netdev - Re: [PATCH net-next] tcp: syncookies: extend validity range

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Date:	Thu, 14 May 2015 17:12:54 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, Florian Westphal <fw@...len.de>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] tcp: syncookies: extend validity range

On Thu, May 14, 2015 at 2:26 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> Now we allow storing more request socks per listener, we might
> hit syncookie mode less often and hit following bug in our stack :
>
> When we send a burst of syncookies, then exit this mode,
> tcp_synq_no_recent_overflow() can return false if the ACK packets coming
> from clients are coming three seconds after the end of syncookie
> episode.
>
> This is a way too strong requirement and conflicts with rest of
> syncookie code which allows ACK to be aged up to 2 minutes.
>
> Perfectly valid ACK packets are dropped just because clients might be
> in a crowded wifi environment or on another planet.
>
> So let's fix this, and also change tcp_synq_overflow() to not
> dirty a cache line for every syncookie we send, as we are under attack.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Yuchung Cheng <ycheng@...gle.com>

Hopefully this reduces phony LINUX_MIB_SYNCOOKIESFAILED counts!

> ---
> As this is an old bug, I chose net-next tree.
>
>  include/net/tcp.h |   38 ++++++++++++++++++++++++--------------
>  1 file changed, 24 insertions(+), 14 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index b8ea12880fd9..7ace6acbf5fd 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -326,18 +326,6 @@ static inline bool tcp_too_many_orphans(struct sock *sk, int shift)
>
>  bool tcp_check_oom(struct sock *sk, int shift);
>
> -/* syncookies: remember time of last synqueue overflow */
> -static inline void tcp_synq_overflow(struct sock *sk)
> -{
> -       tcp_sk(sk)->rx_opt.ts_recent_stamp = jiffies;
> -}
> -
> -/* syncookies: no recent synqueue overflow on this listening socket? */
> -static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
> -{
> -       unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
> -       return time_after(jiffies, last_overflow + TCP_TIMEOUT_FALLBACK);
> -}
>
>  extern struct proto tcp_prot;
>
> @@ -483,13 +471,35 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb);
>   * i.e. a sent cookie is valid only at most for 2*60 seconds (or less if
>   * the counter advances immediately after a cookie is generated).
>   */
> -#define MAX_SYNCOOKIE_AGE 2
> +#define MAX_SYNCOOKIE_AGE      2
> +#define TCP_SYNCOOKIE_PERIOD   (60 * HZ)
> +#define TCP_SYNCOOKIE_VALID    (MAX_SYNCOOKIE_AGE * TCP_SYNCOOKIE_PERIOD)
> +
> +/* syncookies: remember time of last synqueue overflow
> + * But do not dirty this field too often (once per second is enough)
> + */
> +static inline void tcp_synq_overflow(struct sock *sk)
> +{
> +       unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
> +       unsigned long now = jiffies;
> +
> +       if (time_after(now, last_overflow + HZ))
> +               tcp_sk(sk)->rx_opt.ts_recent_stamp = now;
> +}
> +
> +/* syncookies: no recent synqueue overflow on this listening socket? */
> +static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
> +{
> +       unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
> +
> +       return time_after(jiffies, last_overflow + TCP_SYNCOOKIE_VALID);
> +}
>
>  static inline u32 tcp_cookie_time(void)
>  {
>         u64 val = get_jiffies_64();
>
> -       do_div(val, 60 * HZ);
> +       do_div(val, TCP_SYNCOOKIE_PERIOD);
>         return val;
>  }
>
>
>
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