Add --tls-version-max

Because using TLS 1.2 breaks certain setups, a user might want to enforce
a maximum TLS version to use. This patch adds that option.

This patch removes a number of #ifdefs from ssl_polarssl.c, because the
polarssl versions we currently support (polar 1.2 for openvpn 2.3, and
polar 1.3 for openvpn-master) have all versions unconditionally enabled.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <544EC052.3080809@fox-it.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9210
Signed-off-by: Gert Doering <gert@greenie.muc.de>

6 files changed, 100 insertions, 37 deletions

diff --git a/src/openvpn/options.c b/src/openvpn/options.c
index 84eb6ed..64ded09 100644
--- a/src/openvpn/options.c
+++ b/src/openvpn/options.c
@@ -572,6 +572,7 @@ static const char usage_message[] =
   "--tls-version-min <version> ['or-highest'] : sets the minimum TLS version we\n"
   "    will accept from the peer.  If version is unrecognized and 'or-highest'\n"
   "    is specified, require max TLS version supported by SSL implementation.\n"
+  "--tls-version-max <version> : sets the maximum TLS version we will use.\n"
 #ifndef ENABLE_CRYPTO_POLARSSL
   "--pkcs12 file   : PKCS#12 file containing local private key, local certificate\n"
   "                  and optionally the root CA certificate.\n"
@@ -6577,14 +6578,29 @@ add_option (struct options *options,
     {
       int ver;
       VERIFY_PERMISSION (OPT_P_GENERAL);
-      ver = tls_version_min_parse(p[1], p[2]);
+      ver = tls_version_parse(p[1], p[2]);
       if (ver == TLS_VER_BAD)
 	{
 	  msg (msglevel, "unknown tls-version-min parameter: %s", p[1]);
           goto err;
 	}
-      options->ssl_flags &= ~(SSLF_TLS_VERSION_MASK << SSLF_TLS_VERSION_SHIFT);
-      options->ssl_flags |= (ver << SSLF_TLS_VERSION_SHIFT);
+      options->ssl_flags &=
+	  ~(SSLF_TLS_VERSION_MIN_MASK << SSLF_TLS_VERSION_MIN_SHIFT);
+      options->ssl_flags |= (ver << SSLF_TLS_VERSION_MIN_SHIFT);
+    }
+  else if (streq (p[0], "tls-version-max") && p[1])
+    {
+      int ver;
+      VERIFY_PERMISSION (OPT_P_GENERAL);
+      ver = tls_version_parse(p[1], NULL);
+      if (ver == TLS_VER_BAD)
+	{
+	  msg (msglevel, "unknown tls-version-max parameter: %s", p[1]);
+          goto err;
+	}
+      options->ssl_flags &=
+	  ~(SSLF_TLS_VERSION_MAX_MASK << SSLF_TLS_VERSION_MAX_SHIFT);
+      options->ssl_flags |= (ver << SSLF_TLS_VERSION_MAX_SHIFT);
     }
 #ifndef ENABLE_CRYPTO_POLARSSL
   else if (streq (p[0], "pkcs12") && p[1])
diff --git a/src/openvpn/ssl.c b/src/openvpn/ssl.c
index 3ce1f60..c81659f 100644
--- a/src/openvpn/ssl.c
+++ b/src/openvpn/ssl.c
@@ -454,7 +454,7 @@ ssl_put_auth_challenge (const char *cr_str)
  * return tls_version_max().
  */
 int
-tls_version_min_parse(const char *vstr, const char *extra)
+tls_version_parse(const char *vstr, const char *extra)
 {
   const int max_version = tls_version_max();
   if (!strcmp(vstr, "1.0") && TLS_VER_1_0 <= max_version)
diff --git a/src/openvpn/ssl_backend.h b/src/openvpn/ssl_backend.h
index bfd1549..b0777bf 100644
--- a/src/openvpn/ssl_backend.h
+++ b/src/openvpn/ssl_backend.h
@@ -109,11 +109,12 @@ void tls_clear_error();
  * @return 		One of the TLS_VER_x constants or TLS_VER_BAD
  *                      if a parse error should be flagged.
  */
-#define TLS_VER_BAD   -1
-#define TLS_VER_1_0    0 /* default */
-#define TLS_VER_1_1    1
-#define TLS_VER_1_2    2
-int tls_version_min_parse(const char *vstr, const char *extra);
+#define TLS_VER_BAD    -1
+#define TLS_VER_UNSPEC  0 /* default */
+#define TLS_VER_1_0     1
+#define TLS_VER_1_1     2
+#define TLS_VER_1_2     3
+int tls_version_parse(const char *vstr, const char *extra);
 
