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

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Date:	Sat, 05 Oct 2013 18:07:42 -0500
From:	Rob Landley <rob@...dley.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 0/3] vfs: Detach mounts on unlink.

On 10/04/2013 05:41:25 PM, Eric W. Biederman wrote:
> 
> This patchset is an attempt to address two problems:
> 1) Not all modifications to the filesystems happen through the vfs and
>    since the vfs can not cope with a mount point being unlinked or
>    renamed filesystems whose modifications that do not come through  
> the
>    vfs are required to lie.
> 
> 2) Through an oversight it is now possible for one unprivileged user  
> to
>    mount something on another unprivileged users dentry and make it
>    impossible for the other user to unlink or rename that dentry.
> 
> It is now technically possible to easily lift the restriction on
> unlinking and renaming files with mount points on them, with a
> corresponding reduction in complexity of the vfs semantics and a small
> code side reduction.

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.)

Rob--
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