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

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:

quoted

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.

quoted

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.