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853e164c2b
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: 415b3334a2 ("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>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260722104236.2938082-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
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