Improve validation of field size limits in events.

changelog.d/14664.bugfix

@@ -0,0 +1 @@
+Improve validation of field size limits in events.

synapse/api/constants.py

@@ -152,6 +152,7 @@ class EduTypes:
 
 class RejectedReason:
     AUTH_ERROR: Final = "auth_error"
+    OVERSIZED_EVENT: Final = "oversized_event"
 
 
 class RoomCreationPreset:

synapse/api/errors.py

@@ -424,8 +424,17 @@ def error_dict(self, config: Optional["HomeServerConfig"]) -> "JsonDict":
 class EventSizeError(SynapseError):
     """An error raised when an event is too big."""
 
-    def __init__(self, msg: str):
+    def __init__(self, msg: str, unpersistable: bool):
+        """
+        strict:
+            if True, the PDU must not be persisted, not even as a rejected PDU
+            when received over federation.
+            This is notably true when the entire PDU exceeds the size limit for a PDU,
+            (as opposed to an individual key's size limit being exceeded).
+        """
+
         super().__init__(413, msg, Codes.TOO_LARGE)
+        self.unpersistable = unpersistable
 
 
 class LoginError(SynapseError):

synapse/api/room_versions.py

@@ -58,7 +58,10 @@ class PushRuleRoomFlag:
     EXTENSIBLE_EVENTS = "org.matrix.msc3932.extensible_events"
 
 
-@attr.s(slots=True, frozen=True, auto_attribs=True)
+# eq=False means equality and hashing are done using object identity, which suits
+# our purposes because each RoomVersion is instantiated only once and some of
+# the fields are not hashable.
+@attr.s(slots=True, frozen=True, auto_attribs=True, eq=False)
 class RoomVersion:
     """An object which describes the unique attributes of a room version."""
 

synapse/event_auth.py

@@ -52,6 +52,7 @@
     KNOWN_ROOM_VERSIONS,
     EventFormatVersions,
     RoomVersion,
+    RoomVersions,
 )
 from synapse.storage.databases.main.events_worker import EventRedactBehaviour
 from synapse.types import MutableStateMap, StateMap, UserID, get_domain_from_id
@@ -341,19 +342,79 @@ def check_state_dependent_auth_rules(
     logger.debug("Allowing! %s", event)
 
 
+# Set of room versions where Synapse did not apply event key size limits
+# in bytes, but rather in codepoints.
+# In these room versions, we are more lenient with event size validation.
+LENIENT_EVENT_BYTE_LIMITS_ROOM_VERSIONS = {
+    RoomVersions.V1,
+    RoomVersions.V2,
+    RoomVersions.V3,
+    RoomVersions.V4,
+    RoomVersions.V5,
+    RoomVersions.V6,
+    RoomVersions.MSC2176,
+    RoomVersions.V7,
+    RoomVersions.V8,
+    RoomVersions.V9,
+    RoomVersions.MSC3787,
+    RoomVersions.V10,
+    RoomVersions.MSC2716v4,
+}
+
+
 def _check_size_limits(event: "EventBase") -> None:
+    """
+    Checks the size limits in a PDU.
+
+    The entire size limit of the PDU is checked first.
+    Then the size of fields is checked, first in codepoints and then in bytes.
+
+    The codepoint size limits are only for Synapse compatibility.
+
+    Raises:
+        EventSizeError:
+            when a size limit has been violated.
+
+            strict=True if Synapse never would have accepted the event and
+                the PDU must NOT be persisted.
+
+            strict=False if a prior version of Synapse would have accepted the
+                event and so the PDU must be persisted as rejected to avoid
+                breaking the room.
+    """
+
+    # Whole PDU check
+    if len(encode_canonical_json(event.get_pdu_json())) > MAX_PDU_SIZE:
+        raise EventSizeError("event too large", unpersistable=True)
+
+    # Codepoint size check: Synapse always enforced these limits, so apply
+    # them strictly.
     if len(event.user_id) > 255:
-        raise EventSizeError("'user_id' too large")
+        raise EventSizeError("'user_id' too large", unpersistable=True)
     if len(event.room_id) > 255:
-        raise EventSizeError("'room_id' too large")
+        raise EventSizeError("'room_id' too large", unpersistable=True)
     if event.is_state() and len(event.state_key) > 255:
-        raise EventSizeError("'state_key' too large")
+        raise EventSizeError("'state_key' too large", unpersistable=True)
     if len(event.type) > 255:
-        raise EventSizeError("'type' too large")
+        raise EventSizeError("'type' too large", unpersistable=True)
     if len(event.event_id) > 255:
-        raise EventSizeError("'event_id' too large")
-    if len(encode_canonical_json(event.get_pdu_json())) > MAX_PDU_SIZE:
-        raise EventSizeError("event too large")
+        raise EventSizeError("'event_id' too large", unpersistable=True)
+
+    strict_byte_limits = (
+        event.room_version not in LENIENT_EVENT_BYTE_LIMITS_ROOM_VERSIONS
+    )
+
+    # Byte size check: if these fail, then be lenient to avoid breaking rooms.
+    if len(event.user_id.encode("utf-8")) > 255:
+        raise EventSizeError("'user_id' too large", unpersistable=strict_byte_limits)
+    if len(event.room_id.encode("utf-8")) > 255:
+        raise EventSizeError("'room_id' too large", unpersistable=strict_byte_limits)
+    if event.is_state() and len(event.state_key.encode("utf-8")) > 255:
+        raise EventSizeError("'state_key' too large", unpersistable=strict_byte_limits)
+    if len(event.type.encode("utf-8")) > 255:
+        raise EventSizeError("'type' too large", unpersistable=strict_byte_limits)
+    if len(event.event_id.encode("utf-8")) > 255:
+        raise EventSizeError("'event_id' too large", unpersistable=strict_byte_limits)
 
 
 def _check_create(event: "EventBase") -> None:

synapse/handlers/federation_event.py

@@ -43,6 +43,7 @@
 from synapse.api.errors import (
     AuthError,
     Codes,
+    EventSizeError,
     FederationError,
     FederationPullAttemptBackoffError,
     HttpResponseException,
@@ -1736,6 +1737,16 @@ async def prep(event: EventBase) -> None:
                 except AuthError as e:
                     logger.warning("Rejecting %r because %s", event, e)
                     context.rejected = RejectedReason.AUTH_ERROR
+                except EventSizeError as e:
+                    if e.unpersistable:
+                        # A strict event size error means the event is completely
+                        # unpersistable.
+                        raise e
+                    # Otherwise, for non-strict errors, we just persist the event
+                    # as rejected, for moderate compatibility with older Synapse
+                    # versions.
+                    logger.warning("While validating received event %r: %s", event, e)
+                    context.rejected = RejectedReason.OVERSIZED_EVENT
 
             events_and_contexts_to_persist.append((event, context))
 
@@ -1781,6 +1792,17 @@ async def _check_event_auth(
             # TODO: use a different rejected reason here?
             context.rejected = RejectedReason.AUTH_ERROR
             return
+        except EventSizeError as e:
+            if e.unpersistable:
+                # A strict event size error means the event is completely
+                # unpersistable.
+                raise e
+            # Otherwise, for non-strict errors, we just persist the event
+            # as rejected, for moderate compatibility with older Synapse
+            # versions.
+            logger.warning("While validating received event %r: %s", event, e)
+            context.rejected = RejectedReason.OVERSIZED_EVENT
+            return
 
         # next, check that we have all of the event's auth events.
         #