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

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Date:   Fri, 23 Jun 2017 16:09:56 -0400
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Stefan Berger <stefanb@...ux.vnet.ibm.com>
Cc:     ebiederm@...ssion.com, containers@...ts.linux-foundation.org,
        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, amir73il@...il.com,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH 0/3] Enable namespaced file capabilities

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@...=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@...=1000 will be shown. Inside the
> namespace only 'security.capability', with the value of
> security.capability@...=1000, is visible.

Hi Stefan,

Got a question. If child usernamespace sets a
security.capability@...=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@...=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