[PATCH 3.16 039/136] net: ipv4: avoid mixed n_redirects and rate_tokens usage

[Ben Hutchings]

3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paolo Abeni <pabeni@xxxxxxxxxx>

commit b406472b5ad79ede8d10077f0c8f05505ace8b6d upstream.

Since commit c09551c6ff7f ("net: ipv4: use a dedicated counter
for icmp_v4 redirect packets") we use 'n_redirects' to account
for redirect packets, but we still use 'rate_tokens' to compute
the redirect packets exponential backoff.

If the device sent to the relevant peer any ICMP error packet
after sending a redirect, it will also update 'rate_token' according
to the leaking bucket schema; typically 'rate_token' will raise
above BITS_PER_LONG and the redirect packets backoff algorithm
will produce undefined behavior.

Fix the issue using 'n_redirects' to compute the exponential backoff
in ip_rt_send_redirect().

Note that we still clear rate_tokens after a redirect silence period,
to avoid changing an established behaviour.

The root cause predates git history; before the mentioned commit in
the critical scenario, the kernel stopped sending redirects, after
the mentioned commit the behavior more randomic.

Reported-by: Xiumei Mu <xmu@xxxxxxxxxx>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: c09551c6ff7f ("net: ipv4: use a dedicated counter for icmp_v4 redirect packets")
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Acked-by: Lorenzo Bianconi <lorenzo.bianconi@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/ipv4/route.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -891,16 +891,15 @@ void ip_rt_send_redirect(struct sk_buff
 	if (peer->rate_tokens == 0 ||
 	    time_after(jiffies,
 		       (peer->rate_last +
-			(ip_rt_redirect_load << peer->rate_tokens)))) {
+			(ip_rt_redirect_load << peer->n_redirects)))) {
 		__be32 gw = rt_nexthop(rt, ip_hdr(skb)->daddr);
 
 		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)
+		    peer->n_redirects == ip_rt_redirect_number)
 			net_warn_ratelimited("host %pI4/if%d ignores redirects for %pI4 to %pI4\n",
 					     &ip_hdr(skb)->saddr, inet_iif(skb),
 					     &ip_hdr(skb)->daddr, &gw);