Many people have complained about CONFIG_USER_NS missing in Ubuntu [1] [2] [3] [4] [5].

Apparently the reason is that it's not compatible with XFS filesystem until 3.12 [6], and even though nobody uses [7] the XFS filesystem, backwards compatibility takes precedence over new features.

This is changed in 3.12, but it looks like the patch just missed the boat for Ubuntu Saucy [8].

[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/509808

[2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1085684

[3] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1191600

[4] http://lwn.net/Articles/541787/

[5] http://www.mail-archive.com/ubuntu-bugs@lists.ubuntu.com/msg...

[6] https://news.ycombinator.com/item?id=6761847

[7] By "nobody uses," I mean "I don't use it"

[8] http://lists.debian.org/debian-kernel/2013/09/msg00356.html