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

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Date:	Sat, 5 Oct 2013 17:37:24 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rob Landley <rob@...dley.net>
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 Sat, Oct 5, 2013 at 5:18 PM, Rob Landley <rob@...dley.net> wrote:
>
> 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...

Yeah, chroot() really doesn't cut it if you allow root access - and
thus internal chroot() calls - as you noticed (I didn't realize that
your example was meant to be run _inside_ a chroot).

There are generally other ways to escape chroot too if you're root.
It's just too hard to plug. That doesn't make chroot() useless - it
just means that the uses are elsewhere (it's useful for various
non-security issues like development environments, but it can also be
useful as one small _part_ of some bigger model, like a VM etc).

pivot_root() does end up being a "better chroot than chroot" if you're
looking for containment. It may not be a pretty system call, but it
does avoid at least the most obvious gotchas with chroot()..

            Linus
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