linux-kernel - Re: [PATCH 0/3] Enable namespaced file capabilities

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Date:   Fri, 23 Jun 2017 10:07:21 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        lkp@...org, xiaolong.ye@...el.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Tycho Andersen <tycho@...ker.com>,
        christian.brauner@...lbox.org, Vivek Goyal <vgoyal@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 0/3] Enable namespaced file capabilities

On Fri, 2017-06-23 at 11:30 -0500, Serge E. Hallyn wrote:
> Quoting Casey Schaufler (casey@...aufler-ca.com):
> > Or maybe just security.ns.capability, taking James' comment into
> > account.
> 
> That last one may be suitable as an option, useful for his particular
> (somewhat barbaric :) use case, but it's not ok for the general
> solution.
> 
> If uid 1000 was delegated the subuids 100000-199999, it should be 
> able to write a file capability for use by his subuids, but that file
> capability must not apply to other subuids.

I don't think it's barbaric, I think it's the common use case.  Let me
give a more comprehensible answer in terms of docker and IMA.  Lets
suppose I'm running docker locally and in a test cloud both with userns
enabled.

I build an image locally, mapping my uid (1000) to root.  If I begin
with a standard base, each of the files has a security.ima signature. 
 Now I add my layer, which involves updating a file, so I need to write
a new signature to security.ima.  Because I'm running user namespaced,
the update gets written at security.ima@...=1000 when I do a docker
save. 

Now supposing I deploy that image to a cloud.  As a tenant, the cloud
gives me real uid 4531 and maps that to root.  Execution of the binary
fails because it tries to use the underlying signature (in
security.ima) as there is no xattr named security.ima@...=4531

So my essential point is that building the real kuid into the permanent
record of the xattr damages image portability, which is touted as one
of the real advantages of container images.

James