 /**
  * Return the maximum TLS version (as a TLS_VER_x constant)
diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
index 04ba789..2719b41 100644
--- a/src/openvpn/ssl_common.h
+++ b/src/openvpn/ssl_common.h
@@ -296,8 +296,10 @@ struct tls_options
 # define SSLF_AUTH_USER_PASS_OPTIONAL  (1<<2)
 # define SSLF_OPT_VERIFY               (1<<4)
 # define SSLF_CRL_VERIFY_DIR           (1<<5)
-# define SSLF_TLS_VERSION_SHIFT        6
-# define SSLF_TLS_VERSION_MASK         0xF /* (uses bit positions 6 to 9) */
+# define SSLF_TLS_VERSION_MIN_SHIFT    6
+# define SSLF_TLS_VERSION_MIN_MASK     0xF /* (uses bit positions 6 to 9) */
+# define SSLF_TLS_VERSION_MAX_SHIFT    10
+# define SSLF_TLS_VERSION_MAX_MASK     0xF /* (uses bit positions 10 to 13) */
   unsigned int ssl_flags;
 
 #ifdef MANAGEMENT_DEF_AUTH
diff --git a/src/openvpn/ssl_openssl.c b/src/openvpn/ssl_openssl.c
index adf3ae6..6782a95 100644
--- a/src/openvpn/ssl_openssl.c
+++ b/src/openvpn/ssl_openssl.c
@@ -184,15 +184,23 @@ tls_ctx_set_options (struct tls_root_ctx *ctx, unsigned int ssl_flags)
   /* process SSL options including minimum TLS version we will accept from peer */
   {
     long sslopt = SSL_OP_SINGLE_DH_USE | SSL_OP_NO_TICKET | SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3;
-    const int tls_version_min = (ssl_flags >> SSLF_TLS_VERSION_SHIFT) & SSLF_TLS_VERSION_MASK;
-    if (tls_version_min > TLS_VER_1_0)
+    int tls_ver_max = TLS_VER_UNSPEC;
+    const int tls_ver_min =
+	(ssl_flags >> SSLF_TLS_VERSION_MIN_SHIFT) & SSLF_TLS_VERSION_MIN_MASK;
+
+    tls_ver_max =
+	(ssl_flags >> SSLF_TLS_VERSION_MAX_SHIFT) & SSLF_TLS_VERSION_MAX_MASK;
+    if (tls_ver_max <= TLS_VER_UNSPEC)
+	tls_ver_max = tls_version_max();
+
+    if (tls_ver_min > TLS_VER_1_0 || tls_ver_max < TLS_VER_1_0)
       sslopt |= SSL_OP_NO_TLSv1;
 #ifdef SSL_OP_NO_TLSv1_1
-    if (tls_version_min > TLS_VER_1_1)
+    if (tls_ver_min > TLS_VER_1_1 || tls_ver_max < TLS_VER_1_1)
       sslopt |= SSL_OP_NO_TLSv1_1;
 #endif
 #ifdef SSL_OP_NO_TLSv1_2
-    if (tls_version_min > TLS_VER_1_2)
+    if (tls_ver_min > TLS_VER_1_2 || tls_ver_max < TLS_VER_1_2)
       sslopt |= SSL_OP_NO_TLSv1_2;
 #endif
     SSL_CTX_set_options (ctx->ctx, sslopt);
diff --git a/src/openvpn/ssl_polarssl.c b/src/openvpn/ssl_polarssl.c
index 387e636..b026a17 100644
--- a/src/openvpn/ssl_polarssl.c
+++ b/src/openvpn/ssl_polarssl.c
@@ -685,6 +685,40 @@ tls_version_max(void)
 #endif
 }
 
