netdev - [PATCH net-next] ipv6: Don't use dst gateway directly in ip6_confirm

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Date:   Mon,  9 Sep 2019 22:44:06 +0200
From:   Stefano Brivio <sbrivio@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     Guillaume Nault <gnault@...hat.com>, Julian Anastasov <ja@....bg>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        David Ahern <dsahern@...il.com>, netdev@...r.kernel.org
Subject: [PATCH net-next] ipv6: Don't use dst gateway directly in ip6_confirm_neigh()

This is the equivalent of commit 2c6b55f45d53 ("ipv6: fix neighbour
resolution with raw socket") for ip6_confirm_neigh(): we can send a
packet with MSG_CONFIRM on a raw socket for a connected route, so the
gateway would be :: here, and we should pick the next hop using
rt6_nexthop() instead.

This was found by code review and, to the best of my knowledge, doesn't
actually fix a practical issue: the destination address from the packet
is not considered while confirming a neighbour, as ip6_confirm_neigh()
calls choose_neigh_daddr() without passing the packet, so there are no
similar issues as the one fixed by said commit.

A possible source of issues with the existing implementation might come
from the fact that, if we have a cached dst, we won't consider it,
while rt6_nexthop() takes care of that. I might just not be creative
enough to find a practical problem here: the only way to affect this
with cached routes is to have one coming from an ICMPv6 redirect, but
if the next hop is a directly connected host, there should be no
topology for which a redirect applies here, and tests with redirected
routes show no differences for MSG_CONFIRM (and MSG_PROBE) packets on
raw sockets destined to a directly connected host.

However, directly using the dst gateway here is not consistent anymore
with neighbour resolution, and, in general, as we want the next hop,
using rt6_nexthop() looks like the only sane way to fetch it.

Reported-by: Guillaume Nault <gnault@...hat.com>
Signed-off-by: Stefano Brivio <sbrivio@...hat.com>
---
 net/ipv6/route.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 7a5d331cdefa..874641d4d2a1 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -227,7 +227,7 @@ static void ip6_confirm_neigh(const struct dst_entry *dst, const void *daddr)
 	struct net_device *dev = dst->dev;
 	struct rt6_info *rt = (struct rt6_info *)dst;

-	daddr = choose_neigh_daddr(&rt->rt6i_gateway, NULL, daddr);
+	daddr = choose_neigh_daddr(rt6_nexthop(rt, &in6addr_any), NULL, daddr);
 	if (!daddr)
 		return;
 	if (dev->flags & (IFF_NOARP | IFF_LOOPBACK))
-- 
2.20.1