Date: Thu, 22 Jun 2017 18:32:45 -0500
From: "Serge E. Hallyn" <serge@hallyn.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Stefan Berger <stefanb@linux.vnet.ibm.com>, 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,
        christian.brauner@mailbox.org, vgoyal@redhat.com, amir73il@gmail.com,
        linux-security-module@vger.kernel.org
Subject: Re: [PATCH 0/3] Enable namespaced file capabilities
Message-ID: <20170622233245.GB2894@mail.hallyn.com>
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Quoting James Bottomley (James.Bottomley@HansenPartnership.com):
> On Thu, 2017-06-22 at 14:59 -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.
> 
> I'm a bit bothered by the @uid=1000 suffix.  What if I want to use this
> capability but am dynamically mapping the namespaces (i.e. I know I
> want unprivileged root, but I'm going to dynamically select the range
> to map based on what's currently available on the orchestration
> system).  If we stick with the @uid=X suffix, then dynamic mapping
> won't work because X is potentially different each time and there'll be

Note that if you just set a 'security.capability' xattr, it will apply
to all namespaces.

Does that address your concern?

> a name mismatch in my xattrs.  Why not just make the suffix @uid, which
> means if root is mapped to any unprivileged uid then we pick this up
> otherwise we go with the unsuffixed property?
> 
> As far as I can see there's no real advantage to discriminating userns
> specific xattrs based on where root is mapped to, unless there's a use
> case I'm missing?
> 
> James
>