Pull usernamespace mount fixes from Eric Biederman:
 "Way back in October Andrey Vagin reported that umount(MNT_DETACH)
  could be used to defeat MNT_LOCKED.  As I worked to fix this I
  discovered that combined with mount propagation and an appropriate
  selection of shared subtrees a reference to a directory on an
  unmounted filesystem is not necessary.

  That MNT_DETACH is allowed in user namespace in a form that can break
  MNT_LOCKED comes from my early misunderstanding what MNT_DETACH does.

  To avoid breaking existing userspace the conflict between MNT_DETACH
  and MNT_LOCKED is fixed by leaving mounts that are locked to their
  parents in the mount hash table until the last reference goes away.

  While investigating this issue I also found an issue with
  __detach_mounts.  The code was unnecessarily and incorrectly
  triggering mount propagation.  Resulting in too many mounts going away
  when a directory is deleted, and too many cpu cycles are burned while
  doing that.

  Looking some more I realized that __detach_mounts by only keeping
  mounts connected that were MNT_LOCKED it had the potential to still
  leak information so I tweaked the code to keep everything locked
  together that possibly could be.

  This code was almost ready last cycle but Al invented fs_pin which
  slightly simplifies this code but required rewrites and retesting, and
  I have not been in top form for a while so it took me a while to get
  all of that done.  Similiarly this pull request is late because I have
  been feeling absolutely miserable all week.

  The issue of being able to escape a bind mount has not yet been
  addressed, as the fixes are not yet mature"

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
  mnt: Update detach_mounts to leave mounts connected
  mnt: Fix the error check in __detach_mounts
  mnt: Honor MNT_LOCKED when detaching mounts
  fs_pin: Allow for the possibility that m_list or s_list go unused.
  mnt: Factor umount_mnt from umount_tree
  mnt: Factor out unhash_mnt from detach_mnt and umount_tree
  mnt: Fail collect_mounts when applied to unmounted mounts
  mnt: Don't propagate unmounts to locked mounts
  mnt: On an unmount propagate clearing of MNT_LOCKED
  mnt: Delay removal from the mount hash.
  mnt: Add MNT_UMOUNT flag
  mnt: In umount_tree reuse mnt_list instead of mnt_hash
  mnt: Don't propagate umounts in __detach_mounts
  mnt: Improve the umount_tree flags
  mnt: Use hlist_move_list in namespace_unlock