Re: [PATCH net] ipv4: icmp: reject non-unicast 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
> local, broadcast, multicast and unreachable addresses it still succeeds
> and returns an input route whose dst.output is set to ip_rt_bug().  The
> existing check only rejects RTN_LOCAL routes, so the other 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 ]------------
> 
> If the original source address is unroutable, ip_route_input() returns a
> RTN_UNREACHABLE input route with err == 0, and the ICMP reply is then sent
> through ip_rt_bug().
> 
> Only RTN_UNICAST input routes have dst.output set to ip_output() and are
> safe to use for output.  Reject any input route that is not RTN_UNICAST.
> 
> Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support")
> Signed-off-by: Dong Chenchen <[email protected]>
> ---
>   net/ipv4/icmp.c | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> index 0caedfc7ca92..d58ac2f23f9f 100644
> --- a/net/ipv4/icmp.c
> +++ b/net/ipv4/icmp.c
> @@ -584,12 +584,14 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4,
>   		 * 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.
> +		 * concurrently, or if fl4_dec.daddr is otherwise unroutable.
> +		 * In that case, ip_route_input() returns an input route
> +		 * (RTN_LOCAL, RTN_BROADCAST, RTN_UNREACHABLE or
> +		 * RTN_MULTICAST) with dst.output set to ip_rt_bug(), which
> +		 * must not be used for output.
>   		 */
> -		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_UNICAST) {
sorry, this judgement will reject bforward packet.
broadcast and multicast rt with ip_rt_bug has been reject after
icmp_route_lookup.We only need to add a judgment for RTN_UNREACHABLE
v2 will be sent.

best regards
Dong Chenchen> +			net_warn_ratelimited("detected unusable input route 
for %pI4 during ICMP sending, src %pI4\n",
>   					     &fl4_dec.daddr, &fl4_dec.saddr);
>   			dst_release(&rt2->dst);
>   			err = -EINVAL;