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

[Theodore Ts'o]

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:

   security.foo at uid=15000

It's not like there will be:

   security.foo at uid=1000
   security.foo at uid=2000

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 security.foo at uid=1000 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*:

   security.foo

exists, *or*

   security.foo at uid=BAR

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

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.

	  	    		      	   -Ted
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