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

From: Serge E. Hallyn
Date: Fri Jun 23 2017 - 14:34:37 EST

Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
> On 06/23/2017 01:07 PM, James Bottomley wrote:
> >On Fri, 2017-06-23 at 11:30 -0500, Serge E. Hallyn wrote:
> >>Quoting Casey Schaufler (casey@xxxxxxxxxxxxxxxx):
> >>>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.
> >I don't think it's barbaric, I think it's the common use case. Let me
> >give a more comprehensible answer in terms of docker and IMA. Lets
> >suppose I'm running docker locally and in a test cloud both with userns
> >enabled.
> >
> >I build an image locally, mapping my uid (1000) to root. If I begin
> >with a standard base, each of the files has a security.ima signature.
> > Now I add my layer, which involves updating a file, so I need to write
> >a new signature to security.ima. Because I'm running user namespaced,
> >the update gets written at security.ima@uid=1000 when I do a docker
> >save.
> >
> >Now supposing I deploy that image to a cloud. As a tenant, the cloud
> >gives me real uid 4531 and maps that to root. Execution of the binary
> >fails because it tries to use the underlying signature (in
> >security.ima) as there is no xattr named security.ima@uid=4531
> Yes. An answer would be to have Docker rewrite these on the fly. It
> knows what uid the container was running as and specifically looks
> for security.ima@uid=1000 or security.ima, takes the former if it
> finds, otherwise the latter or nothing.

I know many people hate this answer, but I just want to point out that
on my little laptop, while untarring a 500M images takes 9.5 seconds,
remapping all uids and gids and restoring setuid+setgid on that image
takes .01s.

It's high cpu utilization, and it's not zero time, but it's very fast,
and it's 100% safe (when done the right way, not "sudo domychown").