[PATCH 0/5][SRU][B/C] CVE-2018-18397 - tmpfs permissions bypass

[Tyler Hicks]

https://people.canonical.com/~ubuntu-security/cve/2018/CVE-2018-18397.html

 The userfaultfd implementation in the Linux kernel before 4.19.7 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.

All but one of these patches are clean cherry picks to Cosmic and Bionic. The
one that required manual backporting was due to minor context changes due to
upstream commit 2cf855837b89d92996cf264713f3bed2bf9b0b4f missing in those
kernels.

I've successfully regression tested these changes by running the
tools/testing/selftests/vm/run_vmtests kernel selftests, which excercise
userfaultfd.

Tyler