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add SpamChecker callback for silently dropping inbound federated events (#12744)

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Signed-off-by: jesopo <github@lolnerd.net>
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Jess Porter 2022-05-23 17:36:21 +01:00 committed by GitHub
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5 changed files with 108 additions and 4 deletions
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1
changelog.d/12744.feature Normal file
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@ -0,0 +1 @@
Add a `drop_federated_event` callback to `SpamChecker` to disregard inbound federated events before they take up much processing power, in an emergency.

18
docs/modules/spam_checker_callbacks.md
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@ -249,6 +249,24 @@ callback returns `False`, Synapse falls through to the next one. The value of th
callback that does not return `False` will be used. If this happens, Synapse will not call
any of the subsequent implementations of this callback.
### `should_drop_federated_event`
_First introduced in Synapse v1.60.0_
```python
async def should_drop_federated_event(event: "synapse.events.EventBase") -> bool
```
Called when checking whether a remote server can federate an event with us. **Returning
`True` from this function will silently drop a federated event and split-brain our view
of a room's DAG, and thus you shouldn't use this callback unless you know what you are
doing.**
If multiple modules implement this callback, they will be considered in order. If a
callback returns `False`, Synapse falls through to the next one. The value of the first
callback that does not return `False` will be used. If this happens, Synapse will not call
any of the subsequent implementations of this callback.
## Example
The example below is a module that implements the spam checker callback

40
synapse/events/spamcheck.py
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@ -44,6 +44,10 @@ CHECK_EVENT_FOR_SPAM_CALLBACK = Callable[
["synapse.events.EventBase"],
Awaitable[Union[bool, str]],
]
SHOULD_DROP_FEDERATED_EVENT_CALLBACK = Callable[
["synapse.events.EventBase"],
Awaitable[Union[bool, str]],
]
USER_MAY_JOIN_ROOM_CALLBACK = Callable[[str, str, bool], Awaitable[bool]]
USER_MAY_INVITE_CALLBACK = Callable[[str, str, str], Awaitable[bool]]
USER_MAY_SEND_3PID_INVITE_CALLBACK = Callable[[str, str, str, str], Awaitable[bool]]
@ -168,6 +172,9 @@ class SpamChecker:
self.clock = hs.get_clock()
self._check_event_for_spam_callbacks: List[CHECK_EVENT_FOR_SPAM_CALLBACK] = []
self._should_drop_federated_event_callbacks: List[
SHOULD_DROP_FEDERATED_EVENT_CALLBACK
] = []
self._user_may_join_room_callbacks: List[USER_MAY_JOIN_ROOM_CALLBACK] = []
self._user_may_invite_callbacks: List[USER_MAY_INVITE_CALLBACK] = []
self._user_may_send_3pid_invite_callbacks: List[
@ -191,6 +198,9 @@ class SpamChecker:
def register_callbacks(
self,
check_event_for_spam: Optional[CHECK_EVENT_FOR_SPAM_CALLBACK] = None,
should_drop_federated_event: Optional[
SHOULD_DROP_FEDERATED_EVENT_CALLBACK
] = None,
user_may_join_room: Optional[USER_MAY_JOIN_ROOM_CALLBACK] = None,
user_may_invite: Optional[USER_MAY_INVITE_CALLBACK] = None,
user_may_send_3pid_invite: Optional[USER_MAY_SEND_3PID_INVITE_CALLBACK] = None,
@ -209,6 +219,11 @@ class SpamChecker:
if check_event_for_spam is not None:
self._check_event_for_spam_callbacks.append(check_event_for_spam)
if should_drop_federated_event is not None:
self._should_drop_federated_event_callbacks.append(
should_drop_federated_event
)
if user_may_join_room is not None:
self._user_may_join_room_callbacks.append(user_may_join_room)
@ -268,6 +283,31 @@ class SpamChecker:
return False
async def should_drop_federated_event(
self, event: "synapse.events.EventBase"
) -> Union[bool, str]:
"""Checks if a given federated event is considered "spammy" by this
server.
If the server considers an event spammy, it will be silently dropped,
and in doing so will split-brain our view of the room's DAG.
Args:
event: the event to be checked
Returns:
True if the event should be silently dropped
"""
for callback in self._should_drop_federated_event_callbacks:
with Measure(
self.clock, "{}.{}".format(callback.__module__, callback.__qualname__)
):
res: Union[bool, str] = await delay_cancellation(callback(event))
if res:
return res
return False
async def user_may_join_room(
self, user_id: str, room_id: str, is_invited: bool
) -> bool:

