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Date:   Mon, 1 Jul 2019 20:20:49 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     David Howells <dhowells@redhat.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [PATCH] vfs: move_mount: reject moving kernel internal mounts
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On Mon, Jul 01, 2019 at 07:22:39PM +0100, Al Viro wrote:

> FWIW, it's not just move_mount(2) - I'd expect
> 
> 	int fds[2];
> 	char s[80];
> 
> 	pipe(fds);
> 	sprintf(s, "/dev/fd/%d", fds[0]);
> 	mount(s, "/dev/null", NULL, MS_MOVE, 0);
> 
> to step into exactly the same thing.  mount(2) does follow symlinks -
> always had...

The same goes for e.g.

#define _GNU_SOURCE
#include <sched.h>
#include <sys/mount.h>
#include <stdio.h>
#include <sys/epoll.h>

main()
{
	char s[80];
	unshare(CLONE_NEWNS);	// so nobody else gets confused
 	sprintf(s, "/dev/fd/%d", epoll_create1(0));
 	mount(s, "/dev/null", NULL, MS_MOVE, 0);	// see if it oopses
}

modulo error-checking, etc.