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Respond with useful error codes when Content-Length header/s are invalid (#19212)

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Related to https://github.com/element-hq/synapse/issues/17035, when
Synapse receives a request that is larger than the maximum size allowed,
it aborts the connection without ever sending back a HTTP response.
I dug into our usage of twisted and how best to try and report such an
error and this is what I came up with.

It would be ideal to be able to report the status from within
`handleContentChunk` but that is called too early on in the twisted http
handling code, before things have been setup enough to be able to
properly write a response.
I tested this change out locally (both with C-S and S-S apis) and they
do receive a 413 response now in addition to the connection being
closed.

Hopefully this will aid in being able to quickly detect when
https://github.com/element-hq/synapse/issues/17035 is occurring as the
current situation makes it very hard to narrow things down to that
specific issue without making a lot of assumptions.

This PR also responds with more meaningful error codes now in the case
of:
- multiple `Content-Length` headers
- invalid `Content-Length` header value
- request content size being larger than the `Content-Length` value

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [X] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Eric Eastwood <erice@element.io>
This commit is contained in:
Devon Hudson
2025-12-08 21:39:18 +00:00
committed by GitHub
parent 09fd2645c2
commit 8b0083cad9
10 changed files with 336 additions and 50 deletions
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102
tests/http/test_site.py
View File

@@ -22,6 +22,7 @@
from twisted.internet.address import IPv6Address
from twisted.internet.testing import MemoryReactor, StringTransport
from synapse.app._base import max_request_body_size
from synapse.app.homeserver import SynapseHomeServer
from synapse.server import HomeServer
from synapse.util.clock import Clock
@@ -143,3 +144,104 @@ class SynapseRequestTestCase(HomeserverTestCase):
# we should get a 415
self.assertRegex(transport.value().decode(), r"^HTTP/1\.1 415 ")
def test_content_length_too_large(self) -> None:
"""HTTP requests with Content-Length exceeding max size should be rejected with 413"""
self.hs.start_listening()
# find the HTTP server which is configured to listen on port 0
(port, factory, _backlog, interface) = self.reactor.tcpServers[0]
self.assertEqual(interface, "::")
self.assertEqual(port, 0)
# complete the connection and wire it up to a fake transport
client_address = IPv6Address("TCP", "::1", 2345)
protocol = factory.buildProtocol(client_address)
transport = StringTransport()
protocol.makeConnection(transport)
# Send a request with Content-Length header that exceeds the limit.
# Default max is 50MB (from media max_upload_size), so send something larger.
oversized_length = 1 + max_request_body_size(self.hs.config)
protocol.dataReceived(
b"POST / HTTP/1.1\r\n"
b"Connection: close\r\n"
b"Content-Length: " + str(oversized_length).encode() + b"\r\n"
b"\r\n"
b"" + b"x" * oversized_length + b"\r\n"
b"\r\n"
)
# Advance the reactor to process the request
while not transport.disconnecting:
self.reactor.advance(1)
# We should get a 413 Content Too Large
response = transport.value().decode()
self.assertRegex(response, r"^HTTP/1\.1 413 ")
self.assertSubstring("M_TOO_LARGE", response)
def test_too_many_content_length_headers(self) -> None:
"""HTTP requests with multiple Content-Length headers should be rejected with 400"""
self.hs.start_listening()
# find the HTTP server which is configured to listen on port 0
(port, factory, _backlog, interface) = self.reactor.tcpServers[0]
self.assertEqual(interface, "::")
self.assertEqual(port, 0)
# complete the connection and wire it up to a fake transport
client_address = IPv6Address("TCP", "::1", 2345)
protocol = factory.buildProtocol(client_address)
transport = StringTransport()
protocol.makeConnection(transport)
protocol.dataReceived(
b"POST / HTTP/1.1\r\n"
b"Connection: close\r\n"
b"Content-Length: " + str(5).encode() + b"\r\n"
b"Content-Length: " + str(5).encode() + b"\r\n"
b"\r\n"
b"" + b"xxxxx" + b"\r\n"
b"\r\n"
)
# Advance the reactor to process the request
while not transport.disconnecting:
self.reactor.advance(1)
# We should get a 400
response = transport.value().decode()
self.assertRegex(response, r"^HTTP/1\.1 400 ")
def test_invalid_content_length_headers(self) -> None:
"""HTTP requests with invalid Content-Length header should be rejected with 400"""
self.hs.start_listening()
# find the HTTP server which is configured to listen on port 0
(port, factory, _backlog, interface) = self.reactor.tcpServers[0]
self.assertEqual(interface, "::")
self.assertEqual(port, 0)
# complete the connection and wire it up to a fake transport
client_address = IPv6Address("TCP", "::1", 2345)
protocol = factory.buildProtocol(client_address)
transport = StringTransport()
protocol.makeConnection(transport)
protocol.dataReceived(
b"POST / HTTP/1.1\r\n"
b"Connection: close\r\n"
b"Content-Length: eight\r\n"
b"\r\n"
b"" + b"xxxxx" + b"\r\n"
b"\r\n"
)
# Advance the reactor to process the request
while not transport.disconnecting:
self.reactor.advance(1)
# We should get a 400
response = transport.value().decode()
self.assertRegex(response, r"^HTTP/1\.1 400 ")

