Subject: Re: [PATCH v2] xattr: Enable security.capability in user namespaces
From: Mimi Zohar <zohar@linux.vnet.ibm.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Stefan Berger <stefanb@linux.vnet.ibm.com>,
        Mimi Zohar <zohar@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
        "Theodore Ts'o" <tytso@mit.edu>, containers@lists.linux-foundation.org,
        lkp@01.org, linux-kernel@vger.kernel.org, tycho@docker.com,
        vgoyal@redhat.com, christian.brauner@mailbox.org, amir73il@gmail.com,
        linux-security-module@vger.kernel.org, casey@schaufler-ca.com
Date: Fri, 14 Jul 2017 16:03:39 -0400
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On Fri, 2017-07-14 at 11:52 -0700, James Bottomley wrote:
> On Fri, 2017-07-14 at 14:48 -0400, Mimi Zohar wrote:
> > The concern is with a shared filesystems.  In that case, for IMA it
> > would make sense to support a native and a namespace xattr.  If due
> > to xattr space limitations we have to limit the number of xattrs,
> > then we should limit it to two - a native and a namespace version,
> > with a "uid=" tag - first namespace gets permission to write the
> > namespace xattr.  Again, like in the layered case, if the namespace
> > xattr doesn't exist, fall back to using the native xattr.
> 
> Just on this point: if we're really concerned about the need on shared
> filesystems to have multiple IMA signatures per file, might it not make
> sense simply to support multiple signatures within the security.ima
> xattr? The rules for writing signature updates within user namespaces
> would be somewhat complex (say only able to replace a signature for
> which you demonstrate you possess the key) but it would lead to an
> implementation which would work for traditional shared filesystems
> (like NFS) as well as containerised bind mounts.

Writing security.ima requires being root with CAP_SYS_ADMIN
privileges.  I wouldn't want to give root within the namespace
permission to over write or just extend the native security.ima.

Mimi