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Date: Sun, 6 Oct 2013 00:19:15 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Rob Landley <rob@landley.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Linux-Fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC][PATCH 0/3] vfs: Detach mounts on unlink.
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 <1381014462.1974.162@driftwood>
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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.