Fix remotely-triggerable ASSERT() on malformed IPv6 packet.

Correct sanity checks on IPv6 packet length in mss_fixup_ipv6(),
and change the ASSERT() check in mss_fixup_dowork() into a simple
"return" (= the TCP header will simply not be inspected further).

CVE-2017-7508 has been assigned due to the serious nature of the
bug: it can be used to remotely shutdown an openvpn server or
client, if IPv6 and --mssfix are enabled and the IPv6 networks used
inside the VPN are known.

Found by Guido Vranken <guidovranken@gmail.com>.

v2: style changes

CVE: 2017-7508
Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20170613200832.15027-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/search?l=mid&q=20170613200832.15027-1-gert@greenie.muc.de
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit c3f4707)

src/openvpn/mss.c

@@ -110,8 +110,12 @@ mss_fixup_ipv6 (struct buffer *buf, int maxmss)
   if ( pip6->nexthdr != OPENVPN_IPPROTO_TCP )
     return;
 
+  /* skip IPv6 header (40 bytes),
+   * verify remainder is large enough to contain a full TCP header
+   */
   newbuf = *buf;
-  if ( buf_advance( &newbuf, 40 ) )
+  if (buf_advance( &newbuf, 40 )
+      && BLEN(&newbuf) >= (int) sizeof(struct openvpn_tcphdr))
     {
       struct openvpn_tcphdr *tc = (struct openvpn_tcphdr *) BPTR (&newbuf);
       if (tc->flags & OPENVPN_TCPH_SYN_MASK)
@@ -133,7 +137,10 @@ mss_fixup_dowork (struct buffer *buf, uint16_t maxmss)
   int accumulate;
   struct openvpn_tcphdr *tc;
 
-  ASSERT (BLEN (buf) >= (int) sizeof (struct openvpn_tcphdr));
+  if (BLEN(buf) < (int) sizeof(struct openvpn_tcphdr))
+    {
+	return;
+    }
 
   verify_align_4 (buf);
   tc = (struct openvpn_tcphdr *) BPTR (buf);