Re: [PATCH net v3] ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Aug 25, 2026 at 05:45:23PM +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.

Nit: Also mention that the 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")
> Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>
> ---
> v2:
> - RTN_UNICAST judgement will reject bforward packet. Only reject RTN_UNREACHABLE
> v3:
> - Modify the comments to make them more accurate.
> - RTN_UNREACHABLE dont need warn log. 
> ---
>  net/ipv4/icmp.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index 0caedfc7ca92..7350f0380be1 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -581,16 +581,15 @@ 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.
> +		 * At this point, fl4_dec.daddr should NOT be local (the
> +		 * address can be added to the interface concurrently) or unroutable.
> +		 * In that case, ip_route_input() returns a LOCAL or UNREACHABLE
> +		 * route with dst.output=ip_rt_bug, which must not be used for output.
>  		 */

The code is fine, but the comment is a bit misleading (the "should NOT"
part is only correct for "local"). I find this clearer:

"
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.
"

Thanks!