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

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Message-Id: <d288ea69-adec-f257-30cb-b1d9c57c996b@linux.vnet.ibm.com>
Date:   Fri, 23 Jun 2017 13:38:45 -0400
From:   Stefan Berger <stefanb@...ux.vnet.ibm.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Cc:     Amir Goldstein <amir73il@...il.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 06/23/2017 01:07 PM, James Bottomley wrote:
> 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

Yes. An answer would be to have Docker rewrite these on the fly. It 
knows what uid the container was running as and specifically looks for 
security.ima@...=1000 or security.ima, takes the former if it finds, 
otherwise the latter or nothing.

    Stefan

>
> 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
>