The do_shmat function in ipc/shm.c in the Linux kernel through 4.9.12 does not restrict the address calculated by a certain rounding operation, which allows local users to map page zero, and consequently bypass a protection mechanism that exists for the mmap system call, by making crafted shmget and shmat system calls in a privileged context.
https://usn.ubuntu.com/3583-2/
https://usn.ubuntu.com/3583-1/
https://github.com/torvalds/linux/commit/e1d35d4dc7f089e6c9c080d556feedf9c706f0c7
https://github.com/torvalds/linux/commit/95e91b831f87ac8e1f8ed50c14d709089b4e01b8
https://bugzilla.kernel.org/show_bug.cgi?id=192931
http://www.securitytracker.com/id/1037918