netdev - Re: [PATCH net 3/4] tcp: add tcp_min_snd

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Date:   Mon, 17 Jun 2019 10:15:46 -0700
From:   "Jonathan Lemon" <jonathan.lemon@...il.com>
To:     "Eric Dumazet" <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        "Eric Dumazet" <eric.dumazet@...il.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Jonathan Looney" <jtl@...flix.com>,
        "Neal Cardwell" <ncardwell@...gle.com>,
        "Tyler Hicks" <tyhicks@...onical.com>,
        "Yuchung Cheng" <ycheng@...gle.com>,
        "Bruce Curtis" <brucec@...flix.com>
Subject: Re: [PATCH net 3/4] tcp: add tcp_min_snd_mss sysctl



On 17 Jun 2019, at 10:03, Eric Dumazet wrote:

> Some TCP peers announce a very small MSS option in their SYN and/or
> SYN/ACK messages.
>
> This forces the stack to send packets with a very high network/cpu
> overhead.
>
> Linux has enforced a minimal value of 48. Since this value includes
> the size of TCP options, and that the options can consume up to 40
> bytes, this means that each segment can include only 8 bytes of payload.
>
> In some cases, it can be useful to increase the minimal value
> to a saner value.
>
> We still let the default to 48 (TCP_MIN_SND_MSS), for compatibility
> reasons.
>
> Note that TCP_MAXSEG socket option enforces a minimal value
> of (TCP_MIN_MSS). David Miller increased this minimal value
> in commit c39508d6f118 ("tcp: Make TCP_MAXSEG minimum more correct.")
> from 64 to 88.
>
> We might in the future merge TCP_MIN_SND_MSS and TCP_MIN_MSS.
>
> CVE-2019-11479 -- tcp mss hardcoded to 48
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Suggested-by: Jonathan Looney <jtl@...flix.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Tyler Hicks <tyhicks@...onical.com>
> Cc: Bruce Curtis <brucec@...flix.com>
> Cc: Jonathan Lemon <jonathan.lemon@...il.com>
Acked-by: Jonathan Lemon <jonathan.lemon@...il.com>