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

From: Serge E. Hallyn
Date: Fri Jun 23 2017 - 12:30:53 EST

Quoting Casey Schaufler (casey@xxxxxxxxxxxxxxxx):
> On 6/23/2017 9:00 AM, Serge E. Hallyn wrote:
> > Quoting Amir Goldstein (amir73il@xxxxxxxxx):
> >> On Thu, Jun 22, 2017 at 9:59 PM, Stefan Berger
> >> <stefanb@xxxxxxxxxxxxxxxxxx> 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.
> >>>
> >> Am I the only one who thinks that suffix is perhaps not the best grammar
> >> to use for this namespace?
> > You're the only one to have mentioned it so far.
> >
> >> xattrs are clearly namespaced by prefix, so it seems right to me to keep
> >> it that way - define a new special xattr namespace "ns" and only if that
> >> prefix exists, the @uid suffix will be parsed.
> >> This could be either or
> >> The latter seems more correct to me,
> >> because then we will be able to namespace any xattr without having to
> >> protect from "unprivileged xattr injection", i.e.:
> >> setfattr -n ""
> > I like it for simplifying the parser code. One concern I have is that,
> > since ns.* is currently not gated, one could write ns.* on an older
> > kernel and then exploit it on a newer one.
> security.ns.capability@uid=1000, then?

That loses the advantage of simpler parsing though. (Really it's not much
of a simplification anyway.) So I'm not sure what advantage remains.

> Or maybe just security.ns.capability, taking James' comment into account.

That last one may be suitable as an option, useful for his particular
(somewhat barbaric :) use case, but it's not ok for the general solution.

If uid 1000 was delegated the subuids 100000-199999, it should be able
to write a file capability for use by his subuids, but that file capability
must not apply to other subuids.