vti[6]: fix packet tx through bpf_redirect()

With an ebpf program that redirects packets through a vti[6] interface,
the packets are dropped because no dst is attached.

This could also be reproduced with an AF_PACKET socket, with the following
python script (vti1 is an ip_vti interface):

 import socket
 send_s = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, 0)
 # scapy
 # p = IP(src='10.100.0.2', dst='10.200.0.1')/ICMP(type='echo-request')
 # raw(p)
 req = b'E\x00\x00\x1c\x00\x01\x00\x00@\x01e\xb2\nd\x00\x02\n\xc8\x00\x01\x08\x00\xf7\xff\x00\x00\x00\x00'
 send_s.sendto(req, ('vti1', 0x800, 0, 0))
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/ip_vti.c

@@ -187,8 +187,17 @@ static netdev_tx_t vti_xmit(struct sk_buff *skb, struct net_device *dev,
 	int mtu;
 
 	if (!dst) {
-		dev->stats.tx_carrier_errors++;
-		goto tx_error_icmp;
+		struct rtable *rt;
+
+		fl->u.ip4.flowi4_oif = dev->ifindex;
+		fl->u.ip4.flowi4_flags |= FLOWI_FLAG_ANYSRC;
+		rt = __ip_route_output_key(dev_net(dev), &fl->u.ip4);
+		if (IS_ERR(rt)) {
+			dev->stats.tx_carrier_errors++;
+			goto tx_error_icmp;
+		}
+		dst = &rt->dst;
+		skb_dst_set(skb, dst);
 	}
 
 	dst_hold(dst);

net/ipv6/ip6_vti.c

@@ -449,8 +449,17 @@ vti6_xmit(struct sk_buff *skb, struct net_device *dev, struct flowi *fl)
 	int err = -1;
 	int mtu;
 
-	if (!dst)
-		goto tx_err_link_failure;
+	if (!dst) {
+		fl->u.ip6.flowi6_oif = dev->ifindex;
+		fl->u.ip6.flowi6_flags |= FLOWI_FLAG_ANYSRC;
+		dst = ip6_route_output(dev_net(dev), NULL, &fl->u.ip6);
+		if (dst->error) {
+			dst_release(dst);
+			dst = NULL;
+			goto tx_err_link_failure;
+		}
+		skb_dst_set(skb, dst);
+	}
 
 	dst_hold(dst);
 	dst = xfrm_lookup(t->net, dst, fl, NULL, 0);