Limit the size of HTTP responses read over federation.

changelog.d/9833.bugfix

@@ -0,0 +1 @@
+Limit the size of HTTP responses read over federation.

synapse/http/client.py

@@ -33,6 +33,7 @@
 from canonicaljson import encode_canonical_json
 from netaddr import AddrFormatError, IPAddress, IPSet
 from prometheus_client import Counter
+from typing_extensions import Protocol
 from zope.interface import implementer, provider
 
 from OpenSSL import SSL
@@ -754,6 +755,16 @@ def _timeout_to_request_timed_out_error(f: Failure):
     return f
 
 
+class ByteWriteable(Protocol):
+    """The type of object which must be passed into read_body_with_max_size.
+
+    Typically this is a file object.
+    """
+
+    def write(self, data: bytes) -> int:
+        pass
+
+
 class BodyExceededMaxSize(Exception):
     """The maximum allowed size of the HTTP body was exceeded."""
 
@@ -790,7 +801,7 @@ class _ReadBodyWithMaxSizeProtocol(protocol.Protocol):
     transport = None  # type: Optional[ITCPTransport]
 
     def __init__(
-        self, stream: BinaryIO, deferred: defer.Deferred, max_size: Optional[int]
+        self, stream: ByteWriteable, deferred: defer.Deferred, max_size: Optional[int]
     ):
         self.stream = stream
         self.deferred = deferred
@@ -830,7 +841,7 @@ def connectionLost(self, reason: Failure = connectionDone) -> None:
 
 
 def read_body_with_max_size(
-    response: IResponse, stream: BinaryIO, max_size: Optional[int]
+    response: IResponse, stream: ByteWriteable, max_size: Optional[int]
 ) -> defer.Deferred:
     """
     Read a HTTP response body to a file-object. Optionally enforcing a maximum file size.

synapse/http/matrixfederationclient.py

@@ -1,5 +1,4 @@
-# Copyright 2014-2016 OpenMarket Ltd
-# Copyright 2018 New Vector Ltd
+# Copyright 2014-2021 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -13,11 +12,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import cgi
+import codecs
 import logging
 import random
 import sys
+import typing
 import urllib.parse
-from io import BytesIO
+from io import BytesIO, StringIO
 from typing import Callable, Dict, List, Optional, Tuple, Union
 
 import attr
@@ -72,6 +73,9 @@
     "synapse_http_matrixfederationclient_responses", "", ["method", "code"]
 )
 
+# a federation response can be rather large (eg a big state_ids is 50M or so), so we
+# need a generous limit here.
+MAX_RESPONSE_SIZE = 100 * 1024 * 1024
 
 MAX_LONG_RETRIES = 10
 MAX_SHORT_RETRIES = 3
@@ -167,12 +171,27 @@ async def _handle_json_response(
     try:
         check_content_type_is_json(response.headers)
 
-        # Use the custom JSON decoder (partially re-implements treq.json_content).
-        d = treq.text_content(response, encoding="utf-8")
-        d.addCallback(json_decoder.decode)
+        buf = StringIO()
+        d = read_body_with_max_size(response, BinaryIOWrapper(buf), MAX_RESPONSE_SIZE)
         d = timeout_deferred(d, timeout=timeout_sec, reactor=reactor)
 
+        def parse(_len: int):
+            return json_decoder.decode(buf.getvalue())
+
+        d.addCallback(parse)
+
         body = await make_deferred_yieldable(d)
+    except BodyExceededMaxSize as e:
+        # The response was too big.
+        logger.warning(
+            "{%s} [%s] JSON response exceeded max size %i - %s %s",
+            request.txn_id,
+            request.destination,
+            MAX_RESPONSE_SIZE,
+            request.method,
+            request.uri.decode("ascii"),
+        )
+        raise RequestSendFailed(e, can_retry=False) from e
     except ValueError as e:
         # The JSON content was invalid.
         logger.warning(
@@ -218,6 +237,18 @@ async def _handle_json_response(
     return body
 
 
+class BinaryIOWrapper:
+    """A wrapper for a TextIO which converts from bytes on the fly."""
+
+    def __init__(self, file: typing.TextIO, encoding="utf-8", errors="strict"):
+        self.decoder = codecs.getincrementaldecoder(encoding)(errors)
+        self.file = file
+
+    def write(self, b: Union[bytes, bytearray]) -> int:
+        self.file.write(self.decoder.decode(b))
+        return len(b)
+
+
 class MatrixFederationHttpClient:
     """HTTP client used to talk to other homeservers over the federation
     protocol. Send client certificates and signs requests.

tests/http/test_fedclient.py

@@ -26,6 +26,7 @@
 
 from synapse.api.errors import RequestSendFailed
 from synapse.http.matrixfederationclient import (
+    MAX_RESPONSE_SIZE,
     MatrixFederationHttpClient,
     MatrixFederationRequest,
 )
@@ -560,3 +561,61 @@ def test_json_error(self, return_value):
 
         f = self.failureResultOf(test_d)
         self.assertIsInstance(f.value, RequestSendFailed)
+
+    def test_too_big(self):
+        """
+        Test what happens if a huge response is returned from the remote endpoint.
+        """
+
+        test_d = defer.ensureDeferred(self.cl.get_json("testserv:8008", "foo/bar"))
+
+        self.pump()
+
+        # Nothing happened yet
+        self.assertNoResult(test_d)
+
+        # Make sure treq is trying to connect
+        clients = self.reactor.tcpClients
+        self.assertEqual(len(clients), 1)
+        (host, port, factory, _timeout, _bindAddress) = clients[0]
+        self.assertEqual(host, "1.2.3.4")
+        self.assertEqual(port, 8008)
+
+        # complete the connection and wire it up to a fake transport
+        protocol = factory.buildProtocol(None)
+        transport = StringTransport()
+        protocol.makeConnection(transport)
+
+        # that should have made it send the request to the transport
+        self.assertRegex(transport.value(), b"^GET /foo/bar")
+        self.assertRegex(transport.value(), b"Host: testserv:8008")
+
+        # Deferred is still without a result
+        self.assertNoResult(test_d)
+
+        # Send it a huge HTTP response
+        protocol.dataReceived(
+            b"HTTP/1.1 200 OK\r\n"
+            b"Server: Fake\r\n"
+            b"Content-Type: application/json\r\n"
+            b"\r\n"
+        )
+
+        self.pump()
+
+        # should still be waiting
+        self.assertNoResult(test_d)
+
+        sent = 0
+        chunk_size = 1024 * 512
+        while not test_d.called:
+            protocol.dataReceived(b"a" * chunk_size)
+            sent += chunk_size
+            self.assertLessEqual(sent, MAX_RESPONSE_SIZE)
+
+        self.assertEqual(sent, MAX_RESPONSE_SIZE)
+
+        f = self.failureResultOf(test_d)
+        self.assertIsInstance(f.value, RequestSendFailed)
+
+        self.assertTrue(transport.disconnecting)