Add a test for an empty NextProto message

It is valid according to the spec for a NextProto message to have no
protocols listed in it. The OpenSSL implementation however does not allow
us to create such a message. In order to check that we work as expected
when communicating with a client that does generate such messages we have
to use a TLSProxy test.

Follow on from CVE-2024-5535

test/recipes/70-test_npn.t

@@ -0,0 +1,67 @@
+#! /usr/bin/env perl
+# Copyright 2024 The OpenSSL Project Authors. All Rights Reserved.
+#
+# Licensed under the Apache License 2.0 (the "License").  You may not use
+# this file except in compliance with the License.  You can obtain a copy
+# in the file LICENSE in the source distribution or at
+# https://www.openssl.org/source/license.html
+
+use strict;
+use OpenSSL::Test qw/:DEFAULT cmdstr srctop_file/;
+use OpenSSL::Test::Utils;
+
+use TLSProxy::Proxy;
+
+my $test_name = "test_npn";
+setup($test_name);
+
+plan skip_all => "TLSProxy isn't usable on $^O"
+    if $^O =~ /^(VMS)$/;
+
+plan skip_all => "$test_name needs NPN enabled"
+    if disabled("nextprotoneg");
+
+plan skip_all => "$test_name needs TLSv1.2 enabled"
+    if disabled("tls1_2");
+
+my $proxy = TLSProxy::Proxy->new(
+    undef,
+    cmdstr(app(["openssl"]), display => 1),
+    srctop_file("apps", "server.pem"),
+    (!$ENV{HARNESS_ACTIVE} || $ENV{HARNESS_VERBOSE})
+);
+
+$proxy->start() or plan skip_all => "Unable to start up Proxy for tests";
+plan tests => 1;
+
+my $npnseen = 0;
+
+#Test 1: Check sending an empty NextProto message from the client works. This is
+#        valid as per the spec, but OpenSSL does not allow you to send it.
+#        Therefore we must be prepared to receive such a message but we cannot
+#        generate it except via TLSProxy
+$proxy->clear();
+$proxy->filter(\&npn_filter);
+$proxy->clientflags("-nextprotoneg foo -no_tls1_3");
+$proxy->serverflags("-nextprotoneg foo");
+$proxy->start();
+ok($npnseen && TLSProxy::Message->success(), "Empty NPN message");
+
+sub npn_filter
+{
+    my $proxy = shift;
+    my $message;
+
+    # The NextProto message always appears in flight 2
+    return if $proxy->flight != 2;
+
+    foreach my $message (@{$proxy->message_list}) {
+        if ($message->mt == TLSProxy::Message::MT_NEXT_PROTO) {
+            #Our TLSproxy NextProto message support doesn't support parsing of
+            #the message. If we repack it just creates an empty NextProto
+            #message - which is exactly the scenario we want to test here.
+            $message->repack();
+            $npnseen = 1;
+        }
+    }
+}

util/perl/TLSProxy/Message.pm

@@ -384,6 +384,15 @@ sub create_message
             [@message_frag_lens]
         );
         $message->parse();
+    }  elsif ($mt == MT_NEXT_PROTO) {
+        $message = TLSProxy::NextProto->new(
+            $server,
+            $data,
+            [@message_rec_list],
+            $startoffset,
+            [@message_frag_lens]
+        );
+        $message->parse();
     } else {
         #Unknown message type
         $message = TLSProxy::Message->new(

util/perl/TLSProxy/NextProto.pm

@@ -0,0 +1,54 @@
+# Copyright 2024 The OpenSSL Project Authors. All Rights Reserved.
+#
+# Licensed under the Apache License 2.0 (the "License").  You may not use
+# this file except in compliance with the License.  You can obtain a copy
+# in the file LICENSE in the source distribution or at
+# https://www.openssl.org/source/license.html
+
+use strict;
+
+package TLSProxy::NextProto;
+
+use vars '@ISA';
+push @ISA, 'TLSProxy::Message';
+
+sub new
+{
+    my $class = shift;
+    my ($server,
+        $data,
+        $records,
+        $startoffset,
+        $message_frag_lens) = @_;
+
+    my $self = $class->SUPER::new(
+        $server,
+        TLSProxy::Message::MT_NEXT_PROTO,
+        $data,
+        $records,
+        $startoffset,
+        $message_frag_lens);
+
+    return $self;
+}
+
+sub parse
+{
+    #We don't support parsing at the moment
+}
+
+#This is supposed to reconstruct the on-the-wire message data following changes.
+#For now though since we don't support parsing we just create an empty NextProto
+#message - this capability is used in test_npn
+sub set_message_contents
+{
+    my $self = shift;
+    my $data;
+
+    $data = pack("C32", 0x00, 0x1e, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                 0x00, 0x00, 0x00);
+    $self->data($data);
+}
+1;

util/perl/TLSProxy/Proxy.pm

@@ -23,6 +23,7 @@ use TLSProxy::CertificateRequest;
 use TLSProxy::CertificateVerify;
 use TLSProxy::ServerKeyExchange;
 use TLSProxy::NewSessionTicket;
+use TLSProxy::NextProto;
 
 my $have_IPv6;
 my $IP_factory;