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

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>