[PATCH net] ipv6: fix neighbour resolution with raw socket

* [PATCH net] ipv6: fix neighbour resolution with raw socket
@ 2019-06-20 12:34 Nicolas Dichtel
  2019-06-20 15:12 ` David Ahern
                   ` (2 more replies)
  0 siblings, 3 replies; 21+ messages in thread
From: Nicolas Dichtel @ 2019-06-20 12:34 UTC (permalink / raw)
  To: davem; +Cc: netdev, Nicolas Dichtel

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.

Note that rt and in6addr_any are const in ip6_dst_lookup_neigh(), thus
let's constify rt6_nexthop() to avoid ugly cast.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
---
 include/net/ip6_route.h | 4 ++--
 net/ipv6/ip6_output.c   | 2 +-
 net/ipv6/route.c        | 3 ++-
 3 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 4790beaa86e0..ee7405e759ba 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -262,8 +262,8 @@ static inline bool ip6_sk_ignore_df(const struct sock *sk)
 	       inet6_sk(sk)->pmtudisc == IPV6_PMTUDISC_OMIT;
 }
 
-static inline struct in6_addr *rt6_nexthop(struct rt6_info *rt,
-					   struct in6_addr *daddr)
+static inline const struct in6_addr *rt6_nexthop(const struct rt6_info *rt,
+						 const struct in6_addr *daddr)
 {
 	if (rt->rt6i_flags & RTF_GATEWAY)
 		return &rt->rt6i_gateway;
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 834475717110..21efcd02f337 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -59,8 +59,8 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
 {
 	struct dst_entry *dst = skb_dst(skb);
 	struct net_device *dev = dst->dev;
+	const struct in6_addr *nexthop;
 	struct neighbour *neigh;
-	struct in6_addr *nexthop;
 	int ret;
 
 	if (ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr)) {
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 11ad62effd56..b6449bc03f11 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -218,7 +218,8 @@ 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)
-- 
2.21.0


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