openssl: fix overflow check for long --tls-cipher option

The length check in tls_ctx_restrict_ciphers() did not check for overflow,
which could lead to a stack buffer overflow.

This has no real-world impact, because --tls-cipher can only be specified
by entities that are allowed to supply config settings.  Since those
entities can also change --script-security and call scripts and/or
plugins, these users already have code execution at the level of the
openvpn process.  In other words: the attacker would not gain any
capabilities.  Nevertheless, a nasty bug that we should fix.

This bug was discovered and reported to the OpenVPN security team by
Guido Vranken.

Signed-off-by: Steffan Karger <[email protected]>
Acked-by: Gert Doering <[email protected]>
Message-Id: <[email protected]>
URL: https://www.mail-archive.com/[email protected]/msg14716.html
Signed-off-by: Gert Doering <[email protected]>

src/openvpn/ssl_openssl.c

@@ -352,7 +352,8 @@ tls_ctx_restrict_ciphers(struct tls_root_ctx *ctx, const char *ciphers)
         }
 
         /* Make sure new cipher name fits in cipher string */
-        if (((sizeof(openssl_ciphers)-1) - openssl_ciphers_len) < current_cipher_len)
+        if ((SIZE_MAX - openssl_ciphers_len) < current_cipher_len
+            || ((sizeof(openssl_ciphers)-1) < openssl_ciphers_len + current_cipher_len))
         {
             msg(M_FATAL,
                 "Failed to set restricted TLS cipher list, too long (>%d).",