ASA-2018-00070 – Linux kernel: userfaultfd bypasses tmpfs file permission

The userfaultfd implementation in the Linux kernel mishandles access control for certain UFFDIO_ ioctl calls, as demonstrated by allowing local users to write data into holes in a tmpfs file (if the user has read-only access to that file, and that file contains holes), related to fs/userfaultfd.c and mm/userfaultfd.c.