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

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Message-Id: <596f808b-e21d-8296-5fef-23c1ce7ab778@linux.vnet.ibm.com>
Date:   Fri, 14 Jul 2017 11:22:28 -0400
From:   Stefan Berger <stefanb@...ux.vnet.ibm.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Theodore Ts'o" <tytso@....edu>,
        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 09:34 AM, Serge E. Hallyn wrote:
> Quoting Stefan Berger (stefanb@...ux.vnet.ibm.com):
>> 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? For limiting the number of xattrs
> Not really.  If the file is owned by a uid mapped into the container,
> then the container root can chown the file which will clear the file
> capability, after which he can set a new one.  If the file is not
> owned by a uid mapped into the container, then container root could
> not set a filecap anyway.

Let's say I installed a container where all files are signed and thus 
have security.ima. Now for some reason I want to re-sign some or all 
files inside that container. How would I do that ? Would I need to get 
rid of security.ima first, possibly by copying each file, deleting the 
original file, and renaming the copied file to the original name, or 
should I just be able to write out a new signature, thus creating 
security.ima@...=1000 besides the security.ima ?

    Stefan