Re: [PATCH net v2] ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup

On Sat, Aug 15, 2026 at 03:51:02PM +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 any input route that is RTN_UNREACHABLE to fix it.
> 
> Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support")
> Signed-off-by: Dong Chenchen <[email protected]>

FYI, there's a new policy regarding AI review:

"
Patch authors are expected to proactively look into the AI-generated
reviews and handle such feedback as any other kind of review: either
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"

https://docs.kernel.org/next/process/maintainer-netdev.html#review-timelines

> ---
>  net/ipv4/icmp.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index 0caedfc7ca92..4703d8dd6b47 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -585,11 +585,11 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4,
>  		 * 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.
> +		 * LOCAL or UNREACHABLE route with dst.output=ip_rt_bug,
> +		 * which must not be used for output.
>  		 */

Why would the addition of an address lead to RTN_UNREACHABLE? This seems
to be true for RTN_LOCAL, but not for RTN_UNREACHABLE. Similarly, it
doesn't seem like RTN_UNREACHABLE is the result of a race condition.

The comment needs to be reworded to reflect that.

> -		if (!err && rt2 && rt2->rt_type == RTN_LOCAL) {
> -			net_warn_ratelimited("detected local route for %pI4 during ICMP sending, src %pI4\n",
> +		if (!err && rt2 && (rt2->rt_type == RTN_LOCAL || rt2->rt_type == RTN_UNREACHABLE)) {
> +			net_warn_ratelimited("detected unusable input route for %pI4 during ICMP sending, src %pI4\n",
>  					     &fl4_dec.daddr, &fl4_dec.saddr);

The check is fine, but if RTN_UNREACHABLE is not the result of a race
condition, then net_warn_ratelimited() should only be emitted for
RTN_LOCAL.

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