Don't assert out on receiving too-large control packets (CVE-2017-7478)

Don't assert out on receiving too-large control packets (CVE-2017-7478)

Commit 3c1b19e0 changed the maximum size of accepted control channel
packets.  This was needed for crypto negotiation (which is needed for a
nice transition to a new default cipher), but exposed a DoS
vulnerability.  The vulnerability was found during the OpenVPN 2.4 code
audit by Quarkslab (commisioned by OSTIF).

To fix the issue, we should not ASSERT() on external input (in this case
the received packet size), but instead gracefully error out and drop the
invalid packet.

CVE: 2017-7478
Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1494358209-4568-2-git-send-email-steffan.karger@fox-it.com>
URL: http://www.mail-archive.com/search?l=mid&q=1494358209-4568-2-git-send-email-steffan.karger@fox-it.com
Signed-off-by: David Sommerseth <davids@openvpn.net>