linux-kernel - [PATCH 3.16 10/10] tcp: enforce tcp_min_snd_mss in tcp_mtu

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Date:   Tue, 18 Jun 2019 15:28:02 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "David S. Miller" <davem@...emloft.net>,
        "Jonathan Lemon" <jonathan.lemon@...il.com>,
        "Jonathan Looney" <jtl@...flix.com>,
        "Eric Dumazet" <edumazet@...gle.com>,
        "Yuchung Cheng" <ycheng@...gle.com>,
        "Bruce Curtis" <brucec@...flix.com>,
        "Neal Cardwell" <ncardwell@...gle.com>,
        "Tyler Hicks" <tyhicks@...onical.com>
Subject: [PATCH 3.16 10/10] tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

3.16.69-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@...gle.com>

commit 967c05aee439e6e5d7d805e195b3a20ef5c433d6 upstream.

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>
Signed-off-by: David S. Miller <davem@...emloft.net>
[Salvatore Bonaccorso: Backport for context changes in 4.9.168]
[bwh: Backported to 3.16: The sysctl is global]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/ipv4/tcp_timer.c | 1 +
 1 file changed, 1 insertion(+)

--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -113,6 +113,7 @@ static void tcp_mtu_probing(struct inet_
 			mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
 			mss = min(sysctl_tcp_base_mss, mss);
 			mss = max(mss, 68 - tp->tcp_header_len);
+			mss = max(mss, sysctl_tcp_min_snd_mss);
 			icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
 			tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 		}