[PATCH 0/1][B/C/D] CVE-2018-18955 - Privilege escalation within a nested user namespace

[Tyler Hicks]

CVE-2018-18955
BugLink: https://launchpad.net/bugs/1801924

Jann Horn reported that nested user namespaces with more than five mappings
allow gaining privilege over an inode.

These patches cleanly cherry-pick back to Bionic. Releases older than Bionic
are not affected.

Tyler