i tried to restrict root from using "su" to become any user, but was unable to do so. the following piece of policy does not allow root to use su at all:
subject /bin/su dp
user_transition_deny nobody
/dev/log rw
complains of missing CAP_SETUID, CAP_SETGID, however this allows root to so to anybody (including nobody):
subject /bin/su dp
user_transition_deny nobody
/dev/log rw
+CAP_SETUID
+CAP_SETGID
missing feature, bug, misconfiguration?