Fix key verification when key stored with null valid_until_ms

Some keys are stored in the synapse database with a null valid_until_ms
which caused an exception to be thrown when using that key. We fix this
by treating nulls as zeroes, i.e. they keys will match verification
requests with a minimum_valid_until_ms of zero (i.e. don't validate ts)
but will not match requests with a non-zero minimum_valid_until_ms.

Fixes #5391.

synapse/storage/keys.py

@@ -80,6 +80,14 @@ class KeyStore(SQLBaseStore):
 
             for row in txn:
                 server_name, key_id, key_bytes, ts_valid_until_ms = row
+
+                if ts_valid_until_ms is None:
+                    # Old keys may be stored with a ts_valid_until_ms of null,
+                    # in which case we treat this as if it was set to `0`, i.e.
+                    # it won't match key requests that define a minimum
+                    # `ts_valid_until_ms`.
+                    ts_valid_until_ms = 0
+
                 res = FetchKeyResult(
                     verify_key=decode_verify_key_bytes(key_id, bytes(key_bytes)),
                     valid_until_ts=ts_valid_until_ms,

tests/crypto/test_keyring.py

@@ -25,7 +25,11 @@ from twisted.internet import defer
 
 from synapse.api.errors import SynapseError
 from synapse.crypto import keyring
-from synapse.crypto.keyring import PerspectivesKeyFetcher, ServerKeyFetcher
+from synapse.crypto.keyring import (
+    PerspectivesKeyFetcher,
+    ServerKeyFetcher,
+    StoreKeyFetcher,
+)
 from synapse.storage.keys import FetchKeyResult
 from synapse.util import logcontext
 from synapse.util.logcontext import LoggingContext
@@ -219,6 +223,50 @@ class KeyringTestCase(unittest.HomeserverTestCase):
         # self.assertFalse(d.called)
         self.get_success(d)
 
+    def test_verify_json_for_server_with_null_valid_until_ms(self):
+        """Tests that we correctly handle key requests for keys we've stored
+        with a null `ts_valid_until_ms`
+        """
+        mock_fetcher = keyring.KeyFetcher()
+        mock_fetcher.get_keys = Mock(return_value=defer.succeed({}))
+
+        kr = keyring.Keyring(
+            self.hs, key_fetchers=(StoreKeyFetcher(self.hs), mock_fetcher)
+        )
+
+        key1 = signedjson.key.generate_signing_key(1)
+        r = self.hs.datastore.store_server_verify_keys(
+            "server9",
+            time.time() * 1000,
+            [("server9", get_key_id(key1), FetchKeyResult(get_verify_key(key1), None))],
+        )
+        self.get_success(r)
+
+        json1 = {}
+        signedjson.sign.sign_json(json1, "server9", key1)
+
+        # should fail immediately on an unsigned object
+        d = _verify_json_for_server(kr, "server9", {}, 0, "test unsigned")
+        self.failureResultOf(d, SynapseError)
+
+        # should fail on a signed object with a non-zero minimum_valid_until_ms,
+        # as it tries to refetch the keys and fails.
+        d = _verify_json_for_server(
+            kr, "server9", json1, 500, "test signed non-zero min"
+        )
+        self.get_failure(d, SynapseError)
+
+        # We expect the keyring tried to refetch the key once.
+        mock_fetcher.get_keys.assert_called_once_with(
+            {"server9": {get_key_id(key1): 500}}
+        )
+
+        # should succeed on a signed object with a 0 minimum_valid_until_ms
+        d = _verify_json_for_server(
+            kr, "server9", json1, 0, "test signed with zero min"
+        )
+        self.get_success(d)
+
     def test_verify_json_dedupes_key_requests(self):
         """Two requests for the same key should be deduped."""
         key1 = signedjson.key.generate_signing_key(1)