Bug 1683710 - Add a means to disable ALPN, r=bbeurdouche

We've recently learned the value of ALPN and SNI when it comes to protecting
against cross-protocol attacks.  However, some protocols don't have ALPN yet.
For servers that terminate connections for those connections, validating that
the client has not offered ALPN provides a way to protect against cross-protocol
attacks.  If the cross-protocol attack uses a protocol that does include ALPN,
being able to reject those connections safely reduces exposure.

This modifies SSL_SetNextProtoNego() to accept a zero-length buffer as an
argument.  Previously, this would have crashed.  Now it causes the server to
reject a handshake if ALPN is offered by the client.

It was always possible to implement this by passing a function that always
returns SECFailure to SSL_SetNextProtoCallback(). This approach has the
advantage that the server generates a no_application_protocol alert, which is
not something that user-provided code can do.

Differential Revision: https://phabricator.services.mozilla.com/D110887

--HG--
extra : moz-landing-system : lando

gtests/ssl_gtest/ssl_extension_unittest.cc

@@ -326,6 +326,27 @@ TEST_P(TlsExtensionTestGeneric, AlpnMismatch) {
   ClientHelloErrorTest(nullptr, kTlsAlertNoApplicationProtocol);
 }
 
+TEST_P(TlsExtensionTestGeneric, AlpnDisabledServer) {
+  const uint8_t client_alpn[] = {0x01, 0x61};
+  client_->EnableAlpn(client_alpn, sizeof(client_alpn));
+  server_->EnableAlpn(nullptr, 0);
+
+  ClientHelloErrorTest(nullptr, kTlsAlertUnsupportedExtension);
+}
+
+TEST_P(TlsConnectGeneric, AlpnDisabled) {
+  server_->EnableAlpn(nullptr, 0);
+  Connect();
+
+  SSLNextProtoState state;
+  uint8_t buf[255] = {0};
+  unsigned int buf_len = 3;
+  EXPECT_EQ(SECSuccess, SSL_GetNextProto(client_->ssl_fd(), &state, buf,
+                                         &buf_len, sizeof(buf)));
+  EXPECT_EQ(SSL_NEXT_PROTO_NO_SUPPORT, state);
+  EXPECT_EQ(0U, buf_len);
+}
+
 // Many of these tests fail in TLS 1.3 because the extension is encrypted, which
 // prevents modification of the value from the ServerHello.
 TEST_P(TlsExtensionTestPre13, AlpnReturnedEmptyList) {

lib/ssl/ssl.h

@@ -403,7 +403,14 @@ SSL_IMPORT SECStatus SSL_SetNextProtoCallback(PRFileDesc *fd,
  * preferred. The other protocols should be in preference order.
  *
  * The supported protocols are specified in |data| in wire-format (8-bit
- * length-prefixed). For example: "\010http/1.1\006spdy/2". */
+ * length-prefixed). For example: "\010http/1.1\006spdy/2".
+ *
+ * An empty value (i.e., where |length| is 0 and |data| is any value,
+ * including NULL) forcibly disables ALPN.  In this mode, the server will
+ * reject any ClientHello that includes the ALPN extension.
+ *
+ * Calling this function overrides the callback previously set by
+ * SSL_SetNextProtoCallback. */
 SSL_IMPORT SECStatus SSL_SetNextProtoNego(PRFileDesc *fd,
                                           const unsigned char *data,
                                           unsigned int length);

lib/ssl/ssl3exthandle.c

@@ -308,7 +308,7 @@ ssl3_SelectAppProtocol(const sslSocket *ss, TLSExtensionData *xtnData,
     if (rv != SECSuccess) {
         ssl3_ExtSendAlert(ss, alert_fatal, decode_error);
         PORT_SetError(SSL_ERROR_NEXT_PROTOCOL_DATA_INVALID);
-        return rv;
+        return SECFailure;
     }
 
     PORT_Assert(ss->nextProtoCallback);

lib/ssl/sslsock.c

@@ -2212,12 +2212,18 @@ ssl_NextProtoNegoCallback(void *arg, PRFileDesc *fd,
 {
     unsigned int i, j;
     sslSocket *ss = ssl_FindSocket(fd);
-
     if (!ss) {
         SSL_DBG(("%d: SSL[%d]: bad socket in ssl_NextProtoNegoCallback",
                  SSL_GETPID(), fd));
         return SECFailure;
     }
+    if (ss->opt.nextProtoNego.len == 0) {
+        SSL_DBG(("%d: SSL[%d]: ssl_NextProtoNegoCallback ALPN disabled",
+                 SSL_GETPID(), fd));
+        SSL3_SendAlert(ss, alert_fatal, unsupported_extension);
+        return SECFailure;
+    }
+
     PORT_Assert(protoMaxLen <= 255);
     if (protoMaxLen > 255) {
         PORT_SetError(SEC_ERROR_OUTPUT_LEN);
@@ -2257,7 +2263,7 @@ SSL_SetNextProtoNego(PRFileDesc *fd, const unsigned char *data,
         return SECFailure;
     }
 
-    if (ssl3_ValidateAppProtocol(data, length) != SECSuccess) {
+    if (length > 0 && ssl3_ValidateAppProtocol(data, length) != SECSuccess) {
         return SECFailure;
     }
 
@@ -2266,11 +2272,13 @@ SSL_SetNextProtoNego(PRFileDesc *fd, const unsigned char *data,
      * first protocol to the end of the list. */
     ssl_GetSSL3HandshakeLock(ss);
     SECITEM_FreeItem(&ss->opt.nextProtoNego, PR_FALSE);
-    SECITEM_AllocItem(NULL, &ss->opt.nextProtoNego, length);
-    size_t firstLen = data[0] + 1;
-    /* firstLen <= length is ensured by ssl3_ValidateAppProtocol. */
-    PORT_Memcpy(ss->opt.nextProtoNego.data + (length - firstLen), data, firstLen);
-    PORT_Memcpy(ss->opt.nextProtoNego.data, data + firstLen, length - firstLen);
+    if (length > 0) {
+        SECITEM_AllocItem(NULL, &ss->opt.nextProtoNego, length);
+        size_t firstLen = data[0] + 1;
+        /* firstLen <= length is ensured by ssl3_ValidateAppProtocol. */
+        PORT_Memcpy(ss->opt.nextProtoNego.data + (length - firstLen), data, firstLen);
+        PORT_Memcpy(ss->opt.nextProtoNego.data, data + firstLen, length - firstLen);
+    }
     ssl_ReleaseSSL3HandshakeLock(ss);
 
     return SSL_SetNextProtoCallback(fd, ssl_NextProtoNegoCallback, NULL);