[SRU][disco][PATCH 2/2] ipv6: fix neighbour resolution with raw socket

[Kamal Mostafa]

From: Nicolas Dichtel <nicolas.dichtel at 6wind.com>

BugLink: https://bugs.launchpad.net/bugs/1834465

The scenario is the following: the user uses a raw socket to send an ipv6
packet, destinated to a not-connected network, and specify a connected nh.
Here is the corresponding python script to reproduce this scenario:

 import socket
 IPPROTO_RAW = 255
 send_s = socket.socket(socket.AF_INET6, socket.SOCK_RAW, IPPROTO_RAW)
 # scapy
 # p = IPv6(src='fd00:100::1', dst='fd00:200::fa')/ICMPv6EchoRequest()
 # str(p)
 req = b'`\x00\x00\x00\x00\x08:@\xfd\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\xfd\x00\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xfa\x80\x00\x81\xc0\x00\x00\x00\x00'
 send_s.sendto(req, ('fd00:175::2', 0, 0, 0))

fd00:175::/64 is a connected route and fd00:200::fa is not a connected
host.

With this scenario, the kernel starts by sending a NS to resolve
fd00:175::2. When it receives the NA, it flushes its queue and try to send
the initial packet. But instead of sending it, it sends another NS to
resolve fd00:200::fa, which obvioulsy fails, thus the packet is dropped. If
the user sends again the packet, it now uses the right nh (fd00:175::2).

The problem is that ip6_dst_lookup_neigh() uses the rt6i_gateway, which is
:: because the associated route is a connected route, thus it uses the dst
addr of the packet. Let's use rt6_nexthop() to choose the right nh.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel at 6wind.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 2c6b55f45d53420d8310d41310e0e2cd41fe073f)
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 net/ipv6/route.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 0be7158d1816..d00c58ae21fa 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -221,7 +221,8 @@ static struct neighbour *ip6_dst_neigh_lookup(const struct dst_entry *dst,
 {
 	const struct rt6_info *rt = container_of(dst, struct rt6_info, dst);
 
-	return ip6_neigh_lookup(&rt->rt6i_gateway, dst->dev, skb, daddr);
+	return ip6_neigh_lookup(rt6_nexthop(rt, &in6addr_any),
+				dst->dev, skb, daddr);
 }
 
 static void ip6_confirm_neigh(const struct dst_entry *dst, const void *daddr)
-- 
2.17.1