The add_free_nid function in fs/f2fs/node.c in the Linux kernel before 4.12 does not properly track an allocated nid, which allows local users to cause a denial of service (race condition) or possibly have unspecified other impact via concurrent threads.
git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=30a61ddf8117c26ac5b295e1233eaa9629a94ca3
www.securitytracker.com/id/1041432
github.com/torvalds/linux/commit/30a61ddf8117c26ac5b295e1233eaa9629a94ca3
lists.debian.org/debian-lts-announce/2019/03/msg00017.html
usn.ubuntu.com/3932-1/
usn.ubuntu.com/3932-2/