From: Miklos Szeredi <mszeredi@suse.cz>
This patchset adds support for keeping mount ownership information in the

kernel, and allow unprivileged mount(2) and umount(2) in certain cases.
The mount owner has the following privileges:
  - unmount the owned mount

  - create a submount under the owned mount
The sysadmin can set the owner explicitly on mount and remount.  When an

unprivileged user creates a mount, then the owner is automatically set to the

user.
The following use cases are envisioned:
1) Private namespace, with selected mounts owned by user.  E.g.

   /home/$USER is a good candidate for allowing unpriv mounts and unmounts

   within.
2) Private namespace, with all mounts owned by user and having the "nosuid"

   flag.  User can mount and umount anywhere within the namespace, but suid

   programs will not work.
3) Global namespace, with a designated directory, which is a mount owned by

   the user.  E.g.  /mnt/users/$USER is set up so that it is bind mounted onto

   itself, and set to be owned by $USER.  The user can add/remove mounts only

   under this directory.
The following extra security measures are taken for unprivileged mounts:
 - usermounts are limited by a sysctl tunable

 - force "nosuid,nodev" mount options on the created mount
For testing unprivileged mounts (and for other purposes) simple

mount/umount utilities are available from:
  http://www.kernel.org/pub/linux/kernel/people/mszeredi/mmount/
I'll also submit a patch to util-linux, to add the same functionality

to mount(8) and umount(8).
This patch:
A new mount flag, MS_SETUSER is used to make a mount owned by a user.  If this

flag is specified, then the owner will be set to the current fsuid and the

mount will be marked with the MNT_USER flag.  On remount don't preserve

previous owner, and treat MS_SETUSER as for a new mount.  The MS_SETUSER flag

is ignored on mount move.
The MNT_USER flag is not copied on any kind of mount cloning: namespace

creation, binding or ...
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