Re: [PATCH v4] Introduce v3 namespaced file capabilities

["Serge E. Hallyn" <serge-A9i7LUbDfNHQT0dZR+AlfA@public.gmane.org>]

On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
> On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
> >Quoting Stefan Berger (stefanb-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org):
> >>  If all extended
> >>attributes were to support this model, maybe the 'uid' could be
> >>associated with the 'name' of the xattr rather than its 'value' (not
> >>sure whether that's possible).
> >Right, I missed that in your original email when I saw it this morning.
> >It's not what my patch does, but it's an interesting idea.  Do you have
> >a patch to that effect?  We might even be able to generalize that to
> 
> No, I don't have a patch. It may not be possible to implement it.
> The xattr_handler's  take the name of the xattr as input to get().

That may be ok though.  Assume the host created a container with
100000 as the uid for root, which created a container with 130000 as
uid for root.  If root in the nested container tries to read the
xattr, the kernel can check for security.foo[130000] first, then
security.foo[100000], then security.foo.  Or, it can do a listxattr
and look for those.  Am I overlooking one?

> So one could try to encode the mapped uid in the name. However, that

I thought that's exactly what you were suggesting in your original
email?  "security.capability[uid=2000]"

> could lead to problems with stale xattrs in a shared filesystem over
> time unless one could limit the number of xattrs with the same
> prefix, e.g., security.capability*. So I doubt that it would work.

Hm.  Yeah.  But really how many setups are there like that?  I.e. if
you launch a regular docker or lxd container, the image doesn't do a
bind mount of a shared image, it layers something above it or does a
copy.  What setups do you know of where multiple containers in different
user namespaces mount the same filesystem shared and writeable?

> Otherwise it would be good if the value was wrapped in a data
> structure use by all xattrs, but that doesn't seem to be the case,
> either. So I guess we have to go into each type of value structure
> and add a uid field there.
> 
> >namespace any security.* xattrs.  Wouldn't be automatically enabled
> >for anything but ima and capabilities, but we could make the infrastructure
> >generic and re-usable.
> >