gh-146080: fix a crash in SNI callbacks when the SSL object is gone (#146573)

Author: picnixz

Lib/test/test_ssl.py

@@ -1,5 +1,6 @@
 # Test the support for SSL and sockets
 
+import contextlib
 import sys
 import unittest
 import unittest.mock
@@ -47,12 +48,13 @@
 
 PROTOCOLS = sorted(ssl._PROTOCOL_NAMES)
 HOST = socket_helper.HOST
+IS_AWS_LC = "AWS-LC" in ssl.OPENSSL_VERSION
 IS_OPENSSL_3_0_0 = ssl.OPENSSL_VERSION_INFO >= (3, 0, 0)
 CAN_GET_SELECTED_OPENSSL_GROUP = ssl.OPENSSL_VERSION_INFO >= (3, 2)
 CAN_IGNORE_UNKNOWN_OPENSSL_GROUPS = ssl.OPENSSL_VERSION_INFO >= (3, 3)
 CAN_GET_AVAILABLE_OPENSSL_GROUPS = ssl.OPENSSL_VERSION_INFO >= (3, 5)
 CAN_GET_AVAILABLE_OPENSSL_SIGALGS = ssl.OPENSSL_VERSION_INFO >= (3, 4)
-CAN_SET_CLIENT_SIGALGS = "AWS-LC" not in ssl.OPENSSL_VERSION
+CAN_SET_CLIENT_SIGALGS = not IS_AWS_LC
 CAN_IGNORE_UNKNOWN_OPENSSL_SIGALGS = ssl.OPENSSL_VERSION_INFO >= (3, 3)
 CAN_GET_SELECTED_OPENSSL_SIGALG = ssl.OPENSSL_VERSION_INFO >= (3, 5)
 PY_SSL_DEFAULT_CIPHERS = sysconfig.get_config_var('PY_SSL_DEFAULT_CIPHERS')
@@ -383,6 +385,20 @@ def testing_context(server_cert=SIGNED_CERTFILE, *, server_chain=True,
     return client_context, server_context, hostname
 
 
+def do_ssl_object_handshake(sslobject, outgoing, max_retry=25):
+    """Call do_handshake() on the sslobject and return the sent data.
+
+    If do_handshake() fails more than *max_retry* times, return None.
+    """
+    data, attempt = None, 0
+    while not data and attempt < max_retry:
+        with contextlib.suppress(ssl.SSLWantReadError):
+            sslobject.do_handshake()
+        data = outgoing.read()
+        attempt += 1
+    return data
+
+
 class BasicSocketTests(unittest.TestCase):
 
     def test_constants(self):
@@ -1535,6 +1551,49 @@ def dummycallback(sock, servername, ctx):
         ctx.set_servername_callback(None)
         ctx.set_servername_callback(dummycallback)
 
+    def test_sni_callback_on_dead_references(self):
+        # See https://github.com/python/cpython/issues/146080.
+        c_ctx = make_test_context()
+        c_inc, c_out = ssl.MemoryBIO(), ssl.MemoryBIO()
+        client = c_ctx.wrap_bio(c_inc, c_out, server_hostname=SIGNED_CERTFILE_HOSTNAME)
+
+        def sni_callback(sock, servername, ctx): pass
+        sni_callback = unittest.mock.Mock(wraps=sni_callback)
+        s_ctx = make_test_context(server_side=True, certfile=SIGNED_CERTFILE)
+        s_ctx.set_servername_callback(sni_callback)
+
+        s_inc, s_out = ssl.MemoryBIO(), ssl.MemoryBIO()
+        server = s_ctx.wrap_bio(s_inc, s_out, server_side=True)
+        server_impl = server._sslobj
+
+        # Perform the handshake on the client side first.
+        data = do_ssl_object_handshake(client, c_out)
+        sni_callback.assert_not_called()
+        if data is None:
+            self.skipTest("cannot establish a handshake from the client")
+        s_inc.write(data)
+        sni_callback.assert_not_called()
+        # Delete the server object before it starts doing its handshake
+        # and ensure that we did not call the SNI callback yet.
+        del server
+        gc.collect()
+        # Try to continue the server's handshake by directly using
+        # the internal SSL object. The latter is a weak reference
+        # stored in the server context and has now a dead owner.
+        with self.assertRaises(ssl.SSLError) as cm:
+            server_impl.do_handshake()
+        # The SNI C callback raised an exception before calling our callback.
+        sni_callback.assert_not_called()
+
+        # In AWS-LC, any handshake failures reports SSL_R_PARSE_TLSEXT,
+        # while OpenSSL uses SSL_R_CALLBACK_FAILED on SNI callback failures.
+        if IS_AWS_LC:
+            libssl_error_reason = "PARSE_TLSEXT"
+        else:
+            libssl_error_reason = "callback failed"
+        self.assertIn(libssl_error_reason, str(cm.exception))
+        self.assertEqual(cm.exception.errno, ssl.SSL_ERROR_SSL)
+
     def test_sni_callback_refcycle(self):
         # Reference cycles through the servername callback are detected
         # and cleared.

Misc/NEWS.d/next/Library/2026-03-28-13-19-20.gh-issue-146080.srN12a.rst

@@ -0,0 +1,2 @@
+:mod:`ssl`: fix a crash when an SNI callback tries to use an SSL object that
+has already been garbage-collected. Patch by Bénédikt Tran.

Modules/_ssl.c

@@ -5205,7 +5205,7 @@ _servername_callback(SSL *s, int *al, void *args)
     return ret;
 
 error:
-    Py_DECREF(ssl_socket);
+    Py_XDECREF(ssl_socket);
     *al = SSL_AD_INTERNAL_ERROR;
     ret = SSL_TLSEXT_ERR_ALERT_FATAL;
     PyGILState_Release(gstate);