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

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Message-ID: <20170623201723.GA22857@mail.hallyn.com>
Date:   Fri, 23 Jun 2017 15:17:23 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Stefan Berger <stefanb@...ux.vnet.ibm.com>, 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

Quoting Vivek Goyal (vgoyal@...hat.com):
> 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)

That's not the right xattr, though, does

	setfattr -x security.capability@...=1000 foo.txt

work?

If you are in fact uid=1000 then that should work.  If you are uid 1001,
and 1000 was delegated to you, then you'll need to create a transient
userns with uid 1000 mapped into it in order to delete it (so that you
have privilege over the uid).

If that doesn't work, then it's a bug.

-serge