Re: [PATCH] net: neigh: disallow state transition DELAY->STALE in neigh_update()

[Chunhui He]

Hi,

On Fri, 22 Jul 2016 10:20:01 +0300 (EEST), Julian Anastasov <ja@xxxxxx> wrote:
> 
> 	Hello,
> 
> On Thu, 21 Jul 2016, Chunhui He wrote:
> 
>> If neigh entry was CONNECTED and address is not changed, and if new state is
>> STALE, entry state will not change. Because DELAY is not in CONNECTED, it's
>> possible to change state from DELAY to STALE.
>> 
>> That is bad. Consider a host in IPv4 nerwork, a neigh entry in STALE state
>> is referenced to send packets, so goes to DELAY state. If the entry is not
>> confirmed by upper layer, it goes to PROBE state, and sends ARP request.
>> The neigh host sends ARP reply, then the entry goes to REACHABLE state.
>> But the entry state may be reseted to STALE by broadcast ARP packets, before
>> the entry goes to PROBE state. So it's possible that the entry will never go
>> to REACHABLE state, without external confirmation.
>> 
>> In my case, the gateway refuses to send unicast packets to me, before it sees
>> my ARP request. So it's critical to enter REACHABLE state by sending ARP
>> request, but not by external confirmation.
>> 
>> This fixes neigh_update() not to change to STALE if old state is CONNECTED or
>> DELAY.
>> 
>> Signed-off-by: Chunhui He <hchunhui@xxxxxxxxxxxxxxxx>
>> ---
>>  net/core/neighbour.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
>> index 510cd62..29429eb 100644
>> --- a/net/core/neighbour.c
>> +++ b/net/core/neighbour.c
>> @@ -1152,7 +1152,7 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
>>  		} else {
>>  			if (lladdr == neigh->ha && new == NUD_STALE &&
>>  			    ((flags & NEIGH_UPDATE_F_WEAK_OVERRIDE) ||
>> -			     (old & NUD_CONNECTED))
>> +			     (old & (NUD_CONNECTED | NUD_DELAY)))
>>  			    )
>>  				new = old;
>>  		}
> 
> 	You change looks correct to me. But this place
> has more problems. There is no good reason to set NUD_STALE
> for any state that is NUD_VALID if address is not changed.
> This matches perfectly the comment above this code:
> NUD_STALE should change a NUD_VALID state only when
> address changes. It also means that IPv6 does not need
> to provide NEIGH_UPDATE_F_WEAK_OVERRIDE anymore when
> NEIGH_UPDATE_F_OVERRIDE is also present.
>

The NEIGH_UPDATE_F_WEAK_OVERRIDE is confusing to me, so I choose not to deal
with the flag.

> 	By this way the state machine can continue with
> the resolving: NUD_STALE -> NUD_DELAY (traffic) ->
> NUD_PROBE (retries) -> NUD_REACHABLE (unicast reply)
> while the address is not changed. Your change covers only
> NUD_DELAY, not NUD_PROBE, so it is better to allow more
> retries to send. We should not give up until success (NUD_REACHABLE).
>

I have thought about this.
The origin code allows NUD_DELAY -> NUD_STALE and NUD_PROBE -> NUD_STALE.
This part was imported to kernel since v2.1.79, I don't know clearly why it
allows that.

My analysis:
(1) As shown in my previous mail, NUD_DELAY -> NUD_STALE may cause "dead loop",
so it should be fixed.

(2) But NUD_PROBE -> NUD_STALE is acceptable, because in NUD_PROBE, ARP request
has been sent, it is sufficient to break the "dead loop".
More attempts are accomplished by the following sequence:
NUD_STALE --> NUD_DELAY -(sent req)-> NUD_PROBE -(reset by neigh_update())->
NUD_STALE --> NUD_DELAY -(send req again)-> ... -->
NUD_REACHABLE


But I also agree your change.

> 	Second problem: NEIGH_UPDATE_F_WEAK_OVERRIDE has no
> priority over NEIGH_UPDATE_F_ADMIN. For example, now I can not
> change from NUD_PERMANENT to NUD_STALE:
>
> # ip neigh add 192.168.168.111 lladdr 00:11:22:33:44:55 nud perm dev wlan0
> # ip neigh show to 192.168.168.111
> 192.168.168.111 dev wlan0 lladdr 00:11:22:33:44:55 PERMANENT
> # ip neigh change 192.168.168.111 lladdr 00:11:22:33:44:55 nud stale dev wlan0
> # ip neigh show to 192.168.168.111
> 192.168.168.111 dev wlan0 lladdr 00:11:22:33:44:55 PERMANENT
> 
> 	IMHO, here is how this place should look:
> 
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
> index 5cdc62a..2b1cb91 100644
> --- a/net/core/neighbour.c
> +++ b/net/core/neighbour.c
> @@ -1151,10 +1151,8 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
>  				goto out;
>  		} else {
>  			if (lladdr == neigh->ha && new == NUD_STALE &&
> -			    ((flags & NEIGH_UPDATE_F_WEAK_OVERRIDE) ||
> -			     (old & NUD_CONNECTED))
> -			    )
> -				new = old;
> +			    !(flags & NEIGH_UPDATE_F_ADMIN))
> +				goto out;
>  		}
>  	}
> 
> 	Any thoughts?
>  
> Regards
> 
> --
> Julian Anastasov <ja@xxxxxx>

Regards,
Chunhui He