bpo-37228: Fix loop.create_datagram_endpoint()'s usage of SO_REUSEADDR (GH-17311) (#17529)

(cherry picked from commit ab513a38c98695f271e448fe2cb7c5e39eeaaaaf)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>

2 files changed, 42 insertions, 24 deletions

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index 14b80bd..bfd4011 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -66,6 +66,10 @@ _HAS_IPv6 = hasattr(socket, 'AF_INET6')
 # Maximum timeout passed to select to avoid OS limitations
 MAXIMUM_SELECT_TIMEOUT = 24 * 3600
 
+# Used for deprecation and removal of `loop.create_datagram_endpoint()`'s
+# *reuse_address* parameter
+_unset = object()
+
 
 def _format_handle(handle):
     cb = handle._callback
@@ -1201,7 +1205,7 @@ class BaseEventLoop(events.AbstractEventLoop):
     async def create_datagram_endpoint(self, protocol_factory,
                                        local_addr=None, remote_addr=None, *,
                                        family=0, proto=0, flags=0,
-                                       reuse_address=None, reuse_port=None,
+                                       reuse_address=_unset, reuse_port=None,
                                        allow_broadcast=None, sock=None):
         """Create datagram connection."""
         if sock is not None:
@@ -1210,7 +1214,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                     f'A UDP Socket was expected, got {sock!r}')
             if (local_addr or remote_addr or
                     family or proto or flags or
-                    reuse_address or reuse_port or allow_broadcast):
+                    reuse_port or allow_broadcast):
                 # show the problematic kwargs in exception msg
                 opts = dict(local_addr=local_addr, remote_addr=remote_addr,
                             family=family, proto=proto, flags=flags,
@@ -1277,8 +1281,18 @@ class BaseEventLoop(events.AbstractEventLoop):
 
             exceptions = []
 
-            if reuse_address is None:
-                reuse_address = os.name == 'posix' and sys.platform != 'cygwin'
+            # bpo-37228
+            if reuse_address is not _unset:
+                if reuse_address:
+                    raise ValueError("Passing `reuse_address=True` is no "
+                                     "longer supported, as the usage of "
+                                     "SO_REUSEPORT in UDP poses a significant "
+                                     "security concern.")
+                else:
+                    warnings.warn("The *reuse_address* parameter has been "
+                                  "deprecated as of 3.5.10 and is scheduled "
+                                  "for removal in 3.11.", DeprecationWarning,
+                                  stacklevel=2)
 
             for ((family, proto),
                  (local_address, remote_address)) in addr_pairs_info:
@@ -1287,9 +1301,6 @@ class BaseEventLoop(events.AbstractEventLoop):
                 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:
diff --git a/Lib/test/test_asyncio/test_base_events.py b/Lib/test/test_asyncio/test_base_events.py
index 92fdaf1..3a23059 100644
--- a/Lib/test/test_asyncio/test_base_events.py
+++ b/Lib/test/test_asyncio/test_base_events.py
@@ -1733,10 +1733,6 @@ class BaseEventLoopWithSelectorTests(test_utils.TestCase):
         self.assertRaises(ValueError, self.loop.run_until_complete, fut)
 
         fut = self.loop.create_datagram_endpoint(
-            MyDatagramProto, reuse_address=True, sock=FakeSock())
-        self.assertRaises(ValueError, self.loop.run_until_complete, fut)
-
-        fut = self.loop.create_datagram_endpoint(
             MyDatagramProto, reuse_port=True, sock=FakeSock())
         self.assertRaises(ValueError, self.loop.run_until_complete, fut)
 
@@ -1746,7 +1742,6 @@ class BaseEventLoopWithSelectorTests(test_utils.TestCase):
 
     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(
@@ -1755,18 +1750,8 @@ class BaseEventLoopWithSelectorTests(test_utils.TestCase):
         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(
@@ -1782,13 +1767,12 @@ class BaseEventLoopWithSelectorTests(test_utils.TestCase):
         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:
@@ -1803,6 +1787,29 @@ class BaseEventLoopWithSelectorTests(test_utils.TestCase):
         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(ValueError):
+            self.loop.run_until_complete(coro)
+
+    def test_create_datagram_endpoint_reuse_address_warning(self):
+        # bpo-37228: Deprecate *reuse_address* parameter
+
+        coro = self.loop.create_datagram_endpoint(
+            lambda: MyDatagramProto(create_future=True, loop=self.loop),
+            local_addr=('127.0.0.1', 0),
+            reuse_address=False)
+
+        with self.assertWarns(DeprecationWarning):
+            self.loop.run_until_complete(coro)
+
     @patch_socket
     def test_create_datagram_endpoint_nosoreuseport(self, m_socket):
         del m_socket.SO_REUSEPORT