Apply some limits to depth to counter abuse

* When creating a new event, cap its depth to 2^63 - 1
* When receiving events, reject any without a sensible depth

As per https://docs.google.com/document/d/1I3fi2S-XnpO45qrpCsowZv8P8dHcNZ4fsBsbOW7KABI

synapse/api/constants.py

@@ -16,6 +16,9 @@
 
 """Contains constants from the specification."""
 
+# the "depth" field on events is limited to 2**63 - 1
+MAX_DEPTH = 2**63 - 1
+
 
 class Membership(object):
 

synapse/federation/federation_base.py

@@ -14,7 +14,10 @@
 # limitations under the License.
 import logging
 
-from synapse.api.errors import SynapseError
+import six
+
+from synapse.api.constants import MAX_DEPTH
+from synapse.api.errors import SynapseError, Codes
 from synapse.crypto.event_signing import check_event_content_hash
 from synapse.events import FrozenEvent
 from synapse.events.utils import prune_event
@@ -190,11 +193,23 @@ def event_from_pdu_json(pdu_json, outlier=False):
         FrozenEvent
 
     Raises:
-        SynapseError: if the pdu is missing required fields
+        SynapseError: if the pdu is missing required fields or is otherwise
+            not a valid matrix event
     """
     # we could probably enforce a bunch of other fields here (room_id, sender,
     # origin, etc etc)
-    assert_params_in_request(pdu_json, ('event_id', 'type'))
+    assert_params_in_request(pdu_json, ('event_id', 'type', 'depth'))
+
+    depth = pdu_json['depth']
+    if not isinstance(depth, six.integer_types):
+        raise SynapseError(400, "Depth %r not an intger" % (depth, ),
+                           Codes.BAD_JSON)
+
+    if depth < 0:
+        raise SynapseError(400, "Depth too small", Codes.BAD_JSON)
+    elif depth > MAX_DEPTH:
+        raise SynapseError(400, "Depth too large", Codes.BAD_JSON)
+
     event = FrozenEvent(
         pdu_json
     )

synapse/handlers/message.py

@@ -16,7 +16,7 @@
 from twisted.internet import defer, reactor
 from twisted.python.failure import Failure
 
-from synapse.api.constants import EventTypes, Membership
+from synapse.api.constants import EventTypes, Membership, MAX_DEPTH
 from synapse.api.errors import AuthError, Codes, SynapseError
 from synapse.crypto.event_signing import add_hashes_and_signatures
 from synapse.events.utils import serialize_event
@@ -624,6 +624,10 @@ class EventCreationHandler(object):
 
         if prev_events_and_hashes:
             depth = max([d for _, _, d in prev_events_and_hashes]) + 1
+            # we cap depth of generated events, to ensure that they are not
+            # rejected by other servers (and so that they can be persisted in
+            # the db)
+            depth = min(depth, MAX_DEPTH)
         else:
             depth = 1