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

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Date:   Thu, 3 Nov 2022 10:04:19 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Safonov <dima@...sta.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 Thu, Nov 3, 2022 at 9:53 AM Dmitry Safonov <dima@...sta.com> wrote:
>
> 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 :-)

Agree, this seems fine.