Re: ipv4: icmp: icmp_route_lookup() relookups pick wrong netdev with policy routing + strict rp_filter

[Eric Dumazet]

On Wed, Jul 15, 2026 at 5:43 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Wed, Jul 15, 2026 at 5:30 PM Muhammad Ziad <muhzi100@xxxxxxxxx> wrote:
> >
> > Hello,
> >
> > There appears to be a bug in mainline Linux in ICMP reverse-path
> > relookup logic inside icmp_route_lookup() (called by __icmp_send())
> > when strict rp_filter setting is in place.
> >
> > When Linux forwards a packet between two interfaces and needs to
> > generate an ICMP error, icmp_route_lookup() performs a "secondary"
> > reverse-path lookup to find a suitable route back towards the original
> > source via ip_route_input(). To simulate the reverse path, the kernel
> > derives the incoming netdev by calling ip_route_output_key() with a
> > decoy flow that has *only* daddr assigned in it:
> >
> >     struct flowi4 fl4_2 = {};
> >     fl4_2.daddr = fl4_dec.saddr;
> >     rt2 = ip_route_output_key(net, &fl4_2); /* no saddr */
> >     ...
> >     ip_route_input(skb_in, fl4_dec.daddr, fl4_dec.saddr,
> >                     dscp, rt2->dst.dev);
> >
> > This can lead to a mismatch between rt2->dst.dev and the netdev the
> > real reverse packet would use once routing policy rules are in
> > effect. With strict rp_filter, passing the wrong netdev to
> > ip_route_input() causes the relookup to fail and a "martian source"
> > message to be logged, after which icmp_route_lookup() falls back to
> > the earlier output route lookup (relookup_failed).
> >
> > In such a scenario, I would expect the relookup to use a canonical
> > netdev and the kernel to not produce spurious "martian source" log
> > messages as a result. This suggests the decoy flow would possibly
> > need to carry saddr too so that ip_route_output_key() is able to
> > resolve the right netdev.
> >
> > I tested this on: Ubuntu kernel 6.17.0-35-generic.
> >
> > Here is a reproducer script that sets up two net namespaces: a
> > "forwarder" with two routes to the same dst in separate routing tables
> > picked according to saddr, and a "sender" netns behind it which sends
> > a ping with ttl=1 via the forwarder forcing it to generate an ICMP
> > error as a response which leads to the result explained above:
> >
> >     #!/bin/bash
> >
> >     if [ "${forwarder_ns:-}" != "1" ]; then
> >     exec env forwarder_ns=1 unshare -Urn bash "$0" "$@"
> >     fi
> >
> >     SRC=10.0.1.2
> >     DST=198.51.100.5
> >
> >     # Current netns is the "forwarder".
> >     # Create a second namespace for the sender.
> >     unshare -n sleep 120 &
> >     cpid=$!
> >     trap 'kill "$cpid" 2>/dev/null || true' EXIT
> >     in_ns() { nsenter -t "$cpid" -n "$@"; }
> >
> >     # veth r0(router) <-> s0(src)
> >     ip link add s0 type veth peer name r0
> >     ip link set s0 netns "$cpid"
> >     ip link set lo up
> >     ip link set r0 up
> >     ip addr add 10.0.1.1/24 dev r0
> >     ip link add dumA type dummy
> >     ip addr add 203.0.113.1/24 dev dumA
> >     ip link set dumA up
> >     ip link add dumB type dummy
> >     ip addr add 192.0.2.1/24 dev dumB
> >     ip link set dumB up
> >
> >     sysctl -q -w net.ipv4.ip_forward=1
> >     for c in all default r0 dumA dumB; do
> >     sysctl -q -w "net.ipv4.conf.$c.rp_filter=1"
> >     sysctl -q -w "net.ipv4.conf.$c.log_martians=1"
> >     done
> >
> >     # Destination reachable two ways.
> >     # Policy rule diverts traffic FROM src to dumB
> >     ip route add 198.51.100.0/24 dev dumA
> >     ip route add 198.51.100.0/24 dev dumB table 100
> >     ip rule add from "$SRC" lookup 100
> >
> >     # sender namespace setup.
> >     in_ns ip link set lo up
> >     in_ns ip link set s0 up
> >     in_ns ip addr add 10.0.1.2/24 dev s0
> >     in_ns ip route add default via 10.0.1.1
> >
> >     # This will trigger a "martian source" log.
> >     in_ns ping -q -c1 -W2 -t1 "$DST" &>/dev/null
> >
> >
> > Happy to test patches or provide additional traces, if needed.
> >
> > Thank you,
> > Mohamed Ghazy
>
> Thanks for the report.
>
> It seems we are lacking more than saddr setting :/
>
> Could you test
>
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index 23e921d313b36b00d8ae5e14846527220c9db32b..6277e1bf85f304678ff167e8fd2b9239f155ce42
> 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -548,10 +548,17 @@ 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 = {};
> +               /* TODO: populate
> +                       .flowi4_dscp = dscp,
> +                       .flowi4_mark = mark,
> +                       .flowi4_uid = sock_net_uid(net, NULL),
> +               */
> +               struct flowi4 fl4_2 = {
> +                       .daddr = fl4_dec.saddr,
> +                       .saddr = fl4_dec.daddr,
> +               };
>                 unsigned long orefdst;
>
> -               fl4_2.daddr = fl4_dec.saddr;
>                 rt2 = ip_route_output_key(net, &fl4_2);
>                 if (IS_ERR(rt2)) {
>                         err = PTR_ERR(rt2);

I have tested the following fix, PTAL, thanks.

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 23e921d313b36b00d8ae5e14846527220c9db32b..2259f643cb0d43ea54819dd831c79807413e0d69
100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -548,10 +548,16 @@ 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 = {
+                       .daddr = fl4_dec.saddr,
+                       .saddr = fl4_dec.daddr,
+                       .flowi4_dscp = dscp,
+                       .flowi4_proto = IPPROTO_ICMP,
+                       .flowi4_mark = mark,
+                       .flowi4_uid = sock_net_uid(net, NULL),
+               };
                unsigned long orefdst;

-               fl4_2.daddr = fl4_dec.saddr;
                rt2 = ip_route_output_key(net, &fl4_2);
                if (IS_ERR(rt2)) {
                        err = PTR_ERR(rt2);