US 1976 Copyright Act. SS 304(c)(6)(A)
"A derivative work prepard under authority of the grant before its termination may continue to be utilizd under the trms of the grant after its termination, but this privledge does not extnd to th preparation after the termination of other derivative works based upon the copyrightd work covered by the terminated grant."
IE: Once the permission to modify linux is revoked, no further work on grsecurity may commence: it's over.
(The license of Linux (GPLv2) lacks a no-revocation clause, GPLv3 had to be drafted for a reason (adding one in) (so you could attempt to argue estoppel)) (Licenses are revokable at will barring estoppel)