netfilter: Use l3mdev flow key when re-routing mangled packets

Commit 40867d74 ("net: Add l3mdev index to flow struct and avoid oif
reset for port devices") introduces a flow key specific for layer 3
domains, such as a VRF master device. This allows for explicit VRF domain
selection instead of abusing the oif flow key.

Update ip[6]_route_me_harder() to make use of that new key when re-routing
mangled packets within VRFs instead of setting the flow oif, making it
consistent with other users.
Signed-off-by: Martin Willi <martin@strongswan.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter.c

@@ -45,8 +45,7 @@ int ip_route_me_harder(struct net *net, struct sock *sk, struct sk_buff *skb, un
 	fl4.saddr = saddr;
 	fl4.flowi4_tos = RT_TOS(iph->tos);
 	fl4.flowi4_oif = sk ? sk->sk_bound_dev_if : 0;
-	if (!fl4.flowi4_oif)
-		fl4.flowi4_oif = l3mdev_master_ifindex(dev);
+	fl4.flowi4_l3mdev = l3mdev_master_ifindex(dev);
 	fl4.flowi4_mark = skb->mark;
 	fl4.flowi4_flags = flags;
 	fib4_rules_early_flow_dissect(net, skb, &fl4, &flkeys);

net/ipv6/netfilter.c

@@ -31,6 +31,7 @@ int ip6_route_me_harder(struct net *net, struct sock *sk_partial, struct sk_buff
 	int strict = (ipv6_addr_type(&iph->daddr) &
 		      (IPV6_ADDR_MULTICAST | IPV6_ADDR_LINKLOCAL));
 	struct flowi6 fl6 = {
+		.flowi6_l3mdev = l3mdev_master_ifindex(dev),
 		.flowi6_mark = skb->mark,
 		.flowi6_uid = sock_net_uid(net, sk),
 		.daddr = iph->daddr,
@@ -42,8 +43,6 @@ int ip6_route_me_harder(struct net *net, struct sock *sk_partial, struct sk_buff
 		fl6.flowi6_oif = sk->sk_bound_dev_if;
 	else if (strict)
 		fl6.flowi6_oif = dev->ifindex;
-	else
-		fl6.flowi6_oif = l3mdev_master_ifindex(dev);
 
 	fib6_rules_early_flow_dissect(net, skb, &fl6, &flkeys);
 	dst = ip6_route_output(net, sk, &fl6);