Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the current root

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, Oct 07, 2014 at 02:30:40PM +0100, Al Viro wrote:
> On Tue, Oct 07, 2014 at 04:12:57PM +0400, Andrey Vagin wrote:
> > Another problem is that rootfs can't be hidden from a container, because
> > rootfs can't be moved or umounted.
> 
> ... which is a bug in mntns_install(), AFAICS.

Ability to get to exposed rootfs, that is.

> > Here is an example how to get access to rootfs:
> > fd = open("/proc/self/ns/mnt", O_RDONLY)
> > umount2("/", MNT_DETACH);
> > setns(fd, CLONE_NEWNS)
> > 
> > rootfs may contain data, which should not be avaliable in CT-s.
> 
> Indeed.

... and it looks like the above is what your mangled reproducer in previous
patch had been made of -
	fd = open("/proc/self/ns/mnt", O_RDONLY)
	umount2("/", MNT_DETACH);
	setns(fd, CLONE_NEWNS)
	umount2("/", MNT_DETACH);

IMO what it shows is setns() bug.  This "switch root/cwd, no matter what"
is wrong.