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

From: Casey Schaufler
Date: Fri Jun 23 2017 - 16:30:21 EST

On 6/23/2017 11:35 AM, Serge E. Hallyn wrote:
> Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
>> 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@xxxxxxxxx):
>>>>> On Thu, Jun 22, 2017 at 9:59 PM, Stefan Berger
>>>>> <stefanb@xxxxxxxxxxxxxxxxxx> 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@uid=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@uid=1000 will be shown. Inside the
>>>>>> namespace only 'security.capability', with the value of
>>>>>> security.capability@uid=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 or
>>>>> 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 ""
>>>> 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@uid=1000, then?
>> Imo, '.ns' is redundant and 'encoded' in the '@'.
> So how about
> security.@uid=1000@@capability ?

You're back to messing up the final component of the
attribute name. If you want a namespace component, keep
it separate. I disagree with the ".ns" being redundant.
It's descriptive.


looks right to me.

> Maybe it's not worth it.
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