linux-kernel - Re: [PATCH v4] Introduce v3 namespaced file capabilities

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Date:   Tue, 13 Jun 2017 13:42:24 -0400
From:   Stefan Berger <stefanb@...ux.vnet.ibm.com>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, lkp@...org,
        containers@...ts.linux-foundation.org, xiaolong.ye@...el.com,
        LKML <linux-kernel@...r.kernel.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4] Introduce v3 namespaced file capabilities

On 06/13/2017 01:14 PM, Tycho Andersen wrote:
> Hi Stefan,
>
> On Tue, Jun 13, 2017 at 11:47:26AM -0400, Stefan Berger wrote:
>> On 05/08/2017 02:11 PM, Serge E. Hallyn wrote:
>>> Root in a non-initial user ns cannot be trusted to write a traditional
>>> security.capability xattr.  If it were allowed to do so, then any
>>> unprivileged user on the host could map his own uid to root in a private
>>> namespace, write the xattr, and execute the file with privilege on the
>>> host.
>>>
>>> However supporting file capabilities in a user namespace is very
>>> desirable.  Not doing so means that any programs designed to run with
>>> limited privilege must continue to support other methods of gaining and
>>> dropping privilege.  For instance a program installer must detect
>>> whether file capabilities can be assigned, and assign them if so but set
>>> setuid-root otherwise.  The program in turn must know how to drop
>>> partial capabilities, and do so only if setuid-root.
>> Hi Serge,
>>
>>
>>    I have been looking at patch below primarily to learn how we could apply a
>> similar technique to security.ima and security.evm for a namespaced IMA.
>>  From the paragraphs above I thought that you solved the problem of a shared
>> filesystem where one now can write different security.capability xattrs by
>> effectively supporting for example security.capability[uid=1000] and
>> security.capability[uid=2000] written into the filesystem. Each would then
>> become visible as security.capability if the userns mapping is set
>> appropriately.
> One disadvantage of this approach is that whoever is setting up the
> container would need to go touch the security.ima attribute for each
> file in the contianer, which would slow down container creation time.
> For capabilities this makes sense, because you might want the file to
> have different capabilities in different namespaces, but for IMA,
> since the file hash will be the same in every namespace, it would be
> nice to use a design that avoids touching each file on new ns
> creation.

Actually IMA in appraisal mode also supports signatures in the extended 
attributes. Depending on which (public) keys you put on the IMA keyring 
for a namespaced IMA, you may need a different signature on a file to be 
able to access it (execute it for example). For this to work containers 
would have to be able to ship with security.ima xattrs embedded in them 
and users should be able to apply signatures on files on a running 
container (or while building it). I worked on a prototype for namespaces 
IMA before where one of the issues was the CAP_SYS_ADMIN gate that 
disallows setting of security.ima when dropped. So some form of 
exception would have to be granted to be allowed to set security.ima 
from inside a container while CAP_SYS_ADMIN isn't there. And of course 
we need to protect the host's filesystem from an attack where the user 
just modifies the security.ima signature associated with a file.

   Stefan


> Cheers,
>
> Tycho
>