net: ipv4: use a dedicated counter for icmp_v4 redirect packets

According to the algorithm described in the comment block at the
beginning of ip_rt_send_redirect, the host should try to send
'ip_rt_redirect_number' ICMP redirect packets with an exponential
backoff and then stop sending them at all assuming that the destination
ignores redirects.
If the device has previously sent some ICMP error packets that are
rate-limited (e.g TTL expired) and continues to receive traffic,
the redirect packets will never be transmitted. This happens since
peer->rate_tokens will be typically greater than 'ip_rt_redirect_number'
and so it will never be reset even if the redirect silence timeout
(ip_rt_redirect_silence) has elapsed without receiving any packet
requiring redirects.

Fix it by using a dedicated counter for the number of ICMP redirect
packets that has been sent by the host

I have not been able to identify a given commit that introduced the
issue since ip_rt_send_redirect implements the same rate-limiting
algorithm from commit 1da177e4c3f4 ("Linux-2.6.12-rc2")

Signed-off-by: Lorenzo Bianconi <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/inetpeer.h

···
 
 	u32			metrics[RTAX_MAX];
 	u32			rate_tokens;	/* rate limiting for ICMP */
+	u32			n_redirects;
 	unsigned long		rate_last;
 	/*
 	 * Once inet_peer is queued for deletion (refcnt == 0), following field

net/ipv4/inetpeer.c

···
 			atomic_set(&p->rid, 0);
 			p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
 			p->rate_tokens = 0;
+			p->n_redirects = 0;
 			/* 60*HZ is arbitrary, but chosen enough high so that the first
 			 * calculation of tokens is at its maximum.
 			 */

net/ipv4/route.c

···
 	/* No redirected packets during ip_rt_redirect_silence;
 	 * reset the algorithm.
 	 */
-	if (time_after(jiffies, peer->rate_last + ip_rt_redirect_silence))
+	if (time_after(jiffies, peer->rate_last + ip_rt_redirect_silence)) {
 		peer->rate_tokens = 0;
+		peer->n_redirects = 0;
+	}
 
 	/* Too many ignored redirects; do not send anything
 	 * set dst.rate_last to the last seen redirected packet.
 	 */
-	if (peer->rate_tokens >= ip_rt_redirect_number) {
+	if (peer->n_redirects >= ip_rt_redirect_number) {
 		peer->rate_last = jiffies;
 		goto out_put_peer;
 	}
···
 		icmp_send(skb, ICMP_REDIRECT, ICMP_REDIR_HOST, gw);
 		peer->rate_last = jiffies;
 		++peer->rate_tokens;
+		++peer->n_redirects;
 #ifdef CONFIG_IP_ROUTE_VERBOSE
 		if (log_martians &&
 		    peer->rate_tokens == ip_rt_redirect_number)