6
tests/rest/client/test_login.py
View File

@@ -1728,9 +1728,6 @@ class UsernamePickerTestCase(HomeserverTestCase):
content_is_form=True,
custom_headers=[
("Cookie", "username_mapping_session=" + session_id),
# old versions of twisted don't do form-parsing without a valid
# content-length header.
("Content-Length", str(len(content))),
],
)
self.assertEqual(chan.code, 302, chan.result)
@@ -1818,9 +1815,6 @@ class UsernamePickerTestCase(HomeserverTestCase):
content_is_form=True,
custom_headers=[
("Cookie", "username_mapping_session=" + session_id),
# old versions of twisted don't do form-parsing without a valid
# content-length header.
("Content-Length", str(len(content))),
],
)
self.assertEqual(chan.code, 302, chan.result)

4
tests/rest/client/test_media.py
View File

@@ -2590,7 +2590,6 @@ class AuthenticatedMediaTestCase(unittest.HomeserverTestCase):
self.tok,
shorthand=False,
content_type=b"image/png",
custom_headers=[("Content-Length", str(67))],
)
self.assertEqual(channel.code, 200)
res = channel.json_body.get("content_uri")
@@ -2750,7 +2749,6 @@ class AuthenticatedMediaTestCase(unittest.HomeserverTestCase):
self.tok,
shorthand=False,
content_type=b"image/png",
custom_headers=[("Content-Length", str(67))],
)
self.assertEqual(channel.code, 200)
res = channel.json_body.get("content_uri")
@@ -2909,7 +2907,6 @@ class MediaUploadLimits(unittest.HomeserverTestCase):
access_token=self.tok,
shorthand=False,
content_type=b"text/plain",
custom_headers=[("Content-Length", str(size))],
)
def test_upload_under_limit(self) -> None:
@@ -3074,7 +3071,6 @@ class MediaUploadLimitsModuleOverrides(unittest.HomeserverTestCase):
access_token=tok,
shorthand=False,
content_type=b"text/plain",
custom_headers=[("Content-Length", str(size))],
)
def test_upload_under_limit(self) -> None:

2
tests/rest/client/utils.py
View File

@@ -612,7 +612,6 @@ class RestHelper:
filename: The filename of the media to be uploaded
expect_code: The return code to expect from attempting to upload the media
"""
image_length = len(image_data)
path = "/_matrix/media/r0/upload?filename=%s" % (filename,)
channel = make_request(
self.reactor,
@@ -621,7 +620,6 @@ class RestHelper:
path,
content=image_data,
access_token=tok,
custom_headers=[("Content-Length", str(image_length))],
)
assert channel.code == expect_code, "Expected: %d, got: %d, resp: %r" % (

