user namespaces: require cap_set{ug}id for CLONE_NEWUSER

While ideally CLONE_NEWUSER will eventually require no
privilege, the required permission checks are currently
not there.  As a result, CLONE_NEWUSER has the same effect
as a setuid(0)+setgroups(1,"0").  While we already require
CAP_SYS_ADMIN, requiring CAP_SETUID and CAP_SETGID seems
appropriate.
Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: James Morris <jmorris@namei.org>

kernel/fork.c

@@ -1344,7 +1344,8 @@ long do_fork(unsigned long clone_flags,
 		/* hopefully this check will go away when userns support is
 		 * complete
 		 */
-		if (!capable(CAP_SYS_ADMIN))
+		if (!capable(CAP_SYS_ADMIN) || !capable(CAP_SETUID) ||
+				!capable(CAP_SETGID))
 			return -EPERM;
 	}