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

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Message-Id: <1381018428.1974.165@driftwood>
Date:	Sat, 05 Oct 2013 19:13:48 -0500
From:	Rob Landley <rob@...dley.net>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 0/3] vfs: Detach mounts on unlink.

On 10/05/2013 06:19:15 PM, Al Viro wrote:
> On Sat, Oct 05, 2013 at 06:07:42PM -0500, Rob Landley wrote:
> > A todo item I've had _forever_ is fixing chroot() to not be broken
> > so that you can trivially break out of a chroot via:
> >
> >   chdir("/");
> >   mkdir("sub");
> >   chroot("sub");
> >   chdir("./../../../../../../../..");
> >
> > (Because chroot() affects where "/" points but NOT where "." points
> > to, and chdir does an == check with the dentry "/" points at to know
> > when to stop, so if you move "/" under "." you can back up to the
> > actual root of the tree.)
> >
> > The above is why lxc uses pivot_root() instead of chroot().
> >
> > These days, we have multiple mount trees so there's no reason
> > chroot() can't trim the process local mount tree (creating a new
> > bind mount if necessary). Except my todo list runneth over and I
> > haven't had a chance to dig in and see what would be involved. (Last
> > time I brought this up people were wondering why chroot() didn't
> > just move "." to the new "/" if it wasn't under it. I had no idea,
> > still don't.)
> 
> 1) RTFUNIXFAQ.  chroot() never has been root-proof.
> 
> 2) your "fix" isn't - it will lead to mounts done by chrooted process
> not affecting other processes in the same namespace.

So if I write a lightweight container setup command, I need to use  
pivot_root just like lxc does?

Rob--
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