Follow along here: https://people.canonical.com/~ubuntu-security/cve/2018/CVE-2018-5390.html
More here: https://www.kb.cert.org/vuls/id/962459
Very brief bit from kb.cert.org -
Linux kernel versions 4.9+ can be forced to make very expensive calls to tcp_collapse_ofo_queue() and tcp_prune_ofo_queue() for every incoming packet which can lead to a denial of service. An attacker can induce a denial of service condition by sending specially modified packets within ongoing TCP sessions. Maintaining the denial of service condition requires continuous two-way TCP sessions to a reachable open port. Thus, the attacks cannot be performed using spoofed IP addresses.