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

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Date:   Wed, 22 Feb 2017 07:01:30 -0500
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        Amir Goldstein <amir73il@...il.com>,
        Djalal Harouni <tixxdz@...il.com>, Chris Mason <clm@...com>,
        Theodore Tso <tytso@....edu>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Seth Forshee <seth.forshee@...onical.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Dongsu Park <dongsu@...ocode.com>,
        David Herrmann <dh.herrmann@...glemail.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Alban Crequy <alban.crequy@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Serge E. Hallyn" <serge@...lyn.com>, Phil Estes <estesp@...il.com>
Subject: Re: [RFC 1/1] shiftfs: uid/gid shifting bind mount

On Mon, 2017-02-20 at 17:24 +1300, Eric W. Biederman wrote:
> James Bottomley <James.Bottomley@...senPartnership.com> writes:
> 
> > 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.
> 
> As a statement that it is ok for the unprivileged mapping code to
> operate that seems reasonable.  I don't currently the need for such 
> an ok from the system adminstrator, but if you need it a flag that
> propagates to children and child directories seems reasonable.

The other way to do this is with an extended attribute.  I've played
around with that approach and quite like it: the advantage is that it's
sticky across system reboots; The down side is that it requires
additional VFS code to make sure you can't execute from the non-user_ns
view.

James