bpo-37228: Fix loop.create_datagram_endpoint()'s usage of SO_REUSEADDR

Author: aeros

Doc/library/asyncio-eventloop.rst

@@ -473,6 +473,13 @@ Opening network connections
                         reuse_address=None, reuse_port=None, \
                         allow_broadcast=None, sock=None)
 
+   .. note::
+      The parameter *reuse_address* is no longer supported, as using
+      :py:data:`~sockets.SO_REUSEADDR` poses a significant security concern for
+      UDP. Explicitly passing `reuse_address=True` will raise an exception.
+      For supported platforms, *reuse_port* can be used instead for similar
+      functionality.
+
    Create a datagram connection.
 
    The socket family can be either :py:data:`~socket.AF_INET`,
@@ -501,11 +508,6 @@ Opening network connections
      resolution. If given, these should all be integers from the
      corresponding :mod:`socket` module constants.
 
-   * *reuse_address* tells the kernel to reuse a local socket in
-     ``TIME_WAIT`` state, without waiting for its natural timeout to
-     expire. If not specified will automatically be set to ``True`` on
-     Unix.
-
    * *reuse_port* tells the kernel to allow this endpoint to be bound to the
      same port as other existing endpoints are bound to, so long as they all
      set this flag when being created. This option is not supported on Windows
@@ -527,6 +529,10 @@ Opening network connections
       The *family*, *proto*, *flags*, *reuse_address*, *reuse_port,
       *allow_broadcast*, and *sock* parameters were added.
 
+   .. versionchanged:: 3.5.10
+      The *reuse_address* parameter is no longer supported due to security
+      concerns.
+
    .. versionchanged:: 3.8
       Added support for Windows.
 

Lib/asyncio/base_events.py

@@ -1306,8 +1306,12 @@ async def create_datagram_endpoint(self, protocol_factory,
 
             exceptions = []
 
-            if reuse_address is None:
-                reuse_address = os.name == 'posix' and sys.platform != 'cygwin'
+            if reuse_address:
+                # bpo-37228
+                raise RuntimeError("Passing `reuse_address=True` is no "
+                                   "longer supported, as the usage of "
+                                   "SO_REUSEPORT in UDP poses a significant "
+                                   "security concern.")
 
             for ((family, proto),
                  (local_address, remote_address)) in addr_pairs_info:
@@ -1316,9 +1320,6 @@ async def create_datagram_endpoint(self, protocol_factory,
                 try:
                     sock = socket.socket(
                         family=family, type=socket.SOCK_DGRAM, proto=proto)
-                    if reuse_address:
-                        sock.setsockopt(
-                            socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
                     if reuse_port:
                         _set_reuseport(sock)
                     if allow_broadcast:

Lib/test/test_asyncio/test_base_events.py

@@ -1752,7 +1752,6 @@ class FakeSock:
 
     def test_create_datagram_endpoint_sockopts(self):
         # Socket options should not be applied unless asked for.
-        # SO_REUSEADDR defaults to on for UNIX.
         # SO_REUSEPORT is not available on all platforms.
 
         coro = self.loop.create_datagram_endpoint(
@@ -1761,18 +1760,8 @@ def test_create_datagram_endpoint_sockopts(self):
         transport, protocol = self.loop.run_until_complete(coro)
         sock = transport.get_extra_info('socket')
 
-        reuse_address_default_on = (
-            os.name == 'posix' and sys.platform != 'cygwin')
         reuseport_supported = hasattr(socket, 'SO_REUSEPORT')
 
-        if reuse_address_default_on:
-            self.assertTrue(
-                sock.getsockopt(
-                    socket.SOL_SOCKET, socket.SO_REUSEADDR))
-        else:
-            self.assertFalse(
-                sock.getsockopt(
-                    socket.SOL_SOCKET, socket.SO_REUSEADDR))
         if reuseport_supported:
             self.assertFalse(
                 sock.getsockopt(
@@ -1788,13 +1777,12 @@ def test_create_datagram_endpoint_sockopts(self):
         coro = self.loop.create_datagram_endpoint(
             lambda: MyDatagramProto(create_future=True, loop=self.loop),
             local_addr=('127.0.0.1', 0),
-            reuse_address=True,
             reuse_port=reuseport_supported,
             allow_broadcast=True)
         transport, protocol = self.loop.run_until_complete(coro)
         sock = transport.get_extra_info('socket')
 
-        self.assertTrue(
+        self.assertFalse(
             sock.getsockopt(
                 socket.SOL_SOCKET, socket.SO_REUSEADDR))
         if reuseport_supported:
@@ -1809,6 +1797,18 @@ def test_create_datagram_endpoint_sockopts(self):
         self.loop.run_until_complete(protocol.done)
         self.assertEqual('CLOSED', protocol.state)
 
+    def test_create_datagram_endpoint_reuse_address_error(self):
+        # bpo-37228: Ensure that explicit passing of `reuse_address=True`
+        # raises an error, as it is not safe to use SO_REUSEADDR when using UDP
+
+        coro = self.loop.create_datagram_endpoint(
+            lambda: MyDatagramProto(create_future=True, loop=self.loop),
+            local_addr=('127.0.0.1', 0),
+            reuse_address=True)
+
+        with self.assertRaises(RuntimeError):
+            self.loop.run_until_complete(coro)
+
     @patch_socket
     def test_create_datagram_endpoint_nosoreuseport(self, m_socket):
         del m_socket.SO_REUSEPORT