Re: [PATCH v2] xattr: Enable security.capability in user namespaces

From: Stefan Berger
Date: Thu Jul 13 2017 - 13:34:08 EST

On 07/13/2017 01:14 PM, Eric W. Biederman wrote:
Theodore Ts'o <tytso@xxxxxxx> writes:

On Thu, Jul 13, 2017 at 07:11:36AM -0500, Eric W. Biederman wrote:
The concise summary:

Today we have the xattr security.capable that holds a set of
capabilities that an application gains when executed. AKA setuid root exec
without actually being setuid root.

User namespaces have the concept of capabilities that are not global but
are limited to their user namespace. We do not currently have
filesystem support for this concept.
So correct me if I am wrong; in general, there will only be one
variant of the form:

It's not like there will be:

Except.... if you have an Distribution root directory which is shared
by many containers, you would need to put the xattrs in the overlay
inodes. Worse, each time you launch a new container, with a new
subuid allocation, you will have to iterate over all files with
capabilities and do a copy-up operations on the xattrs in overlayfs.
So that's actually a bit of a disaster.

So for distribution overlays, you will need to do things a different
way, which is to map the distro subdirectory so you know that the
capability with the global uid 0 should be used for the container
"root" uid, right?

So this hack of using is *only* useful when the
subcontainer root wants to create the privileged executable. You
still have to do things the other way.

So can we make perhaps the assertion that *either*:

exists, *or*

exists, but never both? And there BAR is exclusive to only one

Otherwise, I suspect that the architecture is going to turn around and
bite us in the *ss eventually, because someone will want to do
something crazy and the solution will not be scalable.
Yep. That is what it looks like from here.

Which is why I asked the question about scalability of the xattr
implementations. It looks like trying to accomodate the general
case just gets us in trouble, and sets unrealistic expectations.

Which strongly suggests that Serge's previous version that
just reved the format of security.capable so that a uid field could
be added is likely to be the better approach.

I want to see what Serge and Stefan have to say but the case looks
pretty clear cut at the moment.

The approach of virtualizing the xattrs on the name-side, which is what this patch does, provides a more general approach than to virtualizing it on the value side, which is what Serge does in his other patch for security.capability alone. With the virtualizing on-the-value side virtualizing the xattr becomes an exercise that needs to be repeated for every xattr name that one would want to virtualize. With this patch you would just add another xattr name to a list, a one-line patch in the end. Xattr with prefixes like trusted.* need a bit more work but this can be woven in as well (

For virtualizing the xattrs on the 'value' side I was looking for whether there's something like a 'wrapper' structure around the actual value of the xattr so that that wrapper could be extended to support different values at different uids and applied to any xattr. Unfortunately there's no such 'wrapper'.