dovecot - Aug 2017 - [PATCH] Add support for lower TLS version than default

The openssl library in Debian unstable (targeting Buster) supports
TLS1.2 by default. The library itself supports also TLS1.1 and TLS1.0.
If the admin decides to also support TLS1.[01] users he can then enable
the lower protocol version in case the users can't update their system.

Signed-off-by: Sebastian Andrzej Siewior <sebastian at breakpoint.cc>
---
 src/config/all-settings.c                    |    1 +
 src/lib-master/master-service-ssl-settings.c |    2 ++
 src/lib-master/master-service-ssl-settings.h |    1 +
 src/login-common/ssl-proxy-openssl.c         |   15 ++++++++++++++-
 4 files changed, 18 insertions(+), 1 deletion(-)

--- a/src/config/all-settings.c
+++ b/src/config/all-settings.c
@@ -308,6 +308,7 @@ struct master_service_ssl_settings {
 	const char *ssl_cert_username_field;
 	const char *ssl_crypto_device;
 	const char *ssl_options;
+	const char *ssl_lowest_version;
 
 	bool ssl_verify_client_cert;
 	bool ssl_require_crl;
--- a/src/lib-master/master-service-ssl-settings.c
+++ b/src/lib-master/master-service-ssl-settings.c
@@ -26,6 +26,7 @@ static const struct setting_define maste
 	DEF(SET_STR, ssl_protocols),
 	DEF(SET_STR, ssl_cert_username_field),
 	DEF(SET_STR, ssl_crypto_device),
+	DEF(SET_STR, ssl_lowest_version),
 	DEF(SET_BOOL, ssl_verify_client_cert),
 	DEF(SET_BOOL, ssl_require_crl),
 	DEF(SET_BOOL, verbose_ssl),
@@ -54,6 +55,7 @@ static const struct master_service_ssl_s
 	.ssl_protocols = "!SSLv3",
 #endif
 	.ssl_cert_username_field = "commonName",
+	.ssl_lowest_version = NULL,
 	.ssl_crypto_device = "",
 	.ssl_verify_client_cert = FALSE,
 	.ssl_require_crl = TRUE,
--- a/src/lib-master/master-service-ssl-settings.h
+++ b/src/lib-master/master-service-ssl-settings.h
@@ -16,6 +16,7 @@ struct master_service_ssl_settings {
 	const char *ssl_cert_username_field;
 	const char *ssl_crypto_device;
 	const char *ssl_options;
+	const char *ssl_lowest_version;
 
 	bool ssl_verify_client_cert;
 	bool ssl_require_crl;
--- a/src/login-common/ssl-proxy-openssl.c
+++ b/src/login-common/ssl-proxy-openssl.c
@@ -1302,7 +1302,20 @@ ssl_server_context_init(const struct log
 	if (ctx->prefer_server_ciphers)
 		SSL_CTX_set_options(ssl_ctx, SSL_OP_CIPHER_SERVER_PREFERENCE);
 	SSL_CTX_set_options(ssl_ctx, openssl_get_protocol_options(ctx->protocols));
-
+#if OPENSSL_VERSION_NUMBER >= 0x10100000
+	if (ssl_set->ssl_lowest_version) {
+		if (!strcmp(ssl_set->ssl_lowest_version, "TLS1.0"))
+		    SSL_CTX_set_min_proto_version(ssl_ctx, TLS1_VERSION);
+		else if (!strcmp(ssl_set->ssl_lowest_version, "TLS1.1"))
+		    SSL_CTX_set_min_proto_version(ssl_ctx, TLS1_1_VERSION);
+		else if (!strcmp(ssl_set->ssl_lowest_version, "TLS1.2"))
+		    SSL_CTX_set_min_proto_version(ssl_ctx, TLS1_2_VERSION);
+		else
+			i_fatal("TLS min version: '%s' is invalid. Only "
+				"'TLS1.0' and 'TLS1.1' is supported",
+				ssl_set->ssl_lowest_version);
+	}
+#endif
 	if (ctx->pri.cert != NULL && *ctx->pri.cert != '\0'
&&
 	    ssl_proxy_ctx_use_certificate_chain(ctx->ctx, ctx->pri.cert) != 1) {
 		i_fatal("Can't load ssl_cert: %s",

On 26 Aug 2017, at 19.47, Sebastian Andrzej Siewior <sebastian at
breakpoint.cc> wrote:
> 
> The openssl library in Debian unstable (targeting Buster) supports
> TLS1.2 by default. The library itself supports also TLS1.1 and TLS1.0.
> If the admin decides to also support TLS1.[01] users he can then enable
> the lower protocol version in case the users can't update their system.
..
> 	DEF(SET_STR, ssl_protocols),
> 	DEF(SET_STR, ssl_cert_username_field),
> 	DEF(SET_STR, ssl_crypto_device),
> +	DEF(SET_STR, ssl_lowest_version),
Does it really require a new setting? Couldn't it use the existing
ssl_protocols setting?

On 2017-08-27 09:32, Timo Sirainen wrote:
> On 26 Aug 2017, at 19.47, Sebastian Andrzej Siewior <sebastian at
breakpoint.cc> wrote:
>> The openssl library in Debian unstable (targeting Buster) supports
>> TLS1.2 by default. The library itself supports also TLS1.1 and TLS1.0.
>> If the admin decides to also support TLS1.[01] users he can then enable
>> the lower protocol version in case the users can't update their
system.
> ..
>> 	DEF(SET_STR, ssl_protocols),
>> 	DEF(SET_STR, ssl_cert_username_field),
>> 	DEF(SET_STR, ssl_crypto_device),
>> +	DEF(SET_STR, ssl_lowest_version),
> Does it really require a new setting? Couldn't it use the existing
ssl_protocols setting?
I think we should set the min protocol version from ssl_protocols, by 
looking at the lowest SSL protocol field there.

Aki

On 27 August 2017 08:32:06 CEST, Timo Sirainen <tss at iki.fi>
wrote:
>> 	DEF(SET_STR, ssl_protocols),
>> 	DEF(SET_STR, ssl_cert_username_field),
>> 	DEF(SET_STR, ssl_crypto_device),
>> +	DEF(SET_STR, ssl_lowest_version),
>
>Does it really require a new setting? Couldn't it use the existing
>ssl_protocols setting?
You need to set a minimal version. SSL_PROTOLS can be set tls1.0 and tls1.2
which avoids tls1.1. Not saying that it is a good thing to do. Also you set it
to not do sslv2 and sslv3 which then enables tls1.0+.
If you want change its definition to use as a minimal version, be my guest. Or
if you plan to scan the string and match for the lowest version then this could
work, too.

Sebastian