[PATCH 3.16 10/10] tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

From: Ben Hutchings
Date: Tue Jun 18 2019 - 10:34:40 EST

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


From: Eric Dumazet <edumazet@xxxxxxxxxx>

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@xxxxxxxxxx>
Reported-by: Jonathan Lemon <jonathan.lemon@xxxxxxxxx>
Cc: Jonathan Looney <jtl@xxxxxxxxxxx>
Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Cc: Yuchung Cheng <ycheng@xxxxxxxxxx>
Cc: Tyler Hicks <tyhicks@xxxxxxxxxxxxx>
Cc: Bruce Curtis <brucec@xxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[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@xxxxxxxxxxxxxxx>
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);