linux-kernel - Re: [PATCH v2 00/28] user_namespace: introduce fsid mappings

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Message-ID: <87pneesf0a.fsf@mid.deneb.enyo.de>
Date:   Sun, 16 Feb 2020 16:55:49 +0100
From:   Florian Weimer <fw@...eb.enyo.de>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     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>,
        Serge Hallyn <serge@...lyn.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 v2 00/28] user_namespace: introduce fsid mappings

* Christian Brauner:

> 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.

I'm worried that this is a bit of a management nightmare because the
data about the mapping does not live within the file system (it's
externally determined, static, but crucial to the interpretation of
file system content).  I expect that many organizations have
centralized allocation of user IDs, but centralized allocation of the
static mapping does not appear feasible.

Have you considered a more complex design, where untranslated nested
user IDs are store in a file attribute (or something like that)?  This
way, any existing user ID infrastructure can be carried over largely
unchanged.