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

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Date:   Mon, 17 Jun 2019 10:03:54 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        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>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: [PATCH net 4/4] tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

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>
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