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

From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Vivek Goyal <vgoyal@redhat.com>,
	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>,
	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: Fri, 17 Feb 2017 09:19:53 -0800	[thread overview]
Message-ID: <1487351993.4351.17.camel@HansenPartnership.com> (raw)
In-Reply-To: <87a89l7f4y.fsf@xmission.com>

On Fri, 2017-02-17 at 14:57 +1300, Eric W. Biederman wrote:
> I think I am missing something but I completely do not understand 
> that subthread that says use file marks and perform the work in the 
> vfs. The problem is that fundamentally we need multiple mappings and 
> I don't see a mark on a file (even an inherited mark) providing the 
> mapping so I don't see the point.

The point of the mark is that it's a statement by the system
administrator that the underlying subtree is safe to be mounted by an
unprivileged container in the containers user view (i.e. with
current_user_ns() == s_user_ns).  For the unprivileged container
there's no real arbitrary s_user_ns use case because the unprivileged
container must prove it can set up the mapping, so it would likely
always be mounting from within a user_ns with the mapping it wanted.

James