Re: [PATCH net v4] ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup

[Ido Schimmel <idosch@nvidia.com>]

On Sat, Aug 29, 2026 at 05:34:00PM +0800, Dong Chenchen 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:
> 
>  ------------[ 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>
> Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

Regarding [1], no need to filter for RTN_BROADCAST because it is handled
later on:

	rt = icmp_route_lookup(net, &fl4, skb_in, iph, saddr,
			       inet_dsfield_to_dscp(tos), mark, type, code,
			       icmp_param);
	if (IS_ERR(rt))
		goto out_unlock;

	if (rt->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
		goto ende;

[1] https://sashiko.dev/#/patchset/20260829093400.1531398-1-dongchenchen2%40huawei.com

> ---
> 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.
> ---
>  net/ipv4/icmp.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index 0caedfc7ca92..288da11c5de8 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -581,16 +581,17 @@ 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.

You should have wrapped it to avoid going over 80 chars.

>  		 */
> -		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)) {

Same.

NIPA is using --max-line-length=80:

https://github.com/linux-netdev/nipa/blob/main/tests/patch/checkpatch/checkpatch.sh#L21
https://github.com/linux-netdev/nipa/tree/main#running-locally

> +			if (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);

Also here. No need to split the format string itself.

>  			dst_release(&rt2->dst);
>  			err = -EINVAL;
>  		}
> -- 
> 2.25.1
>