+/**
+ * Convert an OpenVPN tls-version variable to PolarSSl format (i.e. a major and
+ * minor ssl version number).
+ *
+ * @param tls_ver	The tls-version variable to convert.
+ * @param major		Returns the TLS major version in polarssl format.
+ * 			Must be a valid pointer.
+ * @param minor		Returns the TLS minor version in polarssl format.
+ * 			Must be a valid pointer.
+ */
+static void tls_version_to_major_minor(int tls_ver, int *major, int *minor) {
+  ASSERT(major);
+  ASSERT(minor);
+
+  switch (tls_ver)
+  {
+    case TLS_VER_1_0:
+      *major = SSL_MAJOR_VERSION_3;
+      *minor = SSL_MINOR_VERSION_1;
+      break;
+    case TLS_VER_1_1:
+      *major = SSL_MAJOR_VERSION_3;
+      *minor = SSL_MINOR_VERSION_2;
+      break;
+    case TLS_VER_1_2:
+      *major = SSL_MAJOR_VERSION_3;
+      *minor = SSL_MINOR_VERSION_3;
+      break;
+    default:
+      msg(M_FATAL, "%s: invalid TLS version %d", __func__, tls_ver);
+      break;
+  }
+}
+
 void key_state_ssl_init(struct key_state_ssl *ks_ssl,
     const struct tls_root_ctx *ssl_ctx, bool is_server, struct tls_session *session)
 {
@@ -743,30 +777,32 @@ void key_state_ssl_init(struct key_state_ssl *ks_ssl,
 
       /* Initialize minimum TLS version */
       {
-	const int tls_version_min = (session->opt->ssl_flags >> SSLF_TLS_VERSION_SHIFT) & SSLF_TLS_VERSION_MASK;
-	int polar_major;
-	int polar_minor;
-	switch (tls_version_min)
+	const int tls_version_min =
+	    (session->opt->ssl_flags >> SSLF_TLS_VERSION_MIN_SHIFT) &
+	    SSLF_TLS_VERSION_MIN_MASK;
+
+	/* default to TLS 1.0 */
+	int major = SSL_MAJOR_VERSION_3;
+	int minor = SSL_MINOR_VERSION_1;
+
+	if (tls_version_min > TLS_VER_UNSPEC)
+	  tls_version_to_major_minor(tls_version_min, &major, &minor);
+
+	ssl_set_min_version(ks_ssl->ctx, major, minor);
+      }
+
+      /* Initialize maximum TLS version */
+      {
+	const int tls_version_max =
+	    (session->opt->ssl_flags >> SSLF_TLS_VERSION_MAX_SHIFT) &
+	    SSLF_TLS_VERSION_MAX_MASK;
+
+	if (tls_version_max > TLS_VER_UNSPEC)
 	  {
-	  case TLS_VER_1_0:
-	  default:
-	    polar_major = SSL_MAJOR_VERSION_3;
-	    polar_minor = SSL_MINOR_VERSION_1;
-	    break;
-#if defined(SSL_MAJOR_VERSION_3) && defined(SSL_MINOR_VERSION_2)
-	  case TLS_VER_1_1:
-	    polar_major = SSL_MAJOR_VERSION_3;
-	    polar_minor = SSL_MINOR_VERSION_2;
-	    break;
-#endif
-#if defined(SSL_MAJOR_VERSION_3) && defined(SSL_MINOR_VERSION_3)
-	  case TLS_VER_1_2:
-	    polar_major = SSL_MAJOR_VERSION_3;
-	    polar_minor = SSL_MINOR_VERSION_3;
-	    break;
-#endif
+	    int major, minor;
+	    tls_version_to_major_minor(tls_version_max, &major, &minor);
+	    ssl_set_max_version(ks_ssl->ctx, major, minor);
 	  }
-	ssl_set_min_version(ks_ssl->ctx, polar_major, polar_minor);
       }
 
       /* Initialise BIOs */