Block clients from sending server ACLs that lock the local server out. (#8708)

Fixes #4042

5 files changed, 80 insertions, 9 deletions

diff --git a/changelog.d/8708.misc b/changelog.d/8708.misc
new file mode 100644
index 0000000000..be679fb0f8
--- /dev/null
+++ b/changelog.d/8708.misc
@@ -0,0 +1 @@
+Block attempts by clients to send server ACLs, or redactions of server ACLs, that would result in the local server being blocked from the room.
diff --git a/mypy.ini b/mypy.ini
index 1ece2ba082..fc9f8d8050 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -13,6 +13,7 @@ files =
   synapse/config,
   synapse/event_auth.py,
   synapse/events/builder.py,
+  synapse/events/validator.py,
   synapse/events/spamcheck.py,
   synapse/federation,
   synapse/handlers/_base.py,
diff --git a/synapse/events/validator.py b/synapse/events/validator.py
index 5f9af8529b..f8f3b1a31e 100644
--- a/synapse/events/validator.py
+++ b/synapse/events/validator.py
@@ -13,20 +13,26 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import Union
+
 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.config.homeserver import HomeServerConfig
+from synapse.events import EventBase
+from synapse.events.builder import EventBuilder
 from synapse.events.utils import validate_canonicaljson
+from synapse.federation.federation_server import server_matches_acl_event
 from synapse.types import EventID, RoomID, UserID
 
 
 class EventValidator:
-    def validate_new(self, event, config):
+    def validate_new(self, event: EventBase, config: HomeServerConfig):
         """Validates the event has roughly the right format
 
         Args:
-            event (FrozenEvent): The event to validate.
-            config (Config): The homeserver's configuration.
+            event: The event to validate.
+            config: The homeserver's configuration.
         """
         self.validate_builder(event)
 
@@ -76,12 +82,18 @@ class EventValidator:
         if event.type == EventTypes.Retention:
             self._validate_retention(event)
 
-    def _validate_retention(self, event):
+        if event.type == EventTypes.ServerACL:
+            if not server_matches_acl_event(config.server_name, event):
+                raise SynapseError(
+                    400, "Can't create an ACL event that denies the local server"
+                )
+
+    def _validate_retention(self, event: EventBase):
         """Checks that an event that defines the retention policy for a room respects the
         format enforced by the spec.
 
         Args:
-            event (FrozenEvent): The event to validate.
+            event: The event to validate.
         """
         if not event.is_state():
             raise SynapseError(code=400, msg="must be a state event")
@@ -116,13 +128,10 @@ class EventValidator:
                 errcode=Codes.BAD_JSON,
             )
 
-    def validate_builder(self, event):
+    def validate_builder(self, event: Union[EventBase, EventBuilder]):
         """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
         fields an event would have
-
-        Args:
-            event (EventBuilder|FrozenEvent)
         """
 
         strings = ["room_id", "sender", "type"]
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index ca5602c13e..c6791fb912 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -1138,6 +1138,9 @@ class EventCreationHandler:
                 if original_event.room_id != event.room_id:
                     raise SynapseError(400, "Cannot redact event from a different room")
 
+                if original_event.type == EventTypes.ServerACL:
+                    raise AuthError(403, "Redacting server ACL events is not permitted")
+
             prev_state_ids = await context.get_prev_state_ids()
             auth_events_ids = self.auth.compute_auth_events(
                 event, prev_state_ids, for_verification=True
diff --git a/tests/handlers/test_message.py b/tests/handlers/test_message.py
index 2e0fea04af..8b57081cbe 100644
--- a/tests/handlers/test_message.py
+++ b/tests/handlers/test_message.py
@@ -154,3 +154,60 @@ class EventCreationTestCase(unittest.HomeserverTestCase):
         # Check that we've deduplicated the events.
         self.assertEqual(len(events), 2)
         self.assertEqual(events[0].event_id, events[1].event_id)
+
+
+class ServerAclValidationTestCase(unittest.HomeserverTestCase):
+    servlets = [
+        admin.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+    ]
+
+    def prepare(self, reactor, clock, hs):
+        self.user_id = self.register_user("tester", "foobar")
+        self.access_token = self.login("tester", "foobar")
+        self.room_id = self.helper.create_room_as(self.user_id, tok=self.access_token)
+
+    def test_allow_server_acl(self):
+        """Test that sending an ACL that blocks everyone but ourselves works.
+        """
+
+        self.helper.send_state(
+            self.room_id,
+            EventTypes.ServerACL,
+            body={"allow": [self.hs.hostname]},
+            tok=self.access_token,
+            expect_code=200,
+        )
+
+    def test_deny_server_acl_block_outselves(self):
+        """Test that sending an ACL that blocks ourselves does not work.
+        """
+        self.helper.send_state(
+            self.room_id,
+            EventTypes.ServerACL,
+            body={},
+            tok=self.access_token,
+            expect_code=400,
+        )
+
+    def test_deny_redact_server_acl(self):
+        """Test that attempting to redact an ACL is blocked.
+        """
+
+        body = self.helper.send_state(
+            self.room_id,
+            EventTypes.ServerACL,
+            body={"allow": [self.hs.hostname]},
+            tok=self.access_token,
+            expect_code=200,
+        )
+        event_id = body["event_id"]
+
+        # Redaction of event should fail.
+        path = "/_matrix/client/r0/rooms/%s/redact/%s" % (self.room_id, event_id)
+        request, channel = self.make_request(
+            "POST", path, content={}, access_token=self.access_token
+        )
+        self.render(request)
+        self.assertEqual(int(channel.result["code"]), 403)