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

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Message-Id: <3404c486-c848-3283-50f7-2283cb631e8e@linux.vnet.ibm.com>
Date:   Fri, 23 Jun 2017 14:08:45 -0400
From:   Stefan Berger <stefanb@...ux.vnet.ibm.com>
To:     Casey Schaufler <casey@...aufler-ca.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Amir Goldstein <amir73il@...il.com>
Cc:     "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 06/23/2017 12:16 PM, Casey Schaufler wrote:
> 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?

Imo, '.ns' is redundant and 'encoded' in the '@'.

    Stefan