icmp: Fix regression in nexthop resolution during replies.

icmp_route_lookup() uses the wrong flow parameters if the reverse
session route lookup isn't used.

So do not commit to the re-decoded flow until we actually make a
final decision to use a real route saved in 'rt2'.
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/icmp.c

@@ -380,6 +380,7 @@ static struct rtable *icmp_route_lookup(struct net *net,
 					struct icmp_bxm *param)
 {
 	struct rtable *rt, *rt2;
+	struct flowi4 fl4_dec;
 	int err;
 
 	memset(fl4, 0, sizeof(*fl4));
@@ -408,19 +409,19 @@ static struct rtable *icmp_route_lookup(struct net *net,
 	} else
 		return rt;
 
-	err = xfrm_decode_session_reverse(skb_in, flowi4_to_flowi(fl4), AF_INET);
+	err = xfrm_decode_session_reverse(skb_in, flowi4_to_flowi(&fl4_dec), AF_INET);
 	if (err)
 		goto relookup_failed;
 
-	if (inet_addr_type(net, fl4->saddr) == RTN_LOCAL) {
-		rt2 = __ip_route_output_key(net, fl4);
+	if (inet_addr_type(net, fl4_dec.saddr) == RTN_LOCAL) {
+		rt2 = __ip_route_output_key(net, &fl4_dec);
 		if (IS_ERR(rt2))
 			err = PTR_ERR(rt2);
 	} else {
 		struct flowi4 fl4_2 = {};
 		unsigned long orefdst;
 
-		fl4_2.daddr = fl4->saddr;
+		fl4_2.daddr = fl4_dec.saddr;
 		rt2 = ip_route_output_key(net, &fl4_2);
 		if (IS_ERR(rt2)) {
 			err = PTR_ERR(rt2);
@@ -428,7 +429,7 @@ static struct rtable *icmp_route_lookup(struct net *net,
 		}
 		/* Ugh! */
 		orefdst = skb_in->_skb_refdst; /* save old refdst */
-		err = ip_route_input(skb_in, fl4->daddr, fl4->saddr,
+		err = ip_route_input(skb_in, fl4_dec.daddr, fl4_dec.saddr,
 				     RT_TOS(tos), rt2->dst.dev);
 
 		dst_release(&rt2->dst);
@@ -440,10 +441,11 @@ static struct rtable *icmp_route_lookup(struct net *net,
 		goto relookup_failed;
 
 	rt2 = (struct rtable *) xfrm_lookup(net, &rt2->dst,
-					    flowi4_to_flowi(fl4), NULL,
+					    flowi4_to_flowi(&fl4_dec), NULL,
 					    XFRM_LOOKUP_ICMP);
 	if (!IS_ERR(rt2)) {
 		dst_release(&rt->dst);
+		memcpy(fl4, &fl4_dec, sizeof(*fl4));
 		rt = rt2;
 	} else if (PTR_ERR(rt2) == -EPERM) {
 		if (rt)