, public, private)
 
+    def test_population_conceals_private_nickname(self) -> None:
+        # Make a private room, and set a nickname within
+        user = self.register_user("aaaa", "pass")
+        user_token = self.login(user, "pass")
+        private_room = self.helper.create_room_as(user, is_public=False, tok=user_token)
+        self.helper.send_state(
+            private_room,
+            EventTypes.Member,
+            state_key=user,
+            body={"membership": Membership.JOIN, "displayname": "BBBB"},
+            tok=user_token,
+        )
+
+        # Rebuild the user directory. Make the rescan of the `users` table a no-op
+        # so we only see the effect of scanning the `room_memberships` table.
+        async def mocked_process_users(*args: Any, **kwargs: Any) -> int:
+            await self.store.db_pool.updates._end_background_update(
+                "populate_user_directory_process_users"
+            )
+            return 1
+
+        with mock.patch.dict(
+            self.store.db_pool.updates._background_update_handlers,
+            populate_user_directory_process_users=mocked_process_users,
+        ):
+            self._purge_and_rebuild_user_dir()
+
+        # Local users are ignored by the scan over rooms
+        users = self.get_success(self.user_dir_helper.get_profiles_in_user_directory())
+        self.assertEqual(users, {})
+
+        # Do a full rebuild including the scan over the `users` table. The local
+        # user should appear with their profile name.
+        self._purge_and_rebuild_user_dir()
+        users = self.get_success(self.user_dir_helper.get_profiles_in_user_directory())
+        self.assertEqual(
+            users, {user: ProfileInfo(display_name="aaaa", avatar_url=None)}
+        )
+
 
 class UserDirectoryStoreTestCase(HomeserverTestCase):
     def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None: