Fix the "default" tls-version-min setting

commit 968569f
defined TLS 1.2 as the minimum version if not set
by user. But the patch introduced two errors:

(i) ssl_flags is overwritten without regard to other
    options set in the flags
(ii) Any tls-version-max set by the user is not taken into
     account.
Makes it impossible to set tls-version-max without also setting
tls-version-min along with loss of other bits set in ssl_flags.

Fix it.

The fix retains the original intent when possible, and tries to
use the maximum possible value when it cannot be set to TLS 1.2
without conflicting with user-specified tls-version-max, if any.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211015043227.10679-1-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22939.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/options.c

@@ -3167,15 +3167,22 @@ options_set_backwards_compatible_options(struct options *o)
     /* TLS min version is not set */
     if ((o->ssl_flags & SSLF_TLS_VERSION_MIN_MASK) == 0)
     {
+        int tls_ver_max = (o->ssl_flags >> SSLF_TLS_VERSION_MAX_SHIFT)
+                          & SSLF_TLS_VERSION_MAX_MASK;
         if (need_compatibility_before(o, 20307))
         {
             /* 2.3.6 and earlier have TLS 1.0 only, set minimum to TLS 1.0 */
-            o->ssl_flags = (TLS_VER_1_0 << SSLF_TLS_VERSION_MIN_SHIFT);
+            o->ssl_flags |= (TLS_VER_1_0 << SSLF_TLS_VERSION_MIN_SHIFT);
         }
-        else
+        else if (tls_ver_max == 0 || tls_ver_max >= TLS_VER_1_2)
         {
             /* Use TLS 1.2 as proper default */
-            o->ssl_flags = (TLS_VER_1_2 << SSLF_TLS_VERSION_MIN_SHIFT);
+            o->ssl_flags |= (TLS_VER_1_2 << SSLF_TLS_VERSION_MIN_SHIFT);
+        }
+        else
+        {
+            /* Maximize the minimum version */
+            o->ssl_flags |= (tls_ver_max << SSLF_TLS_VERSION_MIN_SHIFT);
         }
     }