netdev - Re: [PATCH net] net/smc: fix panic smc_tcp_syn_recv

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Date: Mon, 25 Sep 2023 16:29:53 +0800
From: "D. Wythe" <alibuda@...ux.alibaba.com>
To: Wenjia Zhang <wenjia@...ux.ibm.com>, kgraul@...ux.ibm.com,
 jaka@...ux.ibm.com
Cc: kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
 linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net] net/smc: fix panic smc_tcp_syn_recv_sock() while
 closing listen socket



On 9/22/23 7:59 AM, Wenjia Zhang wrote:
>
>
> On 20.09.23 14:08, D. Wythe wrote:
>> From: "D. Wythe" <alibuda@...ux.alibaba.com>
>>
>> Consider the following scenarios:
>>
>> smc_release
>>     smc_close_active
>> write_lock_bh(&smc->clcsock->sk->sk_callback_lock);
>>         smc->clcsock->sk->sk_user_data = NULL;
>> write_unlock_bh(&smc->clcsock->sk->sk_callback_lock);
>>
>> smc_tcp_syn_recv_sock
>>     smc = smc_clcsock_user_data(sk);
>>     /* now */
>>     /* smc == NULL */
>>
>> Hence, we may read the a NULL value in smc_tcp_syn_recv_sock(). And
>> since we only unset sk_user_data during smc_release, it's safe to
>> drop the incoming tcp reqsock.
>>
>> Fixes:  ("net/smc: net/smc: Limit backlog connections"
>> Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com>
>> ---
>>   net/smc/af_smc.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
>> index bacdd97..b4acf47 100644
>> --- a/net/smc/af_smc.c
>> +++ b/net/smc/af_smc.c
>> @@ -125,6 +125,8 @@ static struct sock *smc_tcp_syn_recv_sock(const 
>> struct sock *sk,
>>       struct sock *child;
>>         smc = smc_clcsock_user_data(sk);
>> +    if (unlikely(!smc))
>> +        goto drop;
>>         if (READ_ONCE(sk->sk_ack_backlog) + 
>> atomic_read(&smc->queued_smc_hs) >
>>                   sk->sk_max_ack_backlog)

Hi Wenjia,

>
> this is unfortunately not sufficient for this fix. You have to make 
> sure that is not a life-time problem. Even so, READ_ONCE() is also 
> needed in this case.
>

Life-time problem? If you means the smc will still be NULL in the 
future,  I don't really think so, smc is a local variable assigned by 
smc_clcsock_user_data.
it's either NULL or a valid and unchanged value.

And READ_ONCE() is needed indeed, considering not make too much change, 
maybe we can protected following

smc = smc_clcsock_user_data(sk);

with sk_callback_lock， which solves the same problem. What do you think?

Best Wishes
D. Wythe