pf: fix syncookies in conjunction with tcp fast port reuse

Basic scenario: we have a closed connection (In TCPS_FIN_WAIT_2), and
get a new connection (i.e. SYN) re-using the tuple.

Without syncookies we look at the SYN, and completely unlink the old,
closed state on the SYN.
With syncookies we send a generated SYN|ACK back, and drop the SYN,
never looking at the state table.

So when the ACK (i.e. the third step in the three way handshake for
connection setup) turns up, we’ve not actually removed the old state, so
we find it, and don’t do the syncookie dance, or allow the new
connection to get set up.

Explicitly check for this in pf_test_state_tcp(). If we find a state in
TCPS_FIN_WAIT_2 and the syncookie is valid we delete the existing state
so we can set up the new state.
Note that when we verify the syncookie in pf_test_state_tcp() we don't
decrement the number of half-open connections to avoid an incorrect
double decrement.

MFC after:      2 weeks
Differential Revision:  https://reviews.freebsd.org/D37919

(cherry picked from commit 9c041b4)

sys/net/pfvar.h

@@ -2108,6 +2108,7 @@ int			 pf_set_syncookies(struct pfioc_nv *);
 int			 pf_synflood_check(struct pf_pdesc *);
 void			 pf_syncookie_send(struct mbuf *m, int off,
 			    struct pf_pdesc *);
+bool			 pf_syncookie_check(struct pf_pdesc *);
 u_int8_t		 pf_syncookie_validate(struct pf_pdesc *);
 struct mbuf *		 pf_syncookie_recreate_syn(uint8_t, int,
 			    struct pf_pdesc *);

sys/netpfil/pf/pf.c

@@ -4976,9 +4976,11 @@ pf_test_state_tcp(struct pf_kstate **state, int direction, struct pfi_kkif *kif,
 	if ((action = pf_synproxy(pd, state, reason)) != PF_PASS)
 		return (action);
 
-	if (((th->th_flags & (TH_SYN|TH_ACK)) == TH_SYN) &&
-	    dst->state >= TCPS_FIN_WAIT_2 &&
-	    src->state >= TCPS_FIN_WAIT_2) {
+	if (dst->state >= TCPS_FIN_WAIT_2 &&
+	    src->state >= TCPS_FIN_WAIT_2 &&
+	    (((th->th_flags & (TH_SYN|TH_ACK)) == TH_SYN) ||
+	    ((th->th_flags & (TH_SYN|TH_ACK|TH_RST)) == TH_ACK &&
+	    pf_syncookie_check(pd) && pd->dir == PF_IN))) {
 		if (V_pf_status.debug >= PF_DEBUG_MISC) {
 			printf("pf: state reuse ");
 			pf_print_state(*state);

sys/netpfil/pf/pf_syncookies.c

@@ -300,8 +300,8 @@ pf_syncookie_send(struct mbuf *m, int off, struct pf_pdesc *pd)
 	    1);
 }
 
-uint8_t
-pf_syncookie_validate(struct pf_pdesc *pd)
+bool
+pf_syncookie_check(struct pf_pdesc *pd)
 {
 	uint32_t		 hash, ack, seq;
 	union pf_syncookie	 cookie;
@@ -314,14 +314,29 @@ pf_syncookie_validate(struct pf_pdesc *pd)
 	cookie.cookie = (ack & 0xff) ^ (ack >> 24);
 
 	/* we don't know oddeven before setting the cookie (union) */
-        if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven])
+	if (atomic_load_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven])
 	    == 0)
-                return (0);
+		return (0);
 
 	hash = pf_syncookie_mac(pd, cookie, seq);
 	if ((ack & ~0xff) != (hash & ~0xff))
+		return (false);
+
+	return (true);
+}
+
+uint8_t
+pf_syncookie_validate(struct pf_pdesc *pd)
+{
+	uint32_t		 ack;
+	union pf_syncookie	 cookie;
+
+	if (! pf_syncookie_check(pd))
 		return (0);
 
+	ack = ntohl(pd->hdr.tcp.th_ack) - 1;
+	cookie.cookie = (ack & 0xff) ^ (ack >> 24);
+
 	counter_u64_add(V_pf_status.lcounters[KLCNT_SYNCOOKIES_VALID], 1);
 	atomic_add_64(&V_pf_status.syncookies_inflight[cookie.flags.oddeven], -1);