Re: [PATCH net 4/4] tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

[Tyler Hicks <tyhicks@canonical.com>]

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@google.com>
> Reported-by: Jonathan Lemon <jonathan.lemon@gmail.com>
> Cc: Jonathan Looney <jtl@netflix.com>
> Acked-by: Neal Cardwell <ncardwell@google.com>
> Cc: Yuchung Cheng <ycheng@google.com>
> Cc: Tyler Hicks <tyhicks@canonical.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@canonical.com>

Tyler

> Cc: Bruce Curtis <brucec@netflix.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
>