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

[YOSHIFUJI Hideaki/åèèæ]

Hi,

Chunhui He wrote:
> 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.

IPv6 depends on WEAK_OVERRIDE.  Please do not change.

> 
>> 	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
> 

-- 
åèèæ <hideaki.yoshifuji@xxxxxxxxxxxxxxxx>
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