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

From James Bottomley <James.Bottomley@HansenPartnership.com>
Newsgroups linux.kernel
Subject Re: [PATCH 0/3] Enable namespaced file capabilities
Date 2017-06-23 02:20 +0200
Message-ID <tVkul-3lB-3@gated-at.bofh.it> (permalink)
References <tVfEl-nS-3@gated-at.bofh.it> <tVjRD-2RU-5@gated-at.bofh.it> <tVjRD-2RU-3@gated-at.bofh.it>
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On Thu, 2017-06-22 at 18:36 -0500, Serge E. Hallyn wrote:
> 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 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?
> 
> Yes, the use case is: to allow root in the container to set the
> privilege itself, without endangering any resources not owned by
> that root.

OK, so you envisage the same filesystem being mounted in different user
namespaces and being able to see their own value for the xattr.  It
still seems a bit weird that they'd be able to change file contents and
have that seen by the other userns but not xattrs.

>   If you're going to have a root owned host-wide
> orchestration system setting up the rootfs, then you don't
> necessary need this at all.

I wasn't thinking it would be root owned, just that it would have a
predefined range of allowed uids and be able to map multiple containers
to subsets of these.

> As you say a @uid to say "any unprivileged userns" might be useful.
> The implication is that root on the host doesn't trust the image
> enough to write a real global file capability, but trusts it enough
> to 'endanger' all containers on the host.  If that's the case, I have
> no objection to adding this as a feature.

Yes, precisely.  The filesystem is certified as permitted to override
the xattr whatever unprivileged mapping for root is in place.

How would we effect the switch?  I suppose some global flag because I
can't see we'd be mixing use cases in a physical system.

James

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[PATCH 0/3] Enable namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-22 21:10 +0200
  [PATCH 1/3] xattr: Enable security.capability in user namespaces Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-22 21:10 +0200
    [PATCH] xattr: fix kstrdup.cocci warnings kbuild test robot <lkp@intel.com> - 2017-06-24 23:10 +0200
    Re: [PATCH 1/3] xattr: Enable security.capability in user namespaces kbuild test robot <lkp@intel.com> - 2017-06-24 23:10 +0200
  Re: [PATCH 0/3] Enable namespaced file capabilities Casey Schaufler <casey@schaufler-ca.com> - 2017-06-22 22:00 +0200
    Re: [PATCH 0/3] Enable namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-22 22:20 +0200
      Re: [PATCH 0/3] Enable namespaced file capabilities Casey Schaufler <casey@schaufler-ca.com> - 2017-06-22 22:40 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-22 23:10 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-22 23:10 +0200
          Re: [PATCH 0/3] Enable namespaced file capabilities Casey Schaufler <casey@schaufler-ca.com> - 2017-06-23 00:50 +0200
            Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 01:10 +0200
  Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 01:40 +0200
    Re: [PATCH 0/3] Enable namespaced file capabilities James Bottomley <James.Bottomley@HansenPartnership.com> - 2017-06-23 02:20 +0200
      Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 03:30 +0200
      Re: [PATCH 0/3] Enable namespaced file capabilities ebiederm@xmission.com (Eric W. Biederman) - 2017-06-23 19:50 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 20:40 +0200
  Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 01:40 +0200
  Re: [PATCH 0/3] Enable namespaced file capabilities James Bottomley <James.Bottomley@HansenPartnership.com> - 2017-06-23 01:40 +0200
  Re: [PATCH 0/3] Enable namespaced file capabilities Amir Goldstein <amir73il@gmail.com> - 2017-06-23 09:10 +0200
    Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 18:10 +0200
      Re: [PATCH 0/3] Enable namespaced file capabilities Casey Schaufler <casey@schaufler-ca.com> - 2017-06-23 18:20 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 18:40 +0200
          Re: [PATCH 0/3] Enable namespaced file capabilities Casey Schaufler <casey@schaufler-ca.com> - 2017-06-23 19:00 +0200
            Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 19:10 +0200
              Re: [PATCH 0/3] Enable namespaced file capabilities ebiederm@xmission.com (Eric W. Biederman) - 2017-06-23 20:00 +0200
                Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 20:40 +0200
          Re: [PATCH 0/3] Enable namespaced file capabilities James Bottomley <James.Bottomley@HansenPartnership.com> - 2017-06-23 19:10 +0200
            Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 19:30 +0200
            Re: [PATCH 0/3] Enable namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-23 19:40 +0200
              Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 20:40 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-23 20:10 +0200
          Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 20:40 +0200
            Re: [PATCH 0/3] Enable namespaced file capabilities Casey Schaufler <casey@schaufler-ca.com> - 2017-06-23 22:40 +0200
            Re: [PATCH 0/3] Enable namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-24 01:10 +0200
              Re: [PATCH 0/3] Enable namespaced file capabilities Casey Schaufler <casey@schaufler-ca.com> - 2017-06-24 02:00 +0200
    Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-28 07:50 +0200
      Re: [PATCH 0/3] Enable namespaced file capabilities Amir Goldstein <amir73il@gmail.com> - 2017-06-28 09:20 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities Stefan Berger <stefanb@linux.vnet.ibm.com> - 2017-06-28 16:10 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-28 16:30 +0200
  Re: [PATCH 0/3] Enable namespaced file capabilities Vivek Goyal <vgoyal@redhat.com> - 2017-06-23 22:20 +0200
    Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 22:20 +0200
      Re: [PATCH 0/3] Enable namespaced file capabilities Vivek Goyal <vgoyal@redhat.com> - 2017-06-23 22:40 +0200
        Re: [PATCH 0/3] Enable namespaced file capabilities "Serge E. Hallyn" <serge@hallyn.com> - 2017-06-23 23:00 +0200

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