GH-95494: Fix transport EOF handling in OpenSSL 3.0 (GH-95495)

davidben · web-flow · commit 420bbb783b43 · 2023-03-22T13:16:26.000+01:00
GH-25309 enabled SSL_OP_IGNORE_UNEXPECTED_EOF by default, with a comment that it restores OpenSSL 1.1.1 behavior, but this wasn't quite right. That option causes OpenSSL to treat transport EOF as the same as close_notify (i.e. SSL_ERROR_ZERO_RETURN), whereas Python actually has distinct SSLEOFError and SSLZeroReturnError exceptions. (The latter is usually mapped to a zero return from read.) In OpenSSL 1.1.1, the ssl module would raise them for transport EOF and close_notify, respectively. In OpenSSL 3.0, both act like close_notify. Fix this by, instead, just detecting SSL_R_UNEXPECTED_EOF_WHILE_READING and mapping that to the other exception type. There doesn't seem to have been any unit test of this error, so fill in the missing one. This had to be done with the BIO path because it's actually slightly tricky to simulate a transport EOF with Python's fd based APIs. (If you instruct the server to close the socket, it gets confused, probably because the server's SSL object is still referencing the now dead fd?)
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
@@ -151,7 +151,6 @@ def data_file(*name):
 OP_SINGLE_ECDH_USE = getattr(ssl, "OP_SINGLE_ECDH_USE", 0)
 OP_CIPHER_SERVER_PREFERENCE = getattr(ssl, "OP_CIPHER_SERVER_PREFERENCE", 0)
 OP_ENABLE_MIDDLEBOX_COMPAT = getattr(ssl, "OP_ENABLE_MIDDLEBOX_COMPAT", 0)
-OP_IGNORE_UNEXPECTED_EOF = getattr(ssl, "OP_IGNORE_UNEXPECTED_EOF", 0)
 
 # Ubuntu has patched OpenSSL and changed behavior of security level 2
 # see https://bugs.python.org/issue41561#msg389003
@@ -958,8 +957,7 @@ def test_options(self):
         # SSLContext also enables these by default
         default |= (OP_NO_COMPRESSION | OP_CIPHER_SERVER_PREFERENCE |
                     OP_SINGLE_DH_USE | OP_SINGLE_ECDH_USE |
-                    OP_ENABLE_MIDDLEBOX_COMPAT |
-                    OP_IGNORE_UNEXPECTED_EOF)
+                    OP_ENABLE_MIDDLEBOX_COMPAT)
         self.assertEqual(default, ctx.options)
         with warnings_helper.check_warnings():
             ctx.options |= ssl.OP_NO_TLSv1
@@ -2120,6 +2118,20 @@ def test_bio_read_write_data(self):
         self.assertEqual(buf, b'foo\n')
         self.ssl_io_loop(sock, incoming, outgoing, sslobj.unwrap)
 
+    def test_transport_eof(self):
+        client_context, server_context, hostname = testing_context()
+        with socket.socket(socket.AF_INET) as sock:
+            sock.connect(self.server_addr)
+            incoming = ssl.MemoryBIO()
+            outgoing = ssl.MemoryBIO()
+            sslobj = client_context.wrap_bio(incoming, outgoing,
+                                             server_hostname=hostname)
+            self.ssl_io_loop(sock, incoming, outgoing, sslobj.do_handshake)
+
+            # Simulate EOF from the transport.
+            incoming.write_eof()
+            self.assertRaises(ssl.SSLEOFError, sslobj.read)
+
 
 @support.requires_resource('network')
 class NetworkedTests(unittest.TestCase):
diff --git a/Misc/NEWS.d/next/Library/2022-07-30-23-01-43.gh-issue-95495.RA-q1d.rst b/Misc/NEWS.d/next/Library/2022-07-30-23-01-43.gh-issue-95495.RA-q1d.rst
@@ -0,0 +1,7 @@
+When built against OpenSSL 3.0, the :mod:`ssl` module had a bug where it
+reported unauthenticated EOFs (i.e. without close_notify) as a clean TLS-level
+EOF. It now raises :exc:`~ssl.SSLEOFError`, matching the behavior in previous
+versions of OpenSSL. The :attr:`~ssl.SSLContext.options` attribute on
+:class:`~ssl.SSLContext` also no longer includes
+:data:`~ssl.OP_IGNORE_UNEXPECTED_EOF` by default. This option may be set to
+specify the previous OpenSSL 3.0 behavior.
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
@@ -660,6 +660,16 @@ PySSL_SetError(PySSLSocket *sslsock, int ret, const char *filename, int lineno)
                     ERR_GET_REASON(e) == SSL_R_CERTIFICATE_VERIFY_FAILED) {
                 type = state->PySSLCertVerificationErrorObject;
             }
+#if defined(SSL_R_UNEXPECTED_EOF_WHILE_READING)
+            /* OpenSSL 3.0 changed transport EOF from SSL_ERROR_SYSCALL with
+             * zero return value to SSL_ERROR_SSL with a special error code. */
+            if (ERR_GET_LIB(e) == ERR_LIB_SSL &&
+                    ERR_GET_REASON(e) == SSL_R_UNEXPECTED_EOF_WHILE_READING) {
+                p = PY_SSL_ERROR_EOF;
+                type = state->PySSLEOFErrorObject;
+                errstr = "EOF occurred in violation of protocol";
+            }
+#endif
             break;
         }
         default:
@@ -3092,10 +3102,6 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
 #endif
 #ifdef SSL_OP_SINGLE_ECDH_USE
     options |= SSL_OP_SINGLE_ECDH_USE;
-#endif
-#ifdef SSL_OP_IGNORE_UNEXPECTED_EOF
-    /* Make OpenSSL 3.0.0 behave like 1.1.1 */
-    options |= SSL_OP_IGNORE_UNEXPECTED_EOF;
 #endif
     SSL_CTX_set_options(self->ctx, options);
 
