BugLink: http://bugs.launchpad.net/bugs/1566505

In a userns mount some on-disk inodes may have ids which do not
map into s_user_ns, in which case the in-kernel inodes are owned
by invalid users. The superblock owner should be able to change
attributes of these inodes but cannot. However it is unsafe to
grant the superblock owner privileged access to all inodes in the
superblock since proc, sysfs, etc. use DAC to protect files which
may not belong to s_user_ns. The problem is restricted to only
inodes where the owner or group is an invalid user.

We can work around this by allowing users with CAP_CHOWN in
s_user_ns to change an invalid owner or group id, so long as the
other id is either invalid or mappable in s_user_ns. After
changing ownership the user will be privileged towards the inode
and thus able to change other attributes.

As an precaution, checks for invalid ids are added to the proc
and kernfs setattr interfaces. These filesystems are not expected
to have inodes with invalid ids, but if it does happen any
setattr operations will return -EPERM.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>