[Devel] Re: Keys and namespaces

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> David Howells <dhowells at redhat.com> writes:
> 
> > On the subject of namespaces: I still need to look at providing a key ID and
> > keyring name namespace.
> >
> > Is it worth me just using the user_namespace?  A number of parameters are
> > per-UID (such as the key quotas), so it might very well make sense to do that.
> 
> Yes.
> 
> > That way, user_namespace could actually be a credentials namespace.
> 
> I have always thought of it as a security token namespace.
> 
> I'm not certain about the term credential as that usually means something
> like password.  In the context of keys where you are caching something
> like a kerberos token it makes more sense.  Even there it isn't a
> secret key that is being cached but more of a capability that says
> that the secret key was known at some point.
> 
> > If that is the case, CLONE_NEWUSER should also set up (clone?) the keys and
> > keyrings attached to the parent.  This possibly needs to be done anyway as the
> > keys have UID and GID references that may be invalid in the new namespace.
> >
> > How do the UIDs and GIDs in different namespaces map, anyway?
> 
> At the simplest level they do not map between namespaces.
> 
> At a practical level the users in a user namespace should have
> no more capabilities than the creator of the user namespace and
> the creator of the user namespace should have the power to kill
> all processes and remove all objects created by users in that
> user namespace.
> 
> For filesystem access where we have talked about this in some detail.
> The current idea is to have a user namespace parameter on struct inode.
> All security credential comparisons will need to become comparisons
> of both user_namespace and uid to match.
> 
> > Furthermore, some keys may actually represent foreign user details; perhaps
> > NTFS or CIFS user IDs for example.  Should those be discarded on CLONE_NEWUSER?
> 
> After CLONE_NEWUSER the user id and guid should be 0, and no keys should be
> held.  With respect to holding credentials it should be essentially the same
> state as when init starts.  At least as a general rule.
> 
> If keys are the kind of thing that can be passed between namespaces
> allowing user space to control which keys get passed makes sense.  We
> allow the controlling tty and process groups to remain the same across
> pid namespace creation and require processes to change them explicitly
> if needed.  It is definitely a requirement to get to the state init
> starts in, either by kernel fiat or normal user space operations.

Ok so if I read this right, you're ok with either one of:

	1. on clone(CLONE_NEWUSER), the new task get empty keyrings.
	   If userspace wants the new task to have its creator's
	   keys, it reloads them.
	2. on clone(CLONE_NEWUSER), the new task gets a copy of
	   the creator's keys.  If userspace wants the new task to
	   have an empty keyring, it can clear it.

I had been thinking 2 might be easier in the face of ecryptfs, but
I think 1 is the way to go.

> A word about foreign user details.
> 
> In my conception a user namespace is essentially a single administration
> domain of uids and gids, and as such can span and does today span several machines.
> user namespaces are unique in that their tokens are assigned administratively
> and the kernel has no control over lifetime of tokens.
> 
> In that concept foreign users doesn't make a lot of sense to me.
> 
> Keys certainly provide the ability to several different implementations
> of user identification.  How that interacts with a web of administration
> of user permissions to objects not living in the kernel I don't know.
> 
> Ultimately I think putting keys in the user namespace is a good match.
> As it is a way to talk about stored user identification either in memory
> or on disk, and how that maps to what a user is allowed to do.
> 
> Eric
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