Re: [PATCH 3/7] fs: Ignore file caps in mounts from other user namespaces

["Serge E. Hallyn" <serge@hallyn.com>]

On Thu, Jul 16, 2015 at 12:04:43AM -0500, Eric W. Biederman wrote:
> > I tend to thing that, if we're not honoring the fcaps, we shouldn't be
> > honoring the setuid bit either.  After all, it's really not a trusted
> > file, even though the only user who could have messed with it really
> > is the apparent owner.
> 
> For the file caps we can't honor them because you don't have the bits
> in struct cred.
> 
> For setuid we can honor it, and setuid is something that the user
> namespace allows.

Setuid is something explicitly tied to the user id.  File capabilities
are MAC, that is, explicitly orthogonal to user id.  So 100% agreed with
honoring setuid in user_ns and, for now, ignoring file caps.

As I've mentioned a few times privately, I'm intending to implement
user-namespaced file capabilities as a new xattr.  Design is not 100%
nailed down, but probably it would support a set of userns_fcaps, each
of which lists the k_uid of the root user in the namespace assigning the
filecaps, followed by three sets.  Then when exec()ing the file, if
the current->userns->root user has a userns_fcap entry, or there is a -1
entry, then use that, else use nothing.  I think this is a very importing
thing to support, to remove a barrier to shipping packages with software
using filecaps.  Without this, any package, say ping, which wants to
support being installed in a (unprivileged) cotainer would need to also
support use without filecaps, meaning that will likely be the only
supported mode.

-serge