[nginx] SSL: enabled TLSv1.3 with BoringSSL.

[Maxim Dounin]

details:   https://hg.nginx.org/nginx/rev/b3a4f6d23e82
branches:  stable-1.14
changeset: 7420:b3a4f6d23e82
user:      Maxim Dounin <mdounin at mdounin.ru>
date:      Tue Aug 07 02:15:28 2018 +0300
description:
SSL: enabled TLSv1.3 with BoringSSL.

BoringSSL currently requires SSL_CTX_set_max_proto_version(TLS1_3_VERSION)
to be able to enable TLS 1.3.  This is because by default max protocol
version is set to TLS 1.2, and the SSL_OP_NO_* options are merely used
as a blacklist within the version range specified using the
SSL_CTX_set_min_proto_version() and SSL_CTX_set_max_proto_version()
functions.

With this change, we now call SSL_CTX_set_max_proto_version() with an
explicit maximum version set.  This enables TLS 1.3 with BoringSSL.
As a side effect, this change also limits maximum protocol version to
the newest protocol we know about, TLS 1.3.  This seems to be a good
change, as enabling unknown protocols might have unexpected results.

Additionally, we now explicitly call SSL_CTX_set_min_proto_version()
with 0.  This is expected to help with Debian system-wide default
of MinProtocol set to TLSv1.2, see
http://mailman.nginx.org/pipermail/nginx-ru/2017-October/060411.html.

Note that there is no SSL_CTX_set_min_proto_version macro in BoringSSL,
so we call SSL_CTX_set_min_proto_version() and SSL_CTX_set_max_proto_version()
as long as the TLS1_3_VERSION macro is defined.

diffstat:

 src/event/ngx_event_openssl.c |  5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diffs (15 lines):

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -330,6 +330,11 @@ ngx_ssl_create(ngx_ssl_t *ssl, ngx_uint_
     }
 #endif
 
+#ifdef TLS1_3_VERSION
+    SSL_CTX_set_min_proto_version(ssl->ctx, 0);
+    SSL_CTX_set_max_proto_version(ssl->ctx, TLS1_3_VERSION);
+#endif
+
 #ifdef SSL_OP_NO_COMPRESSION
     SSL_CTX_set_options(ssl->ctx, SSL_OP_NO_COMPRESSION);
 #endif