netdev - Re: [PATCH net-next 1/1] net/ipv4: disable SMC TCP option with SYN Cookies

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Date:   Tue, 20 Mar 2018 09:43:01 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Ursula Braun <ubraun@...ux.vnet.ibm.com>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, linux-s390@...r.kernel.org,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        raspl@...ux.vnet.ibm.com
Subject: Re: [PATCH net-next 1/1] net/ipv4: disable SMC TCP option with SYN
 Cookies



On 03/20/2018 09:21 AM, Eric Dumazet wrote:
> 
> 
> On 03/20/2018 08:53 AM, Ursula Braun wrote:
>> From: Hans Wippel <hwippel@...ux.vnet.ibm.com>
>>
>> Currently, the SMC experimental TCP option in a SYN packet is lost on
>> the server side when SYN Cookies are active. However, the corresponding
>> SYNACK sent back to the client contains the SMC option. This causes an
>> inconsistent view of the SMC capabilities on the client and server.
>>
>> This patch disables the SMC option in the SYNACK when SYN Cookies are
>> active to avoid this issue.
>>
>> Signed-off-by: Hans Wippel <hwippel@...ux.vnet.ibm.com>
>> Signed-off-by: Ursula Braun <ubraun@...ux.vnet.ibm.com>
>> ---
>>  net/ipv4/tcp_output.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>> index 383cac0ff0ec..22894514feae 100644
>> --- a/net/ipv4/tcp_output.c
>> +++ b/net/ipv4/tcp_output.c
>> @@ -3199,6 +3199,8 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
>>  		/* Under synflood, we do not attach skb to a socket,
>>  		 * to avoid false sharing.
>>  		 */
>> +		if (IS_ENABLED(CONFIG_SMC))
>> +			ireq->smc_ok = 0;
>>  		break;
>>  	case TCP_SYNACK_FASTOPEN:
>>  		/* sk is a const pointer, because we want to express multiple
>>
> 
> I disagree with net-next qualification.
>
> This fixes a bug, so please send it for net tree, and including an appropriate Fixes: tag.
>

Also, please do not add the fix in tcp_make_synack()

tcp_make_synack() builds an skb, and really should not modify ireq, ideally.
The only reason ireq is not const is because of the skb_set_owner_w().

I would clear it in cookie_v4_check()/cookie_v6_check()

(We could have a common helper to allocate a TCP ireq btw, but this will wait a future patch for net-next)