netdev - Re: [PATCH 2/2] net/tcp: Disable TCP-MD5 static key on tcp_md5sig

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Date:   Thu, 3 Nov 2022 16:53:38 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     linux-kernel@...r.kernel.org, David Ahern <dsahern@...nel.org>,
        Bob Gilligan <gilligan@...sta.com>,
        "David S. Miller" <davem@...emloft.net>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Francesco Ruggeri <fruggeri@...sta.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Salam Noureddine <noureddine@...sta.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] net/tcp: Disable TCP-MD5 static key on
 tcp_md5sig_info destruction

On 11/2/22 21:25, Eric Dumazet wrote:
> On Wed, Nov 2, 2022 at 2:14 PM Dmitry Safonov <dima@...sta.com> wrote:
[..]
>> @@ -337,11 +338,13 @@ EXPORT_SYMBOL(tcp_time_wait);
>>  void tcp_twsk_destructor(struct sock *sk)
>>  {
>>  #ifdef CONFIG_TCP_MD5SIG
>> -       if (static_branch_unlikely(&tcp_md5_needed)) {
>> +       if (static_branch_unlikely(&tcp_md5_needed.key)) {
>>                 struct tcp_timewait_sock *twsk = tcp_twsk(sk);
>>
>> -               if (twsk->tw_md5_key)
>> +               if (twsk->tw_md5_key) {
> 
> Orthogonal to this patch, but I wonder why we do not clear
> twsk->tw_md5_key before kfree_rcu()
> 
> It seems a lookup could catch the invalid pointer.
> 
>>                         kfree_rcu(twsk->tw_md5_key, rcu);
>> +                       static_branch_slow_dec_deferred(&tcp_md5_needed);
>> +               }
>>         }

I looked into that, it seems tcp_twsk_destructor() is called from
inet_twsk_free(), which is either called from:
1. inet_twsk_put(), protected by tw->tw_refcnt
2. sock_gen_put(), protected by the same sk->sk_refcnt

So, in result, if I understand correctly, lookups should fail on ref
counter check. Maybe I'm missing something, but clearing here seems not
necessary?

I can add rcu_assign_pointer() just in case the destruction path changes
in v2 if you think it's worth it :-)

Thanks,
          Dmitry