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

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Date:   Fri, 23 Jun 2017 09:16:54 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>,
        Amir Goldstein <amir73il@...il.com>
Cc:     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>,
        James Bottomley <James.Bottomley@...senpartnership.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 6/23/2017 9:00 AM, Serge E. Hallyn wrote:
> Quoting Amir Goldstein (amir73il@...il.com):
>> On Thu, Jun 22, 2017 at 9:59 PM, Stefan Berger
>> <stefanb@...ux.vnet.ibm.com> wrote:
>>> This series of patches primary goal is to enable file capabilities
>>> in user namespaces without affecting the file capabilities that are
>>> effective on the host. This is to prevent that any unprivileged user
>>> on the host maps his own uid to root in a private namespace, writes
>>> the xattr, and executes the file with privilege on the host.
>>>
>>> We achieve this goal by writing extended attributes with a different
>>> name when a user namespace is used. If for example the root user
>>> in a user namespace writes the security.capability xattr, the name
>>> of the xattr that is actually written is encoded as
>>> security.capability@...=1000 for root mapped to uid 1000 on the host.
>>> When listing the xattrs on the host, the existing security.capability
>>> as well as the security.capability@...=1000 will be shown. Inside the
>>> namespace only 'security.capability', with the value of
>>> security.capability@...=1000, is visible.
>>>
>> Am I the only one who thinks that suffix is perhaps not the best grammar
>> to use for this namespace?
> You're the only one to have mentioned it so far.
>
>> xattrs are clearly namespaced by prefix, so it seems right to me to keep
>> it that way - define a new special xattr namespace "ns" and only if that
>> prefix exists, the @uid suffix will be parsed.
>> This could be either  ns.security.capability@...=1000 or
>> ns@...=1000.security.capability. The latter seems more correct to me,
>> because then we will be able to namespace any xattr without having to
>> protect from "unprivileged xattr injection", i.e.:
>> setfattr -n "user.whatever.foo@...=0"
> I like it for simplifying the parser code.  One concern I have is that,
> since ns.* is currently not gated, one could write ns.* on an older
> kernel and then exploit it on a newer one.

security.ns.capability@...=1000, then?

Or maybe just security.ns.capability, taking James' comment into account.

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