[PATCH net v5] ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup

When the forward output route cannot be used in icmp_route_lookup(),
it enters the "reverse path" and calls ip_route_input() on fl4_dec.daddr,
the original packet's source address.

ip_route_input() only returns an error for truly invalid packets. For
unreachable addresses it will succeed and return an input route whose
dst.output is set to ip_rt_bug(). The existing check only rejects
RTN_LOCAL routes, so the RTN_UNREACHABLE route types can still be returned
and later used for output, syzkaller triggering a WARN_ON_ONCE()
in ip_rt_bug() as bellow:

 ------------[ cut here ]------------
 WARNING: net/ipv4/route.c:1273 at ip_rt_bug+0x14/0x20
 RIP: 0010:ip_rt_bug+0x14/0x20
 Call Trace:
  ip_push_pending_frames+0xfa/0x100
  __icmp_send+0x905/0xf10
  ip_options_compile+0xc0/0xd0
  ip_rcv_finish_core+0x321/0xae0
  ip_rcv+0x1de/0x260
  __netif_receive_skb_one_core+0x11a/0x130
  netif_receive_skb+0x7b/0x260
  tun_get_user+0x11bf/0x1c10
 ------------[ cut here ]------------

Reject input route that is RTN_UNREACHABLE to fix it. The net warning
is only printed for RTN_LOCAL, as RTN_UNREACHABLE is not the result of
a race condition.

Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support")
Suggested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Dong Chenchen <redacted>
---
v2:
- RTN_UNICAST judgement will reject bforward packet. Only reject RTN_UNREACHABLE
v3:
- RTN_UNREACHABLE dont need warn log. 
v4:
- Modify the comments and commit msg to make them more accurate.
v5:
- avoid going over 80 chars
---
 net/ipv4/icmp.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 0caedfc7ca92..90c0e22c29be 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c

@@ -581,16 +581,19 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4,
 		skb_dstref_restore(skb_in, orefdst);
 
 		/*
-		 * At this point, fl4_dec.daddr should NOT be local (we
-		 * checked fl4_dec.saddr above). However, a race condition
-		 * may occur if the address is added to the interface
-		 * concurrently. In that case, ip_route_input() returns a
-		 * LOCAL route with dst.output=ip_rt_bug, which must not
-		 * be used for output.
+		 * fl4_dec.daddr is not expected to be local here, but it can be
+		 * added to an interface concurrently, in which case
+		 * ip_route_input() returns a LOCAL route. It can also fail to
+		 * build a forwarding route towards fl4_dec.daddr, for example,
+		 * when forwarding is disabled, and return an UNREACHABLE route.
+		 * Both cases will result in a route with dst.output=ip_rt_bug,
+		 * which must not be used for output.
 		 */
-		if (!err && rt2 && rt2->rt_type == RTN_LOCAL) {
+		if (!err && rt2 && rt2->rt_type == RTN_LOCAL)
 			net_warn_ratelimited("detected local route for %pI4 during ICMP sending, src %pI4\n",
 					     &fl4_dec.daddr, &fl4_dec.saddr);
+		if (!err && rt2 &&
+		    (rt2->rt_type == RTN_LOCAL || rt2->rt_type == RTN_UNREACHABLE)) {
 			dst_release(&rt2->dst);
 			err = -EINVAL;
 		}

-- 
2.25.1

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni [off-list ref]:

On Thu, 10 Sep 2026 22:00:42 +0800 you wrote:

When the forward output route cannot be used in icmp_route_lookup(),
it enters the "reverse path" and calls ip_route_input() on fl4_dec.daddr,
the original packet's source address.

ip_route_input() only returns an error for truly invalid packets. For
unreachable addresses it will succeed and return an input route whose
dst.output is set to ip_rt_bug(). The existing check only rejects
RTN_LOCAL routes, so the RTN_UNREACHABLE route types can still be returned
and later used for output, syzkaller triggering a WARN_ON_ONCE()
in ip_rt_bug() as bellow:

[...]

Here is the summary with links:
  - [net,v5] ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup
    https://git.kernel.org/netdev/net/c/2998147b59c9

You are awesome, thank you!
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