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

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 06/22/2017 04:33 PM, Casey Schaufler wrote:
> On 6/22/2017 1:12 PM, Stefan Berger wrote:
>> On 06/22/2017 03:59 PM, Casey Schaufler wrote:
>>> On 6/22/2017 11:59 AM, Stefan Berger 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.
>>> You need to identify the instance of the user namespace for
>>> this to work right on a system with multiple user namespaces.
>>> If I have a shared filesystem mounted in two different user
>>> namespaces a change by one will affect the other.
>> Two different user namespaces with different uid mappings will not affect each other.
> But two namespaces with the same uid mapping will, and I
> don't think this meets the principle of least astonishment.
> I also object to associating capabilities with UIDs. The
> whole point of capabilities is to disassociate UID 0 from
> privilege. What you've done is explicitly associate a UID
> with the ability to have privilege. That's an architectural
> regression.

It has privilege within the bounding set of the capabilities that it is 
given. Afaik, a process cannot gain additional capabilities through file 
capabilities. Allowing to set a process's file capabilities allows one 
to _restrict_ what it can do, which is useful for shared filesystems 
where I can now set my ping capabilities to cap_net_raw, overriding the 
ones one the host which could be cap_net_admin+cap_net_raw. So I don't 
need to extend my bounding set with cap_net_admin or mess with xattrs on 
the host.


>
>> If root in userns1 mapped to uid 1000 (size 1000) writes security.capability, it will write security.capability@uid=1000 into the fs.
>> If root in userns2 mapped to uid 2000 (size 1000) writes security.capability, it will write security.capability@uid=2000 into the fs.
>>
>> Neither of the two will see each other's security.capability, but each will see their own 'security.capability'.
>>
>> Assume now userns1 has a size of 2000, so overlapping with userns2, it will now see userns2's security.capability@uid=1000 as well as its own 'security.capability'. security.capability@uid=1000 (of userns2) in userns1 will not have an effect on effective file capabilities.
>>
>>> ... unless I'm missing something obvious about namespace behavior.
>>>
>>>> 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.
>>>>
>>>> To maintain compatibility with existing behavior, the value of
>>>> security.capability of the host is shown inside the user namespace
>>>> once the security.capability of the user namespace has been removed
>>>> (which really removes security.capability@uid=1000). Writing to
>>>> an extended attribute inside a user namespace effectively hides the
>>>> extended attribute of the host.
>>>>
>>>> The general framework that is established with these patches can
>>>> be applied to other extended attributes as well, such as security.ima
>>>> or the 'trusted.' prefix . Another extended attribute that needed to
>>>> be enabled here is 'security.selinux,' since otherwise this extended
>>>> attribute would not be shown anymore inside a user namespace.
>>>>
>>>> Regards,
>>>>      Stefan & Serge
>>>>
>>>>
>>>> Stefan Berger (3):
>>>>     xattr: Enable security.capability in user namespaces
>>>>     Enable capabilities of files from shared filesystem
>>>>     Enable security.selinux in user namespaces
>>>>
>>>>    fs/xattr.c               | 472 ++++++++++++++++++++++++++++++++++++++++++++++-
>>>>    security/commoncap.c     |  36 +++-
>>>>    security/selinux/hooks.c |   9 +-
>>>>    3 files changed, 501 insertions(+), 16 deletions(-)
>>>>
>>