Add system test for HTTP/2 SETTINGS frame flood

Send a valid DoH query followed by a flood of SETTINGS frames to
trigger a use-after-free in the write buffer.  Under ASan, named
will abort if the bug is present.

bin/tests/system/doth/tests_malicious.py

@@ -0,0 +1,73 @@
+# Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+#
+# SPDX-License-Identifier: MPL-2.0
+#
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0.  If a copy of the MPL was not distributed with this
+# file, you can obtain one at https://mozilla.org/MPL/2.0/.
+#
+# See the COPYRIGHT file distributed with this work for additional
+# information regarding copyright ownership.
+
+import socket
+import ssl
+
+from h2.config import H2Configuration
+from h2.connection import H2Connection
+from h2.settings import SettingCodes
+
+import dns.message
+
+
+def test_settings_frame_flood(ns1, named_httpsport):
+    msg = dns.message.make_query(".", "SOA")
+    wire = msg.to_wire()
+
+    with socket.create_connection((ns1.ip, named_httpsport), timeout=10) as sock:
+        ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
+        ctx.check_hostname = False
+        ctx.verify_mode = ssl.CERT_NONE
+        ctx.set_alpn_protocols(["h2"])
+
+        with ctx.wrap_socket(sock, server_hostname=ns1.ip) as tls:
+            config = H2Configuration(client_side=True, header_encoding="utf-8")
+            conn = H2Connection(config=config)
+            conn.initiate_connection()
+            tls.sendall(conn.data_to_send())
+
+            stream_id = conn.get_next_available_stream_id()
+            conn.send_headers(
+                stream_id,
+                [
+                    (":method", "POST"),
+                    (":path", "/dns-query"),
+                    (":scheme", "https"),
+                    (":authority", f"{ns1.ip}:{named_httpsport}"),
+                    ("content-type", "application/dns-message"),
+                    ("accept", "application/dns-message"),
+                    ("content-length", str(len(wire))),
+                ],
+            )
+            conn.send_data(stream_id, wire, end_stream=True)
+            tls.sendall(conn.data_to_send())
+
+            for i in range(4096):
+                try:
+                    conn.update_settings(
+                        {
+                            SettingCodes.MAX_CONCURRENT_STREAMS: (i % 100) + 1,
+                            SettingCodes.INITIAL_WINDOW_SIZE: i + 1,
+                        }
+                    )
+                    tls.sendall(conn.data_to_send())
+                except Exception:  # pylint: disable=broad-except
+                    break
+
+                if i % 500 == 0:
+                    tls.settimeout(0.05)
+                    try:
+                        while (data := tls.recv(65535)) != b"":
+                            conn.receive_data(data)
+                            tls.sendall(conn.data_to_send())
+                    except Exception:  # pylint: disable=broad-except
+                        pass