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

Quoting Theodore Ts'o (tytso at mit.edu):

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

quoted

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.

Is that a problem?  Essentially people who would try to do the
above also want to use 'shiftfs' stackable filesystem, which would
presumably eventually do this for you.

  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.

Only if you create the container rootfs as a copy.

Note that generally they would want to walk the fs in that case anyway, to chown
the files into the container.  And said chown would clear out any existing file
capabilities (and suid/sgid bits).

On the other hand, unprivileged lxc containers are created by
untarring the distro image straight into the mapped user namespace.
So no chowning is needed, and - once we we have this properly supported -
the filecaps should be automatically written correctly for the container.

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?

I think that's fine.

-serge
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