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

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Date:   Mon, 20 Feb 2017 17:24:52 +1300
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     James Bottomley <James.Bottomley@...senPartnership.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

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.

Eric