[PATCH 3/3] ipv6/icmp: l3mdev: Perform icmp error route lookup on source device routing table

[Mathieu Desnoyers]

As per RFC4443, the destination address field for ICMPv6 error messages
is copied from the source address field of the invoking packet.

In configurations with Virtual Routing and Forwarding tables, looking up
which routing table to use for sending ICMPv6 error messages is
currently done by using the destination net_device.

If the source and destination interfaces are within separate VRFs, or
one in the global routing table and the other in a VRF, looking up the
source address of the invoking packet in the destination interface's
routing table will fail if the destination interface's routing table
contains no route to the invoking packet's source address.

One observable effect of this issue is that traceroute6 does not work in
the following cases:

- Route leaking between global routing table and VRF
- Route leaking between VRFs

Preferably use the source device routing table when sending ICMPv6 error
messages. If no source device is set, fall-back on the destination
device routing table.

Link: https://tools.ietf.org/html/rfc4443
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: David Ahern <dsahern@xxxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
---
 net/ipv6/icmp.c       | 15 +++++++++++++--
 net/ipv6/ip6_output.c |  2 --
 2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/icmp.c b/net/ipv6/icmp.c
index a4e4912ad607..a971b58b0371 100644
--- a/net/ipv6/icmp.c
+++ b/net/ipv6/icmp.c
@@ -501,8 +501,19 @@ void icmp6_send(struct sk_buff *skb, u8 type, u8 code, __u32 info,
 	if (__ipv6_addr_needs_scope_id(addr_type)) {
 		iif = icmp6_iif(skb);
 	} else {
-		dst = skb_dst(skb);
-		iif = l3mdev_master_ifindex(dst ? dst->dev : skb->dev);
+		struct net_device *route_lookup_dev = NULL;
+
+		/*
+		 * The device used for looking up which routing table to use is
+		 * preferably the source whenever it is set, which should
+		 * ensure the icmp error can be sent to the source host, else
+		 * fallback on the destination device.
+		 */
+		if (skb->dev)
+			route_lookup_dev = skb->dev;
+		else if (skb_dst(skb))
+			route_lookup_dev = skb_dst(skb)->dev;
+		iif = l3mdev_master_ifindex(route_lookup_dev);
 	}
 
 	/*
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index c78e67d7747f..cd623068de53 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -468,8 +468,6 @@ int ip6_forward(struct sk_buff *skb)
 	 *	check and decrement ttl
 	 */
 	if (hdr->hop_limit <= 1) {
-		/* Force OUTPUT device used as source address */
-		skb->dev = dst->dev;
 		icmpv6_send(skb, ICMPV6_TIME_EXCEED, ICMPV6_EXC_HOPLIMIT, 0);
 		__IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS);
 
-- 
2.17.1