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Date: Fri, 23 Jun 2017 16:09:56 -0400
From: Vivek Goyal <vgoyal@redhat.com>
To: Stefan Berger <stefanb@linux.vnet.ibm.com>
Cc: ebiederm@xmission.com, containers@lists.linux-foundation.org,
        lkp@01.org, xiaolong.ye@intel.com, linux-kernel@vger.kernel.org,
        zohar@linux.vnet.ibm.com, serge@hallyn.com, tycho@docker.com,
        James.Bottomley@HansenPartnership.com, christian.brauner@mailbox.org,
        amir73il@gmail.com, linux-security-module@vger.kernel.org
Subject: Re: [PATCH 0/3] Enable namespaced file capabilities
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On Thu, Jun 22, 2017 at 02:59:46PM -0400, 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.
> 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.

Hi Stefan,

Got a question. If child usernamespace sets a
security.capability@uid=1000, can any of the parent namespace remove it?

IOW, I set capability from usernamespace and tried to remove it from
host and that failed. Is that expected.

# Inside usernamespce
$setcap cat_net_raw+ep foo.txt

# outside user namespace
$listxattr foo.txt
 xattr: security.capability@uid=1000
 xattr: security.selinux

# outside user namespace
setfattr -x security.capability@uid foo.txt
setfattr: foo.txt: Invalid argument

Doing a strace shows removexattr() failed. May this will need fixing?

removexattr("testfile.txt", "security.capability@uid") = -1 EINVAL
(Invalid argument)

Vivek