In the AppleTalk subsystem in the Linux kernel before 5.1, there is a potential NULL pointer dereference because register_snap_client may return NULL. This will lead to denial of service in net/appletalk/aarp.c and net/appletalk/ddp.c, as demonstrated by unregister_snap_client, aka CID-9804501fa122.
https://usn.ubuntu.com/4287-2/
https://usn.ubuntu.com/4287-1/
https://usn.ubuntu.com/4258-1/
https://usn.ubuntu.com/4254-2/
https://usn.ubuntu.com/4254-1/
http://lists.opensuse.org/opensuse-security-announce/2019-12/msg00029.html