Date: Thu, 13 Jul 2017 16:17:44 -0500
From: "Serge E. Hallyn" <serge@hallyn.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Stefan Berger <stefanb@linux.vnet.ibm.com>,
        "Theodore Ts'o" <tytso@mit.edu>, "Serge E. Hallyn" <serge@hallyn.com>,
        containers@lists.linux-foundation.org, lkp@01.org,
        linux-kernel@vger.kernel.org, zohar@linux.vnet.ibm.com,
        tycho@docker.com, James.Bottomley@HansenPartnership.com,
        vgoyal@redhat.com, christian.brauner@mailbox.org, amir73il@gmail.com,
        linux-security-module@vger.kernel.org, casey@schaufler-ca.com
Subject: Re: [PATCH v2] xattr: Enable security.capability in user namespaces
Message-ID: <20170713211744.GB6167@mail.hallyn.com>
References: <1499785511-17192-2-git-send-email-stefanb@linux.vnet.ibm.com>
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 <87y3rscz9j.fsf@xmission.com>
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Quoting Eric W. Biederman (ebiederm@xmission.com):
> Stefan Berger <stefanb@linux.vnet.ibm.com> writes:
> If you don't care about the ownership of the files, and read only is
> acceptable, and you still don't want to give these executables
> capabilities in the initial user namespace.  What you can do is
> make everything owned by some non-zero uid including the security
> capability.  Call this non-zero uid image-root.
> 
> When the container starts it creates two nested user namespaces first
> with image-root mapped to 0.  Then with the containers choice of uid
> mapped to 0 image-root unmapped.  This will ensure the capability
> attributes work for all containers that share that root image.  And it
> ensures the file are read-only from the container.
> 
> So I don't think there is ever a case where we would share a filesystem
> image where we would need to set multiple security attributes on a file.

Neat idea.  In fact, you can take it a step further and still have the
files be owned by valid uids in the containers.  The parent ns just
needs to have its *root* map to a common kuid not mapped into the child
namespaces, but the files can be owned by another kuid which *is* mapped
into the child containers.