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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 8/25/26 5:45 PM, 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.
>
> Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support")
> Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>


Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>