linux-kernel - [PATCH 3.12 108/127] ipv4: use new

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Date:   Fri, 25 Nov 2016 09:30:21 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     stable@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Stephen Suryaputra Lin <stephen.suryaputra.lin@...il.com>,
        Stephen Suryaputra Lin <ssurya@...e.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 108/127] ipv4: use new_gw for redirect neigh lookup

From: Stephen Suryaputra Lin <stephen.suryaputra.lin@...il.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

[ Upstream commit 969447f226b451c453ddc83cac6144eaeac6f2e3 ]

In v2.6, 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 commit 5943634fc559 ("ipv4: Maintain redirect and PMTU info in
struct rtable again."), 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.

So, use the new_gw for neigh lookup.

Changes from v1:
 - use __ipv4_neigh_lookup instead (per Eric Dumazet).

Fixes: 5943634fc559 ("ipv4: Maintain redirect and PMTU info in struct rtable again.")
Signed-off-by: Stephen Suryaputra Lin <ssurya@...e.org>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 net/ipv4/route.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index f1631aec4206..fd2811086257 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -764,7 +764,9 @@ static void __ip_do_redirect(struct rtable *rt, struct sk_buff *skb, struct flow
 			goto reject_redirect;
 	}

-	n = ipv4_neigh_lookup(&rt->dst, NULL, &new_gw);
+	n = __ipv4_neigh_lookup(rt->dst.dev, new_gw);
+	if (!n)
+		n = neigh_create(&arp_tbl, &new_gw, rt->dst.dev);
 	if (!IS_ERR(n)) {
 		if (!(n->nud_state & NUD_VALID)) {
 			neigh_event_send(n, NULL);
-- 
2.10.2