[PATCH 0/3] Enable namespaced file capabilities

[Stefan Berger]

On 06/28/2017 03:18 AM, Amir Goldstein wrote:
> On Wed, Jun 28, 2017 at 8:41 AM, Serge E. Hallyn <serge at hallyn.com> wrote:
>> On Fri, Jun 23, 2017 at 10:01:46AM +0300, Amir Goldstein wrote:
>>> On Thu, Jun 22, 2017 at 9:59 PM, Stefan Berger
>>> <stefanb at linux.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 at 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 at uid=1000 will be shown. Inside the
>>>> namespace only 'security.capability', with the value of
>>>> security.capability at uid=1000, is visible.
>>>>
>>> Am I the only one who thinks that suffix is perhaps not the best grammar
>>> to use for this namespace?
>>> 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 at uid=1000 or
>>> ns at uid=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 at uid=0"
>>>
>>> Amir.
>> Hi Amir,
>>
>> I was liking the prefix at first, but I'm actually not sure it's worth
>> it.  THe main advantage would be so that checking for namespace or other
>> tags could be done always at the same offset simplifying the parser.
>> But since we will want to only handle namespacing for some tags, and
>> potentially differently for each task, it won't actually be simpler, I
>> don't think.
>>
>> On the other hand we do want to make sure that the syntax we use is
>> generally usable, so I think simply specifying that >1 tags can each
>> be separate by '@' should suffice.  So for now we'd only have
> Serge,
>
> I am not sure I am parsing what you are saying correctly (pun intended).
> Can you give some examples of xattr names with several @.
>
>>          security.capability at uid=100000
>>
>> soon we'd hopefully have
>>
>>          security.ima at uid=100000
>>
> IIUC, the xattr names above should be parsed as:
>
>          security.(([ima|capability])@(uid=100000)
>
>> and eventually trusted.blarb at foo=bar
>>
> But the trusted xattr name should be parsed as:
>
>          (trusted.blarb)@(uid=100000)
>
> Otherwise it won't be able to pass the xattr_is_trusted() test
> which looks only at the trusted prefix.

To be precise, it looks at 'trusted.', including the dot.

>
> So we can write it like this, if it makes sense for the parser:
>          trusted at uid=100000.blarb

For the parser I think it would be easier to parse what Serge is 
proposing, and it would pass the existing xattr_is_trusted() call.



>
> But I don't think that trusted.foo should have a different
> userns behavior than trusted.bar down the road.
>
> Admittedly, I am not so much of a security developer myself,
> so I prefer to let Casey be the spokesman for the '.ns' prefix.
> Casey's proposal seems right to me:
>
>          security.ns at uid=1000@@.capability
>
> We can also stick to a more conventional syntax of a perfect
> new namespace 'security.ns', which encapsulates the unprivileged
> xattr name completely. This should suffice perfectly for the current
> capability V3 needs and is flexible enough to be extended later:
>
>          security.ns.user.1000.security.capability
> OR:
>          security.ns at uid=1000@@.security.capability
>
> And going forward, just as easy:
>
>          security.ns.user.1000.[trusted|system|user].foo
>
> Amir.
>

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