linux-kernel - Re: [PATCH v3 00/25] user_namespace: introduce fsid mappings

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Date:   Wed, 19 Feb 2020 15:48:37 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Stéphane Graber <stgraber@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>,
        smbarber@...omium.org, Seth Forshee <seth.forshee@...onical.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Phil Estes <estesp@...il.com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 00/25] user_namespace: introduce fsid mappings

On Wed, Feb 19, 2020 at 01:35:58PM -0600, Serge E. Hallyn wrote:
> On Tue, Feb 18, 2020 at 03:33:46PM +0100, Christian Brauner wrote:
> > With fsid mappings we can solve this by writing an id mapping of 0
> > 100000 100000 and an fsid mapping of 0 300000 100000. On filesystem
> > access the kernel will now lookup the mapping for 300000 in the fsid
> > mapping tables of the user namespace. And since such a mapping exists,
> > the corresponding files will have correct ownership.
> 
> So if I have
> 
> /proc/self/uid_map: 0 100000 100000
> /proc/self/fsid_map: 1000 1000 1

Oh, sorry.  Your explanation in 20/25 i think set me straight, though I need
to think through a few more examples.

...

> 3. If I create a new file, as nsuid 1000, what will be the inode owning kuid?

(Note - I edited the quoted txt above to be more precise)

I'm still not quite clear on this.  I believe the fsid mapping will take
precedence so it'll be uid 1000 ?  Per mount behavior would be nice there,
but perhaps unwieldy.