netdev - Re: [PATCH net] tcp: reduce accepted window in NEW_SYN

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Message-ID: <CADVnQy=MGsnuKet_FjXh8a32+bDAE5e3kF716MXb7aNjT0ihFw@mail.gmail.com>
Date: Thu, 23 May 2024 11:37:35 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net] tcp: reduce accepted window in NEW_SYN_RECV state

On Thu, May 23, 2024 at 9:05 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> Jason commit made checks against ACK sequence less strict
> and can be exploited by attackers to establish spoofed flows
> with less probes.
>
> Innocent users might use tcp_rmem[1] == 1,000,000,000,
> or something more reasonable.
>
> An attacker can use a regular TCP connection to learn the server
> initial tp->rcv_wnd, and use it to optimize the attack.
>
> If we make sure that only the announced window (smaller than 65535)
> is used for ACK validation, we force an attacker to use
> 65537 packets to complete the 3WHS (assuming server ISN is unknown)
>
> Fixes: 378979e94e95 ("tcp: remove 64 KByte limit for initial tp->rcv_wnd value")
> Link: https://datatracker.ietf.org/meeting/119/materials/slides-119-tcpm-ghost-acks-00
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Jason Xing <kernelxing@...cent.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> ---

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric!

neal