[RFC PATCH] security, capability: pass object information to security_capable

[Andy Lutomirski]

On Tue, Jul 16, 2019 at 7:03 AM Serge E. Hallyn <serge at hallyn.com> wrote:
>
> On Fri, Jul 12, 2019 at 12:54:02PM -0700, Casey Schaufler wrote:
> > On 7/12/2019 11:25 AM, Stephen Smalley wrote:
> > > On 7/12/19 1:58 PM, Casey Schaufler wrote:
> > >> On 7/12/2019 10:34 AM, Nicholas Franck wrote:
> > >>> At present security_capable does not pass any object information
> > >>> and therefore can neither audit the particular object nor take it
> > >>> into account. Augment the security_capable interface to support
> > >>> passing supplementary data. Use this facility initially to convey
> > >>> the inode for capability checks relevant to inodes. This only
> > >>> addresses capable_wrt_inode_uidgid calls; other capability checks
> > >>> relevant to inodes will be addressed in subsequent changes. In the
> > >>> future, this will be further extended to pass object information for
> > >>> other capability checks such as the target task for CAP_KILL.
> > >>
> > >> This seems wrong to me. The capability system has nothing to do
> > >> with objects. Passing object information through security_capable()
> > >> may be convenient, but isn't relevant to the purpose of the interface.
> > >> It appears that there are very few places where the object information
> > >> is actually useful.
> > >
> > > A fair number of capabilities are checked upon some attempted object access (often right after comparing UIDs or other per-object state), and the particular object can be very helpful in both audit and in access control.  More below.
> >
> > I'm not disagreeing with that. What I'm saying is that the capability
> > check interface is not the right place to pass that information. The
> > capability check has no use for the object information. I would much
>
> I've had to argue this before while doing the namespaced file
> capabilities implementation.  Perhaps this would be worth writing something
> more formal about.  My main argument is, even if we want to claim that the
> capabilities model is and should be object agnostic, the implementation
> of user namespaces (currently) is such that the whole view of the user's
> privilege must include information which is stored with the object.
>
> There are various user namespaces.
>
> The Linux capabilities ( :-) ) model is user namespaced.  It must be, in
> order to be useful.  If we're going to use file capabilities in distros,
> and distros are going to run in containers, then the capabilities must
> be namespaced.  Otherwise, capabilities will not be used, and heck, should
> just be dropped.
>
> The only way to find out which user namespace has privilege over an inode
> is to look at the inode.
>
> Therefore, object information is needed.

Agreed.  The concept in the kernel is "capability over a namespace."

That being said, sticking a flexible object type into ns_capable()
seems prematurely general to me.  How about adding
security_capable_wrt_inode_uidgid() and allowing LSMs to hook that?
The current implementation would go into commoncap.  The obvious
extensions I can think of are security_dac_read_search(..., inode,
...) and security_dac_override(..., inode, ...).  (Or dentry or
whatever is appropriate.)

If this patch were restructured like that, the semantics would be
obvious, and it would arguably be a genuine cleanup instead of a whole
new mechanism of unknown scope.