xfrm: Generate blackhole routes only from route lookup functions

Currently we genarate a blackhole route route whenever we have
matching policies but can not resolve the states. Here we assume
that dst_output() is called to kill the balckholed packets.
Unfortunately this assumption is not true in all cases, so
it is possible that these packets leave the system unwanted.

We fix this by generating blackhole routes only from the
route lookup functions, here we can guarantee a call to
dst_output() afterwards.

Fixes: 2774c131 ("xfrm: Handle blackhole route creation via afinfo.")
Reported-by: Konstantinos Kolelis <k.kolelis@sirrix.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

(cherry picked from commit f92ee619)
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

include/net/dst.h

@@ -510,6 +510,10 @@ struct dst_entry *xfrm_lookup_route(struct net *net, struct dst_entry *dst_orig,
 				    const struct flowi *fl, struct sock *sk,
 				    int flags);
 
+struct dst_entry *xfrm_lookup_route(struct net *net, struct dst_entry *dst_orig,
+				    const struct flowi *fl, struct sock *sk,
+				    int flags);
+
 /* skb attached with this dst needs transformation if dst->xfrm is valid */
 static inline struct xfrm_state *dst_xfrm(const struct dst_entry *dst)
 {