bpf: lwtunnel: Fix reroute supplying invalid dst

The dst in bpf_input() has lwtstate field set. As it is of the
LWTUNNEL_ENCAP_BPF type, lwtstate->data is struct bpf_lwt. When the bpf
program returns BPF_LWT_REROUTE, ip_route_input_noref is directly called on
this skb. This causes invalid memory access, as ip_route_input_slow calls
skb_tunnel_info(skb) that expects the dst->lwstate->data to be
struct ip_tunnel_info. This results to struct bpf_lwt being accessed as
struct ip_tunnel_info.

Drop the dst before calling the IP route input functions (both for IPv4 and
IPv6).

Reported by KASAN.

Fixes: 3bd0b15 ("bpf: add handling of BPF_LWT_REROUTE to lwt_bpf.c")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Peter Oskolkov <posk@google.com>
Link: https://lore.kernel.org/bpf/111664d58fe4e9dd9c8014bb3d0b2dab93086a9e.1570609794.git.jbenc@redhat.com

net/core/lwt_bpf.c

@@ -88,11 +88,16 @@ static int bpf_lwt_input_reroute(struct sk_buff *skb)
 	int err = -EINVAL;
 
 	if (skb->protocol == htons(ETH_P_IP)) {
+		struct net_device *dev = skb_dst(skb)->dev;
 		struct iphdr *iph = ip_hdr(skb);
 
+		dev_hold(dev);
+		skb_dst_drop(skb);
 		err = ip_route_input_noref(skb, iph->daddr, iph->saddr,
-					   iph->tos, skb_dst(skb)->dev);
+					   iph->tos, dev);
+		dev_put(dev);
 	} else if (skb->protocol == htons(ETH_P_IPV6)) {
+		skb_dst_drop(skb);
 		err = ipv6_stub->ipv6_route_input(skb);
 	} else {
 		err = -EAFNOSUPPORT;