netdev - Re: [PATCH net 4/4] tcp: enforce tcp_min_snd_mss in tcp_mtu

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Date:   Mon, 17 Jun 2019 12:18:31 -0500
From:   Tyler Hicks <tyhicks@...onical.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>,
        Yuchung Cheng <ycheng@...gle.com>,
        Bruce Curtis <brucec@...flix.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH net 4/4] tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

On 2019-06-17 10:03:54, Eric Dumazet wrote:
> If mtu probing is enabled tcp_mtu_probing() could very well end up
> with a too small MSS.
> 
> Use the new sysctl tcp_min_snd_mss to make sure MSS search
> is performed in an acceptable range.
> 
> CVE-2019-11479 -- tcp mss hardcoded to 48
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Jonathan Lemon <jonathan.lemon@...il.com>
> Cc: Jonathan Looney <jtl@...flix.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Tyler Hicks <tyhicks@...onical.com>

As mentioned for the other sysctl patch, I've given the two sysctl
patches a close review and some testing.

Acked-by: Tyler Hicks <tyhicks@...onical.com>

Tyler

> Cc: Bruce Curtis <brucec@...flix.com>
> ---
>  net/ipv4/tcp_timer.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index 5bad937ce779ef8dca42a26dcbb5f1d60a571c73..c801cd37cc2a9c11f2dd4b9681137755e501a538 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -155,6 +155,7 @@ static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
>  		mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
>  		mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
>  		mss = max(mss, 68 - tcp_sk(sk)->tcp_header_len);
> +		mss = max(mss, net->ipv4.sysctl_tcp_min_snd_mss);
>  		icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
>  	}
>  	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
> -- 
> 2.22.0.410.gd8fdbe21b5-goog
>