25
tests/server.py
View File

@@ -81,6 +81,7 @@ from twisted.web.http_headers import Headers
from twisted.web.resource import IResource
from twisted.web.server import Request, Site
from synapse.api.constants import MAX_REQUEST_SIZE
from synapse.config.database import DatabaseConnectionConfig
from synapse.config.homeserver import HomeServerConfig
from synapse.events.auto_accept_invites import InviteAutoAccepter
@@ -241,7 +242,6 @@ class FakeChannel:
def loseConnection(self) -> None:
self.unregisterProducer()
self.transport.loseConnection()
# Type ignore: mypy doesn't like the fact that producer isn't an IProducer.
def registerProducer(self, producer: IProducer, streaming: bool) -> None:
@@ -428,18 +428,29 @@ def make_request(
channel = FakeChannel(site, reactor, ip=client_ip)
req = request(channel, site, our_server_name="test_server")
req = request(
channel,
site,
our_server_name="test_server",
max_request_body_size=MAX_REQUEST_SIZE,
)
channel.request = req
req.content = BytesIO(content)
# Twisted expects to be at the end of the content when parsing the request.
req.content.seek(0, SEEK_END)
# Old version of Twisted (<20.3.0) have issues with parsing x-www-form-urlencoded
# bodies if the Content-Length header is missing
req.requestHeaders.addRawHeader(
b"Content-Length", str(len(content)).encode("ascii")
)
# If `Content-Length` was passed in as a custom header, don't automatically add it
# here.
if custom_headers is None or not any(
(k if isinstance(k, bytes) else k.encode("ascii")) == b"Content-Length"
for k, _ in custom_headers
):
# Old version of Twisted (<20.3.0) have issues with parsing x-www-form-urlencoded
# bodies if the Content-Length header is missing
req.requestHeaders.addRawHeader(
b"Content-Length", str(len(content)).encode("ascii")
)
if access_token:
req.requestHeaders.addRawHeader(

60
tests/test_server.py
View File

@@ -212,6 +212,66 @@ class JsonResourceTests(unittest.TestCase):
self.assertEqual(channel.code, 200)
self.assertNotIn("body", channel.result)
def test_content_larger_than_content_length(self) -> None:
"""
HTTP requests with content size exceeding Content-Length should be rejected with 400.
"""
def _callback(
request: SynapseRequest, **kwargs: object
) -> tuple[int, JsonDict]:
return 200, {}
res = JsonResource(self.homeserver)
res.register_paths(
"POST", [re.compile("^/_matrix/foo$")], _callback, "test_servlet"
)
channel = make_request(
self.reactor,
FakeSite(res, self.reactor),
b"POST",
b"/_matrix/foo",
{},
# Set the `Content-Length` value to be smaller than the actual content size
custom_headers=[("Content-Length", "1")],
# The request should disconnect early so don't await the result
await_result=False,
)
self.reactor.advance(0.1)
self.assertEqual(channel.code, 400)
def test_content_smaller_than_content_length(self) -> None:
"""
HTTP requests with content size smaller than Content-Length should be rejected with 400.
"""
def _callback(
request: SynapseRequest, **kwargs: object
) -> tuple[int, JsonDict]:
return 200, {}
res = JsonResource(self.homeserver)
res.register_paths(
"POST", [re.compile("^/_matrix/foo$")], _callback, "test_servlet"
)
channel = make_request(
self.reactor,
FakeSite(res, self.reactor),
b"POST",
b"/_matrix/foo",
{},
# Set the `Content-Length` value to be larger than the actual content size
custom_headers=[("Content-Length", "10")],
# The request should disconnect early so don't await the result
await_result=False,
)
self.reactor.advance(0.1)
self.assertEqual(channel.code, 400)
class OptionsResourceTests(unittest.TestCase):
def setUp(self) -> None: