Allow adding random delay to push (#15516)

This is to discourage timing based profiling on the push gateways.

changelog.d/15516.feature

@@ -0,0 +1 @@
+Add a config option to delay push notifications by a random amount, to discourage time-based profiling.

docs/usage/configuration/config_documentation.md

@@ -3442,6 +3442,9 @@ This option has a number of sub-options. They are as follows:
    user has unread messages in. Defaults to true, meaning push clients will see the number of
    rooms with unread messages in them. Set to false to instead send the number
    of unread messages.
+* `jitter_delay`: Delays push notifications by a random amount up to the given
+  duration. Useful for mitigating timing attacks. Optional, defaults to no
+  delay. _Added in Synapse 1.84.0._
 
 Example configuration:
 ```yaml
@@ -3449,6 +3452,7 @@ push:
   enabled: true
   include_content: false
   group_unread_count_by_room: false
+  jitter_delay: "10s"
 ```
 ---
 ## Rooms

synapse/config/push.py

@@ -42,11 +42,17 @@ class PushConfig(Config):
 
         # Now check for the one in the 'email' section and honour it,
         # with a warning.
-        push_config = config.get("email") or {}
-        redact_content = push_config.get("redact_content")
+        email_push_config = config.get("email") or {}
+        redact_content = email_push_config.get("redact_content")
         if redact_content is not None:
             print(
                 "The 'email.redact_content' option is deprecated: "
                 "please set push.include_content instead"
             )
             self.push_include_content = not redact_content
+
+        # Whether to apply a random delay to outbound push.
+        self.push_jitter_delay_ms = None
+        push_jitter_delay = push_config.get("jitter_delay", None)
+        if push_jitter_delay:
+            self.push_jitter_delay_ms = self.parse_duration(push_jitter_delay)

synapse/push/httppusher.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
+import random
 import urllib.parse
 from typing import TYPE_CHECKING, Dict, List, Optional, Union
 
@@ -114,6 +115,8 @@ class HttpPusher(Pusher):
         )
         self._pusherpool = hs.get_pusherpool()
 
+        self.push_jitter_delay_ms = hs.config.push.push_jitter_delay_ms
+
         self.data = pusher_config.data
         if self.data is None:
             raise PusherConfigException("'data' key can not be null for HTTP pusher")
@@ -327,6 +330,21 @@ class HttpPusher(Pusher):
         event = await self.store.get_event(push_action.event_id, allow_none=True)
         if event is None:
             return True  # It's been redacted
+
+        # Check if we should delay sending out the notification by a random
+        # amount.
+        #
+        # Note: we base the delay off of when the event was sent, rather than
+        # now, to handle the case where we need to send out many notifications
+        # at once. If we just slept the random amount each loop then the last
+        # push notification in the set could be delayed by many times the max
+        # delay.
+        if self.push_jitter_delay_ms:
+            delay_ms = random.randint(1, self.push_jitter_delay_ms)
+            diff_ms = event.origin_server_ts + delay_ms - self.clock.time_msec()
+            if diff_ms > 0:
+                await self.clock.sleep(diff_ms / 1000)
+
         rejected = await self.dispatch_push_event(event, tweaks, badge)
         if rejected is False:
             return False

tests/push/test_http.py

@@ -962,3 +962,40 @@ class HTTPPusherTests(HomeserverTestCase):
             channel.json_body["pushers"][0]["org.matrix.msc3881.device_id"],
             lookup_result.device_id,
         )
+
+    @override_config({"push": {"jitter_delay": "10s"}})
+    def test_jitter(self) -> None:
+        """Tests that enabling jitter actually delays sending push."""
+        user_id, access_token = self._make_user_with_pusher("user")
+        other_user_id, other_access_token = self._make_user_with_pusher("otheruser")
+
+        room = self.helper.create_room_as(user_id, tok=access_token)
+        self.helper.join(room=room, user=other_user_id, tok=other_access_token)
+
+        # Send a message and check that it did not generate a push, as it should
+        # be delayed.
+        self.helper.send(room, body="Hi!", tok=other_access_token)
+        self.assertEqual(len(self.push_attempts), 0)
+
+        # Now advance time past the max jitter, and assert the message was sent.
+        self.reactor.advance(15)
+        self.assertEqual(len(self.push_attempts), 1)
+
+        self.push_attempts[0][0].callback({})
+
+        # Now we send a bunch of messages and assert that they were all sent
+        # within the 10s max delay.
+        for _ in range(10):
+            self.helper.send(room, body="Hi!", tok=other_access_token)
+
+        index = 1
+        for _ in range(11):
+            while len(self.push_attempts) > index:
+                self.push_attempts[index][0].callback({})
+                self.pump()
+                index += 1
+
+            self.reactor.advance(1)
+            self.pump()
+
+        self.assertEqual(len(self.push_attempts), 11)