In the Linux kernel before 4.15, fs/ocfs2/aops.c omits use of a semaphore and consequently has a race condition for access to the extent tree during read operations in DIRECT mode, which allows local users to cause a denial of service (BUG) by modifying a certain e_cpos field.
https://www.debian.org/security/2018/dsa-4188
https://github.com/torvalds/linux/commit/3e4c56d41eef5595035872a2ec5a483f42e8917f