48
synapse/federation/federation_server.py
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@ -110,6 +110,7 @@ class FederationServer(FederationBase):
self.handler = hs.get_federation_handler()
self.storage = hs.get_storage()
self._spam_checker = hs.get_spam_checker()
self._federation_event_handler = hs.get_federation_event_handler()
self.state = hs.get_state_handler()
self._event_auth_handler = hs.get_event_auth_handler()
@ -1019,6 +1020,12 @@ class FederationServer(FederationBase):
except SynapseError as e:
raise FederationError("ERROR", e.code, e.msg, affected=pdu.event_id)
if await self._spam_checker.should_drop_federated_event(pdu):
logger.warning(
"Unstaged federated event contains spam, dropping %s", pdu.event_id
)
return
# Add the event to our staging area
await self.store.insert_received_event_to_staging(origin, pdu)
@ -1032,6 +1039,41 @@ class FederationServer(FederationBase):
pdu.room_id, room_version, lock, origin, pdu
)
async def _get_next_nonspam_staged_event_for_room(
self, room_id: str, room_version: RoomVersion
) -> Optional[Tuple[str, EventBase]]:
"""Fetch the first non-spam event from staging queue.
Args:
room_id: the room to fetch the first non-spam event in.
room_version: the version of the room.
Returns:
The first non-spam event in that room.
"""
while True:
# We need to do this check outside the lock to avoid a race between
# a new event being inserted by another instance and it attempting
# to acquire the lock.
next = await self.store.get_next_staged_event_for_room(
room_id, room_version
)
if next is None:
return None
origin, event = next
if await self._spam_checker.should_drop_federated_event(event):
logger.warning(
"Staged federated event contains spam, dropping %s",
event.event_id,
)
continue
return next
@wrap_as_background_process("_process_incoming_pdus_in_room_inner")
async def _process_incoming_pdus_in_room_inner(
self,
@ -1109,12 +1151,10 @@ class FederationServer(FederationBase):
(self._clock.time_msec() - received_ts) / 1000
)
# We need to do this check outside the lock to avoid a race between
# a new event being inserted by another instance and it attempting
# to acquire the lock.
next = await self.store.get_next_staged_event_for_room(
next = await self._get_next_nonspam_staged_event_for_room(
room_id, room_version
)
if not next:
break

5
synapse/module_api/__init__.py
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@ -47,6 +47,7 @@ from synapse.events.spamcheck import (
CHECK_MEDIA_FILE_FOR_SPAM_CALLBACK,
CHECK_REGISTRATION_FOR_SPAM_CALLBACK,
CHECK_USERNAME_FOR_SPAM_CALLBACK,
SHOULD_DROP_FEDERATED_EVENT_CALLBACK,
USER_MAY_CREATE_ROOM_ALIAS_CALLBACK,
USER_MAY_CREATE_ROOM_CALLBACK,
USER_MAY_INVITE_CALLBACK,
@ -234,6 +235,9 @@ class ModuleApi:
self,
*,
check_event_for_spam: Optional[CHECK_EVENT_FOR_SPAM_CALLBACK] = None,
should_drop_federated_event: Optional[
SHOULD_DROP_FEDERATED_EVENT_CALLBACK
] = None,
user_may_join_room: Optional[USER_MAY_JOIN_ROOM_CALLBACK] = None,
user_may_invite: Optional[USER_MAY_INVITE_CALLBACK] = None,
user_may_send_3pid_invite: Optional[USER_MAY_SEND_3PID_INVITE_CALLBACK] = None,
@ -254,6 +258,7 @@ class ModuleApi:
"""
return self._spam_checker.register_callbacks(
check_event_for_spam=check_event_for_spam,
should_drop_federated_event=should_drop_federated_event,
user_may_join_room=user_may_join_room,
user_may_invite=user_may_invite,
user_may_send_3pid_invite=user_may_send_3pid_invite,