linux-kernel - Re: [RFC][PATCH 0/3] vfs: Detach mounts on unlink.

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Message-Id: <1381018693.1974.166@driftwood>
Date:	Sat, 05 Oct 2013 19:18:13 -0500
From:	Rob Landley <rob@...dley.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFC][PATCH 0/3] vfs: Detach mounts on unlink.

On 10/05/2013 06:22:15 PM, Linus Torvalds wrote:
> On Sat, Oct 5, 2013 at 4:17 PM, Linus Torvalds
> <torvalds@...ux-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--
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