vti6: fix input path

Since commit 1625f4529957, vti6 is broken, all input packets are dropped
(LINUX_MIB_XFRMINNOSTATES is incremented).

XFRM_TUNNEL_SKB_CB(skb)->tunnel.ip6 is set by vti6_rcv() before calling
xfrm6_rcv()/xfrm6_rcv_spi(), thus we cannot set to NULL that value in
xfrm6_rcv_spi().

A new function xfrm6_rcv_tnl() that enables to pass a value to
xfrm6_rcv_spi() is added, so that xfrm6_rcv() is not touched (this function
is used in several handlers).

CC: Alexey Kodanev <[e-mail]>
Fixes: 1625f4529957 ("net/xfrm_input: fix possible NULL deref of tunnel.ip6->parms.i_key")
Signed-off-by: Nicolas Dichtel <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

include/net/xfrm.h

···
 void xfrm4_local_error(struct sk_buff *skb, u32 mtu);
 int xfrm6_extract_header(struct sk_buff *skb);
 int xfrm6_extract_input(struct xfrm_state *x, struct sk_buff *skb);
-int xfrm6_rcv_spi(struct sk_buff *skb, int nexthdr, __be32 spi);
+int xfrm6_rcv_spi(struct sk_buff *skb, int nexthdr, __be32 spi,
+		  struct ip6_tnl *t);
 int xfrm6_transport_finish(struct sk_buff *skb, int async);
+int xfrm6_rcv_tnl(struct sk_buff *skb, struct ip6_tnl *t);
 int xfrm6_rcv(struct sk_buff *skb);
 int xfrm6_input_addr(struct sk_buff *skb, xfrm_address_t *daddr,
 		     xfrm_address_t *saddr, u8 proto);

net/ipv6/ip6_vti.c

···
 			goto discard;
 		}
 
-		XFRM_TUNNEL_SKB_CB(skb)->tunnel.ip6 = t;
-
 		rcu_read_unlock();
 
-		return xfrm6_rcv(skb);
+		return xfrm6_rcv_tnl(skb, t);
 	}
 	rcu_read_unlock();
 	return -EINVAL;

net/ipv6/xfrm6_input.c

···
 	return xfrm6_extract_header(skb);
 }
 
-int xfrm6_rcv_spi(struct sk_buff *skb, int nexthdr, __be32 spi)
+int xfrm6_rcv_spi(struct sk_buff *skb, int nexthdr, __be32 spi,
+		  struct ip6_tnl *t)
 {
-	XFRM_TUNNEL_SKB_CB(skb)->tunnel.ip6 = NULL;
+	XFRM_TUNNEL_SKB_CB(skb)->tunnel.ip6 = t;
 	XFRM_SPI_SKB_CB(skb)->family = AF_INET6;
 	XFRM_SPI_SKB_CB(skb)->daddroff = offsetof(struct ipv6hdr, daddr);
 	return xfrm_input(skb, nexthdr, spi, 0);
···
 	return -1;
 }
 
-int xfrm6_rcv(struct sk_buff *skb)
+int xfrm6_rcv_tnl(struct sk_buff *skb, struct ip6_tnl *t)
 {
 	return xfrm6_rcv_spi(skb, skb_network_header(skb)[IP6CB(skb)->nhoff],
-			     0);
+			     0, t);
+}
+EXPORT_SYMBOL(xfrm6_rcv_tnl);
+
+int xfrm6_rcv(struct sk_buff *skb)
+{
+	return xfrm6_rcv_tnl(skb, NULL);
 }
 EXPORT_SYMBOL(xfrm6_rcv);
-
 int xfrm6_input_addr(struct sk_buff *skb, xfrm_address_t *daddr,
 		     xfrm_address_t *saddr, u8 proto)
 {

net/ipv6/xfrm6_tunnel.c

···
 	__be32 spi;
 
 	spi = xfrm6_tunnel_spi_lookup(net, (const xfrm_address_t *)&iph->saddr);
-	return xfrm6_rcv_spi(skb, IPPROTO_IPV6, spi);
+	return xfrm6_rcv_spi(skb, IPPROTO_IPV6, spi, NULL);
 }
 
 static int xfrm6_tunnel_err(struct sk_buff *skb, struct inet6_skb_parm *opt,