[PATCH v2] xattr: Enable security.capability in user namespaces

[Mimi Zohar]

On Fri, 2017-07-14 at 12:35 -0500, Serge E. Hallyn wrote:
> Quoting Stefan Berger (stefanb at linux.vnet.ibm.com):
> > On 07/14/2017 09:34 AM, Serge E. Hallyn wrote:
> > >Quoting Stefan Berger (stefanb at linux.vnet.ibm.com):
> > >>On 07/13/2017 08:38 PM, Eric W. Biederman wrote:
> > >>>Stefan Berger <stefanb at linux.vnet.ibm.com> writes:
> > >>>
> > >>>>On 07/13/2017 01:49 PM, Eric W. Biederman wrote:
> > >>>>
> > >>>>>My big question right now is can you implement Ted's suggested
> > >>>>>restriction.  Only one security.foo or secuirty.foo at ... attribute ?
> > >>>>We need to raw-list the xattrs and do the check before writing them. I am fairly sure this can be done.
> > >>>>
> > >>>>So now you want to allow security.foo and one security.foo at uid=<> or just a single one security.foo(@[[:print:]]*)?
> > >>>>
> > >>>The latter.
> > >>That case would prevent a container user from overriding the xattr
> > >>on the host. Is that what we want? For limiting the number of xattrs
> > >Not really.  If the file is owned by a uid mapped into the container,
> > >then the container root can chown the file which will clear the file
> > >capability, after which he can set a new one.  If the file is not
> > >owned by a uid mapped into the container, then container root could
> > >not set a filecap anyway.
> > 
> > Let's say I installed a container where all files are signed and
> > thus have security.ima. Now for some reason I want to re-sign some
> > or all files inside that container. How would I do that ? Would I
> > need to get rid of security.ima first, possibly by copying each
> > file, deleting the original file, and renaming the copied file to
> > the original name, or should I just be able to write out a new
> > signature, thus creating security.ima at uid=1000 besides the
> > security.ima ?
> > 
> >    Stefan
> 
> Hi Mimi,
> 
> what do you think makes most sense for IMA?

If I'm understanding the discussion correctly, this isn't an issue for
layered copy on write filesystems, as each fs layer could have it's
own set of xattrs.  The underlying and layered xattrs should be able
to co-exist.  Use the layered xattr if it exists, but fall back to
using the underlying xattr if it doesn't.

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.

This allows most files to use the underlying xattrs, but allows a few
files to be re-signed inside the namespace, as needed.  For the
layered filesystem case, this would allow mutable file hashes to be
written.  (Unclear as to how shared filesystems would work in this
case.)

Mimi

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