The KEYS subsystem in the Linux kernel through 4.13.7 mishandles use of add_key for a key that already exists but is uninstantiated, which allows local users to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact via a crafted system call.
https://access.redhat.com/errata/RHSA-2018:0654
https://bugzilla.redhat.com/show_bug.cgi?id=1498016
https://lists.debian.org/debian-lts-announce/2017/12/msg00004.html
https://marc.info/?t=150654188100001&r=1&w=2
https://marc.info/?t=150783958600011&r=1&w=2