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

[Stefan Berger]

On 06/23/2017 02:35 PM, Serge E. Hallyn wrote:
> Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
> > On 06/23/2017 12:16 PM, Casey Schaufler wrote:
> > > 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  ns.security.capability@uid=1000 or
> > > > > ns@uid=1000.security.capability. 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 "user.whatever.foo@uid=0"
> > > > 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?
> > Imo, '.ns' is redundant and 'encoded' in the '@'.
> So how about
> 	security.@uid=1000@@capability ?
Ouch.
> Maybe it's not worth it.

So the .ns is there to be able to possibly extend it in another 
dimension in the future, like have '.foo' there at some point?


>