netdev - [PATCH net] Fixes: 5943634fc559 ("ipv4: Maintain redirect and PMTU info in struct rtable again.")

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Date:   Mon,  7 Nov 2016 10:04:18 -0500
From:   Stephen Suryaputra Lin <stephen.suryaputra.lin@...il.com>
To:     netdev@...r.kernel.org
Cc:     Stephen Suryaputra Lin <ssurya@...e.org>
Subject: [PATCH net] Fixes: 5943634fc559 ("ipv4: Maintain redirect and PMTU info in struct rtable again.")

ICMP redirects behavior is different after the commit above. An email
requesting the explanation on why the behavior needs to be different
was sent earlier to netdev (https://patchwork.ozlabs.org/patch/687728/).
Since there isn't a reply yet, I decided to prepare this formal patch.

In v2.6 kernel, it used to be that ip_rt_redirect() calls
arp_bind_neighbour() which returns 0 and then the state of the neigh for
the new_gw is checked. If the state isn't valid then the redirected
route is deleted. This behavior is maintained up to v3.5.7 by
check_peer_redirect() because rt->rt_gateway is assigned to
peer->redirect_learned.a4 before calling ipv4_neigh_lookup().

After the commit, ipv4_neigh_lookup() is performed without the
rt_gateway assigned to the new_gw. In the case when rt_gateway (old_gw)
isn't zero, the function uses it as the key. The neigh is most likely valid
since the old_gw is the one that sends the ICMP redirect message. Then the
new_gw is assigned to fib_nh_exception. The problem is: the new_gw ARP may
never gets resolved and the traffic is blackholed.

Signed-off-by: Stephen Suryaputra Lin <ssurya@...e.org>
---
 net/ipv4/route.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 62d4d90c1389..510045cefcab 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -753,7 +753,9 @@ static void __ip_do_redirect(struct rtable *rt, struct sk_buff *skb, struct flow
 			goto reject_redirect;
 	}

+	rt->rt_gateway = 0;
 	n = ipv4_neigh_lookup(&rt->dst, NULL, &new_gw);
+	rt->rt_gateway = old_gw;
 	if (!IS_ERR(n)) {
 		if (!(n->nud_state & NUD_VALID)) {
 			neigh_event_send(n, NULL);
-- 
2.7.4