linux-kernel - Re: [PATCH v2] xattr: Enable security.capability in user namespaces

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Date:   Fri, 14 Jul 2017 08:39:49 -0400
From:   Stefan Berger <stefanb@...ux.vnet.ibm.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "Theodore Ts'o" <tytso@....edu>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        containers@...ts.linux-foundation.org, lkp@...org,
        linux-kernel@...r.kernel.org, zohar@...ux.vnet.ibm.com,
        tycho@...ker.com, James.Bottomley@...senPartnership.com,
        vgoyal@...hat.com, christian.brauner@...lbox.org,
        amir73il@...il.com, linux-security-module@...r.kernel.org,
        casey@...aufler-ca.com
Subject: Re: [PATCH v2] xattr: Enable security.capability in user namespaces

On 07/14/2017 08:04 AM, Eric W. Biederman wrote:
> Stefan Berger <stefanb@...ux.vnet.ibm.com> writes:
>
>> On 07/13/2017 08:38 PM, Eric W. Biederman wrote:
>>> Stefan Berger <stefanb@...ux.vnet.ibm.com> writes:
>>>
>>>> On 07/13/2017 01:49 PM, Eric W. Biederman wrote:
>>>>
>>>>> My big question right now is can you implement Ted's suggested
>>>>> restriction.  Only one security.foo or secuirty.foo@... attribute ?
>>>> We need to raw-list the xattrs and do the check before writing them. I am fairly sure this can be done.
>>>>
>>>> So now you want to allow security.foo and one security.foo@...=<> or just a single one security.foo(@[[:print:]]*)?
>>>>
>>> The latter.
>> That case would prevent a container user from overriding the xattr on
>> the host. Is that what we want?
> Most definitely.  If a more privileged use has set secure.capable that
> is better.
>
>> For limiting the number of xattrs and
>> getting that functionality (override IMA signature for example) the
>> former seems better...
> I don't know about IMA.  But my feeling is that we will only be dealing
> with a single signing key, so I don't see how having multiple IMA xattrs
> make sense.  Could you explain that to me?

Admittedly I would need to construct and example where the user inside 
the container doesn't want to share the public key with the host on a 
file mounted from the host for some reason.

An example related to security.capability could be a Fedora Docker 
container where the container is distributed with the ping tool 
installed. The ping tool is installed with cap_net_admin,cap_net_raw+ep. 
On a normal Fedora container I cannot use this tool due to my 
capabilities bounding set not including cap_net_admin. So, I overwrite 
this and set only cap_net_raw+ep and I can use for pinging. I may loose 
some functionality on the way due to the lost cap_net_admin but I can 
now use the tool. I guess the point is one can override the capabilities 
set of a distributed container if the container is started with less 
capabilities.

    Stefan


>
>> For the former I now have the topmost patch here:
>> https://github.com/stefanberger/linux/commits/xattr_for_userns.v3
> Thank you.
>
> Eric
>