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

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Message-ID: <20170623160026.GA18257@mail.hallyn.com>
Date:   Fri, 23 Jun 2017 11:00:26 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        lkp@...org, xiaolong.ye@...el.com,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Tycho Andersen <tycho@...ker.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        christian.brauner@...lbox.org, Vivek Goyal <vgoyal@...hat.com>,
        LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH 0/3] Enable namespaced file capabilities

Quoting Amir Goldstein (amir73il@...il.com):
> On Thu, Jun 22, 2017 at 9:59 PM, Stefan Berger
> <stefanb@...ux.vnet.ibm.com> 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.
> >
> 
> Am I the only one who thinks that suffix is perhaps not the best grammar
> to use for this namespace?

You're the only one to have mentioned it so far.

> xattrs are clearly namespaced by prefix, so it seems right to me to keep
> it that way - define a new special xattr namespace "ns" and only if that
> prefix exists, the @uid suffix will be parsed.
> This could be either  ns.security.capability@...=1000 or
> ns@...=1000.security.capability. The latter seems more correct to me,
> because then we will be able to namespace any xattr without having to
> protect from "unprivileged xattr injection", i.e.:
> setfattr -n "user.whatever.foo@...=0"

I like it for simplifying the parser code.  One concern I have is that,
since ns.* is currently not gated, one could write ns.* on an older
kernel and then exploit it on a newer one.