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Date:	Tue, 7 Oct 2014 17:40:12 +0400
From:	Andrew Vagin <avagin@...allels.com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Andrey Vagin <avagin@...nvz.org>, <linux-fsdevel@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>
Subject: Re: [PATCH] mnt: don't allow to detach the namespace root

On Tue, Oct 07, 2014 at 02:24:36PM +0100, Al Viro wrote:
> On Tue, Oct 07, 2014 at 04:00:12PM +0400, Andrey Vagin wrote:
> > This patch fixes a bug, which is triggered by following code:
> > while (1) {
> >         if (umount2("/", MNT_DETACH) ||
> > 	    setns(fd, CLONE_NEWNS))
> >                 return break;
> > }
> 
> Excuse me, but that makes no sense whatsoever (not to mention that
> reproducer won't compile - return break; alone is enough to make
> sure of that).
> 
> Could you post the real reproducer?

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <sys/mount.h>

int main(int argc, char **argv)
{
	int fd;

	fd = open("/proc/self/ns/mnt", O_RDONLY);
	if (fd < 0)
		return 1;
	while (1) {
		if (umount2("/", MNT_DETACH) ||
		    setns(fd, CLONE_NEWNS))
			break;
	}

	return 0;
}

root@...ntu:/home/avagin# gcc -Wall nsenter.c -o nsenter
root@...ntu:/home/avagin# strace ./nsenter
execve("./nsenter", ["./nsenter"], [/* 22 vars */]) = 0
...
open("/proc/self/ns/mnt", O_RDONLY)     = 3
umount("/", MNT_DETACH)                 = 0
setns(3, 131072)                        = 0
umount("/", MNT_DETACH

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