On Wed, Jan 27, 2016 at 04:36:02PM -0800, Andy Lutomirski wrote:

On Wed, Jan 27, 2016 at 9:22 AM, Jann Horn [off-list ref] wrote:

quoted

I think it sounds good from a security perspective.

I'm a bit late to the game, but I have a question: why should this be
keyed to the *root* uid of the namespace in particular?  Certainly if
user foo trusts the cap bits on some file, then user foo might trust
those caps to be exerted over any namespace that user foo owns, since
user foo owns the namespace.

...  Tying it to a kuid which represents the userns->owner of any
namespace in which the capability will be honored might be fine
with me.  Is that what you mean?  So if uid 1000 creates a userns
mapping uids 100000-200000, and 100000 in that container puts X=pe
on /bin/foo, uid 101000 in that container runs /bin/foo with privilege
X.  Uid 101000 in someone else's container does not.

Although, if I create two containers and provide them different
uidmaps, it may well be because I want them segragated and want
to minimize the changes of one container breaking out into the
other.  This risks breaking that.

But another option would be to include a list of uids and gids such
that the cap bits on the file are trusted by any namespace that maps
only uids and gids in the list.  After all, the existence of a
namespace with root user foo that also maps bar and baz along with a
file with caps set means that, if baz can get to the file and
permissions are set appropriately, then baz now owns bar (via any
number of fs-related capabilities).  So maybe bar and baz should have
to be listed as well.

But maybe this doesn't matter.

In any event, at the end of the day, the right answer to all of this
is to stop using setuid and stop using cap bits too and start using
privileged daemons or other things that don't use the eternally
fragile grant-privilege-on-execve mechanisms.

Heh, that's why I wrote a p9auth driver a few years ago, but it
was too early for such a thing.