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

From: Serge E. Hallyn
Date: Fri Jun 23 2017 - 12:00:41 EST

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.