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

From: Stefan Berger
Date: Fri Jun 23 2017 - 19:09:45 EST

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 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?
Imo, '.ns' is redundant and 'encoded' in the '@'.
So how about
security.@uid=1000@@capability ?
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?