fs/f2fs/segment.c in the Linux kernel before 4.13 allows local users to cause a denial of service (NULL pointer dereference and panic) by using a noflush_merge option that triggers a NULL value for a flush_cmd_control data structure.
https://www.debian.org/security/2018/dsa-4188
https://www.debian.org/security/2018/dsa-4187
https://usn.ubuntu.com/3910-2/
https://usn.ubuntu.com/3910-1/
https://github.com/torvalds/linux/commit/d4fdf8ba0e5808ba9ad6b44337783bd9935e0982