From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Subject: Re: [PATCH] [RFC] mnt: add ability to clone mntns starting with the
 current root
Date: Tue, 7 Oct 2014 14:33:39 +0100
Message-ID: <20141007133339.GH7996@ZenIV.linux.org.uk>
References: <1412683977-29543-1-git-send-email-avagin@openvz.org>
 <20141007133039.GG7996@ZenIV.linux.org.uk>
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Cc: linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andrey Vagin <avagin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
	"Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>,
	Cyrill Gorcunov <gorcunov-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>,
	Pavel Emelyanov <xemul-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>,
	Serge Hallyn <serge.hallyn-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>,
	Rob Landley <rob-VoJi6FS/r0vR7s880joybQ@public.gmane.org>
To: Andrey Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
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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.