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Date: Sat, 05 Oct 2013 19:18:13 -0500
From: Rob Landley <rob@landley.net>
Subject: Re: [RFC][PATCH 0/3] vfs: Detach mounts on unlink.
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
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        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>
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	(from torvalds@linux-foundation.org on Sat Oct  5 18:22:15 2013)
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On 10/05/2013 06:22:15 PM, Linus Torvalds wrote:
> On Sat, Oct 5, 2013 at 4:17 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > So learn this pattern: every time you use chroot, add a simple
> >
> >   chdir("/");
> >
> > immediately after the chroot call.
> 
> .. btw, also make sure that you close all non-essential file
> descriptors. Having any open directory file descriptors pointing to
> outside the chroot is also a classic escape.
> 
> Even then, escaping chroot is usually fairly easy. Making a
> escape-proof chroot is really quite hard. Basically impossible if you
> allow root.

Which is why containers have all sorts of extra plumbing. But that  
extra plumbing is currently built on pivot_root(), not on chroot(). And  
I'd dismissed pivot_root() as residue from initramfs with all that  
kernel thread reparenting, so it seemed like the wrong tool for the  
job, but obviously I'll take your and Al's word it's not...

Rob