[PATCH 09/19] fs: Refuse uid/gid changes which don't map into s_user_ns

[Seth Forshee]

On Fri, Dec 04, 2015 at 11:27:38AM -0600, Serge E. Hallyn wrote:
> On Wed, Dec 02, 2015 at 09:40:09AM -0600, Seth Forshee wrote:
> > Add checks to inode_change_ok to verify that uid and gid changes
> > will map into the superblock's user namespace. If they do not
> > fail with -EOVERFLOW. This cannot be overriden with ATTR_FORCE.
> > 
> > Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> 
> Acked-by: Serge Hallyn <serge.hallyn at canonical.com>
> 
> ...  although i could see root on the host being upset that it can't
> assign a uid not valid in the mounter's ns.  But it does seem safer.

That change wouldn't be representable in the backing store though, and
that could lead to unexpected behaviour. It's better to tell root that
we can't make the requested change, in my opinion.