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

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Date:   Thu, 22 Jun 2017 13:33:20 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Stefan Berger <stefanb@...ux.vnet.ibm.com>, ebiederm@...ssion.com,
        containers@...ts.linux-foundation.org
Cc:     lkp@...org, xiaolong.ye@...el.com, linux-kernel@...r.kernel.org,
        zohar@...ux.vnet.ibm.com, serge@...lyn.com, tycho@...ker.com,
        James.Bottomley@...senPartnership.com,
        christian.brauner@...lbox.org, vgoyal@...hat.com,
        amir73il@...il.com, linux-security-module@...r.kernel.org
Subject: Re: [PATCH 0/3] Enable namespaced file capabilities

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@...=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.

>
> If root in userns1 mapped to uid 1000 (size 1000) writes security.capability, it will write security.capability@...=1000 into the fs.
> If root in userns2 mapped to uid 2000 (size 1000) writes security.capability, it will write security.capability@...=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@...=1000 as well as its own 'security.capability'. security.capability@...=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@...=1000 will be shown. Inside the
>>> namespace only 'security.capability', with the value of
>>> security.capability@...=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@...=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(-)
>>>
>
>