bpo-30622: Improve NPN support detection (#5859)

The ssl module now detects missing NPN support in LibreSSL.
Co-Authored-By: Bernard Spil <brnrd@FreeBSD.org>
Signed-off-by: Christian Heimes <christian@python.org>

Doc/library/ssl.rst

@@ -2434,6 +2434,23 @@ successful call of :func:`~ssl.RAND_add`, :func:`~ssl.RAND_bytes` or
 :func:`~ssl.RAND_pseudo_bytes` is sufficient.
 
 
+.. ssl-libressl:
+
+LibreSSL support
+----------------
+
+LibreSSL is a fork of OpenSSL 1.0.1. The ssl module has limited support for
+LibreSSL. Some features are not available when the ssl module is compiled
+with LibreSSL.
+
+* LibreSSL >= 2.6.1 no longer supports NPN. The methods
+  :meth:`SSLContext.set_npn_protocols` and
+  :meth:`SSLSocket.selected_npn_protocol` are not available.
+* :meth:`SSLContext.set_default_verify_paths` ignores the env vars
+  :envvar:`SSL_CERT_FILE` and :envvar:`SSL_CERT_PATH` although
+  :func:`get_default_verify_paths` still reports them.
+
+
 .. seealso::
 
    Class :class:`socket.socket`

Misc/NEWS.d/next/Library/2018-02-24-21-40-42.bpo-30622.dQjxSe.rst

+The ssl module now detects missing NPN support in LibreSSL.

Modules/_ssl.c

@@ -160,6 +160,19 @@ static void _PySSLFixErrno(void) {
 # define HAVE_ALPN
 #endif
 
+/* We cannot rely on OPENSSL_NO_NEXTPROTONEG because LibreSSL 2.6.1 dropped
+ * NPN support but did not set OPENSSL_NO_NEXTPROTONEG for compatibility
+ * reasons. The check for TLSEXT_TYPE_next_proto_neg works with
+ * OpenSSL 1.0.1+ and LibreSSL.
+ */
+#ifdef OPENSSL_NO_NEXTPROTONEG
+#  define HAVE_NPN 0
+#elif defined(TLSEXT_TYPE_next_proto_neg)
+#  define HAVE_NPN 1
+#else
+#  define HAVE_NPN 0
+# endif
+
 #ifndef INVALID_SOCKET /* MS defines this */
 #define INVALID_SOCKET (-1)
 #endif
@@ -328,7 +341,7 @@ static unsigned int _ssl_locks_count = 0;
 typedef struct {
     PyObject_HEAD
     SSL_CTX *ctx;
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
+#ifdef HAVE_NPN
     unsigned char *npn_protocols;
     int npn_protocols_len;
 #endif
@@ -1909,7 +1922,7 @@ _ssl__SSLSocket_version_impl(PySSLSocket *self)
     return PyUnicode_FromString(version);
 }
 
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
+#ifdef HAVE_NPN
 /*[clinic input]
 _ssl._SSLSocket.selected_npn_protocol
 [clinic start generated code]*/
@@ -2874,7 +2887,7 @@ _ssl__SSLContext_impl(PyTypeObject *type, int proto_version)
     self->ctx = ctx;
     self->hostflags = X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS;
     self->protocol = proto_version;
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
+#ifdef HAVE_NPN
     self->npn_protocols = NULL;
 #endif
 #ifdef HAVE_ALPN
@@ -3013,7 +3026,7 @@ context_dealloc(PySSLContext *self)
     PyObject_GC_UnTrack(self);
     context_clear(self);
     SSL_CTX_free(self->ctx);
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
+#ifdef HAVE_NPN
     PyMem_FREE(self->npn_protocols);
 #endif
 #ifdef HAVE_ALPN
@@ -3091,7 +3104,7 @@ _ssl__SSLContext_get_ciphers_impl(PySSLContext *self)
 #endif
 
 
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG) || defined(HAVE_ALPN)
+#if defined(HAVE_NPN) || defined(HAVE_ALPN)
 static int
 do_protocol_selection(int alpn, unsigned char **out, unsigned char *outlen,
                       const unsigned char *server_protocols, unsigned int server_protocols_len,
@@ -3117,7 +3130,7 @@ do_protocol_selection(int alpn, unsigned char **out, unsigned char *outlen,
 }
 #endif
 
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
+#ifdef HAVE_NPN
 /* this callback gets passed to SSL_CTX_set_next_protos_advertise_cb */
 static int
 _advertiseNPN_cb(SSL *s,
@@ -3160,7 +3173,7 @@ _ssl__SSLContext__set_npn_protocols_impl(PySSLContext *self,
                                          Py_buffer *protos)
 /*[clinic end generated code: output=72b002c3324390c6 input=319fcb66abf95bd7]*/
 {
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
+#ifdef HAVE_NPN
     PyMem_Free(self->npn_protocols);
     self->npn_protocols = PyMem_Malloc(protos->len);
     if (self->npn_protocols == NULL)
@@ -5705,7 +5718,7 @@ PyInit__ssl(void)
     Py_INCREF(r);
     PyModule_AddObject(m, "HAS_ECDH", r);
 
-#if defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)
+#ifdef HAVE_NPN
     r = Py_True;
 #else
     r = Py_False;

Modules/clinic/_ssl.c.h

@@ -132,7 +132,7 @@ _ssl__SSLSocket_version(PySSLSocket *self, PyObject *Py_UNUSED(ignored))
     return _ssl__SSLSocket_version_impl(self);
 }
 
-#if (defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG))
+#if defined(HAVE_NPN)
 
 PyDoc_STRVAR(_ssl__SSLSocket_selected_npn_protocol__doc__,
 "selected_npn_protocol($self, /)\n"
@@ -151,7 +151,7 @@ _ssl__SSLSocket_selected_npn_protocol(PySSLSocket *self, PyObject *Py_UNUSED(ign
     return _ssl__SSLSocket_selected_npn_protocol_impl(self);
 }
 
-#endif /* (defined(OPENSSL_NPN_NEGOTIATED) && !defined(OPENSSL_NO_NEXTPROTONEG)) */
+#endif /* defined(HAVE_NPN) */
 
 #if defined(HAVE_ALPN)
 
@@ -1175,4 +1175,4 @@ exit:
 #ifndef _SSL_ENUM_CRLS_METHODDEF
     #define _SSL_ENUM_CRLS_METHODDEF
 #endif /* !defined(_SSL_ENUM_CRLS_METHODDEF) */
-/*[clinic end generated code: output=d987411caeb106e7 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=a00fef6a470cfc2c input=a9049054013a1b77]*/