[PATCH v2 net] ipv4: icmp: fill flow parameters in icmp_route_lookup decoy lookup

[Eric Dumazet <edumazet@google.com>]

When Linux forwards a packet and needs to generate an ICMP error,
icmp_route_lookup() performs a reverse-path relookup. For non-local
destinations, it performs a decoy lookup to find the expected egress
interface (rt2->dst.dev) before validating the path with ip_route_input().

Currently, the decoy flow structure (fl4_2) only sets .daddr = fl4_dec.saddr,
leaving .saddr, .flowi4_dscp, .flowi4_proto, .flowi4_mark, .flowi4_oif,
.fl4_sport, .fl4_dport, and .flowi4_uid zeroed out.

When policy routing rules (such as ip rule add from $SRC lookup 100, or
dscp/fwmark/ipproto/port rules, or VRF bindings) are configured:
1. The decoy lookup fails to match the policy rule because saddr and other
   key flow selectors are missing in fl4_2.
2. It resolves a route using the default table instead, returning an incorrect
   egress netdev.
3. Passing the wrong netdev to ip_route_input() causes strict reverse-path
   filtering (rp_filter=1) to fail, logging false-positive "martian source"
   warnings and causing the relookup to fail.

Fix this by initializing fl4_2 from fl4_dec and:
- Swapping source/destination IP addresses.
- Swapping L4 ports for transport protocols with ports (TCP, UDP, SCTP, DCCP)
  so port-based policy routing matches correctly. Non-port protocols (such as
  ICMP or GRE) leave the flowi_uli union fields intact to prevent corruption.
- Setting .flowi4_oif = l3mdev_master_ifindex(route_lookup_dev) to ensure
  VRF routing tables are respected.
- Setting .flowi4_flags |= FLOWI_FLAG_ANYSRC to allow output route lookups
  for non-local source IP addresses.
- Using __ip_route_output_key() instead of ip_route_output_key() for fl4_2
  so that raw FIB routing is used without triggering spurious XFRM policy
  lookups on the decoy flow (the actual XFRM lookup is performed later using
  fl4_dec).

Fixes: 415b3334a21a ("icmp: Fix regression in nexthop resolution during replies.")
Reported-by: Muhammad Ziad <muhzi100@gmail.com>
Closes: https://lore.kernel.org/netdev/CAOAwikA60AYKdFr_UDLyja3oU4hqyAE7uFZWqum5uRdaQsgRYg@mail.gmail.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
v2: addressed Sashiko and Jakub feedback
v1: https://lore.kernel.org/netdev/20260716021049.2124921-1-edumazet@google.com/

 net/ipv4/icmp.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 23e921d313b36b00d8ae5e14846527220c9db32b..0caedfc7ca92f64d146c11c552e73650541b508c 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -548,11 +548,23 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4,
 		if (IS_ERR(rt2))
 			err = PTR_ERR(rt2);
 	} else {
-		struct flowi4 fl4_2 = {};
+		struct flowi4 fl4_2 = fl4_dec;
 		unsigned long orefdst;
 
-		fl4_2.daddr = fl4_dec.saddr;
-		rt2 = ip_route_output_key(net, &fl4_2);
+		swap(fl4_2.daddr, fl4_2.saddr);
+		switch (fl4_2.flowi4_proto) {
+		case IPPROTO_TCP:
+		case IPPROTO_UDP:
+		case IPPROTO_SCTP:
+		case IPPROTO_DCCP:
+			swap(fl4_2.fl4_sport, fl4_2.fl4_dport);
+			break;
+		}
+
+		fl4_2.flowi4_oif = l3mdev_master_ifindex(route_lookup_dev);
+		fl4_2.flowi4_flags |= FLOWI_FLAG_ANYSRC;
+
+		rt2 = __ip_route_output_key(net, &fl4_2);
 		if (IS_ERR(rt2)) {
 			err = PTR_ERR(rt2);
 			goto relookup_failed;
-- 
2.55.0.229.g6434b31f56-goog