Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Amir Goldstein <amir73il@gmail.com>,
	Djalal Harouni <tixxdz@gmail.com>, Chris Mason <clm@fb.com>,
	Theodore Tso <tytso@mit.edu>,
	Josh Triplett <josh@joshtriplett.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Andy Lutomirski <luto@kernel.org>,
	Seth Forshee <seth.forshee@canonical.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	Dongsu Park <dongsu@endocode.com>,
	David Herrmann <dh.herrmann@googlemail.com>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Alban Crequy <alban.crequy@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Phil Estes <estesp@gmail.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount
Date: Thu, 16 Feb 2017 08:58:21 -0800	[thread overview]
Message-ID: <1487264301.2944.51.camel@HansenPartnership.com> (raw)
In-Reply-To: <20170216164233.GC23490@redhat.com>

On Thu, 2017-02-16 at 11:42 -0500, Vivek Goyal wrote:
> On Thu, Feb 16, 2017 at 07:51:58AM -0800, James Bottomley wrote:
> 
> [..]
> > > Two levels of checks will simplify this a bit. Top level inode 
> > > will belong to the user namespace of caller and checks should 
> > > pass. And mounter's creds will have ownership over the real inode 
> > > so no additional namespace shifting required there.
> > 
> > That's the problem: for a marked mount, they don't.
> 
> In this new model it does not fit directly. 
> 
> I was playing with a slightly different approach and modified patches 
> so that real root still does the mounting and takes an mount option
> which specifies which user namespace we want to shift into. Thanks to 
> Eric for the idea.
> 
> mount -t shiftfs -o userns_fd=<fd> source shifted-fs

This is a non-starter because it doesn't work for the unprivileged use
case, which is what I'm really interested in.  For fully unprivileged
containers you don't have an orchestration system to ask to build the
container.  You can get init scripts to set stuff up for you, like the
marks, but ideally it should just work even without that (so an inode
flag following project semantics seems really appealing), but after
that the unprivileged user should be able to build their own
containers.

As you saw from the reply to Eric, this approach (which I have tried)
also opens up a whole can of worms for non-FS_USERNS_MOUNT filesystems.

James

> In this case real-root is mounter and notion of using mounter's creds 
> on real-inode works. 
> This requires a user namespace to be created before shiftfs can be 
> mounted and then container admin should be able to bind mount shifted
> -fs.
> 
> In this model, intervention of real-root is still required to setup
> container and shiftfs. I guess that might not satisfy your needs 
> where unprivileged user should be able to launch container and be 
> able to make use of shiftfs, IIUC.
> 
> Vivek
> 