net: ipv4: fix multipath RTM_GETROUTE behavior when iif is given

[ Upstream commit a8801799c6975601fd58ae62f48964caec2eb83f ]

inet_rtm_getroute synthesizes a skeletal ICMP skb, which is passed to
ip_route_input when iif is given. If a multipath route is present for
the designated destination, ip_multipath_icmp_hash ends up being called,
which uses the source/destination addresses within the skb to calculate
a hash. However, those are not set in the synthetic skb, causing it to
return an arbitrary and incorrect result.

Instead, use UDP, which gets no such special treatment.

Signed-off-by: Florian Larysch <fl@n621.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: florolf

net/ipv4/route.c

@@ -2559,7 +2559,7 @@ static int inet_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr *nlh)
 	skb_reset_network_header(skb);
 
 	/* Bugfix: need to give ip_route_input enough of an IP header to not gag. */
-	ip_hdr(skb)->protocol = IPPROTO_ICMP;
+	ip_hdr(skb)->protocol = IPPROTO_UDP;
 	skb_reserve(skb, MAX_HEADER + sizeof(struct iphdr));
 
 	src = tb[RTA_SRC] ? nla_get_in_addr(tb[RTA_SRC]) : 0;