[RFC PATCH 09/27] vfs: Allow mounting to other namespaces

[Al Viro]

On Fri, Feb 15, 2019 at 04:08:46PM +0000, David Howells wrote:
> Currently sys_move_mount() and sys_mount(MS_MOVE) prevent the caller from
> moving a mount into a namespace not their own.  Relax this such that any
> mount can be mounted onto any given mountpoint provided that the source
> mount is either detached or the same namespace as the destination.
> 
> This permits container namespaces to be built from the outside rather than
> from the inside.

I'm looking forward to your analysis of security implications, as well as
the proof that attach_recursive_mnt() won't get confused by that...