[PATCH review 0/11 Call for testing and review of mount detach fixes (take 2)

Eric W. Biederman ebiederm at xmission.com
Mon Jan 5 20:45:50 UTC 2015

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 burned while doing that.

For those who like to see everything in a single tree the code is at:

   git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace.git for-testing

Eric W. Biederman (11):
      mnt: Improve the umount_tree flags
      mnt: Don't propagate umounts in __detach_mounts
      mnt: In umount_tree reuse mnt_list instead of mnt_hash
      mnt: Add MNT_UMOUNT flag
      mnt: Delay removal from the mount hash.
      mnt: Factor out __detach_mnt from detach_mnt
      mnt: Simplify umount_tree
      mnt: Remove redundant NULL tests in namespace_unlock
      mnt: On an unmount propagate clearing of MNT_LOCKED
      mnt: Don't propagate unmounts to locked mounts
      mnt: Honor MNT_LOCKED when detaching mounts

 fs/namespace.c        | 152 +++++++++++++++++++++++++++++++-------------------
 fs/pnode.c            |  60 +++++++++++++++++---
 fs/pnode.h            |   7 ++-
 include/linux/mount.h |   1 +
 4 files changed, 154 insertions(+), 66 deletions(-)

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