tcp: enforce tcp_min_snd_mss in tcp_mtu_probing()

commit 967c05a 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 <[email protected]>
Reported-by: Jonathan Lemon <[email protected]>
Cc: Jonathan Looney <[email protected]>
Acked-by: Neal Cardwell <[email protected]>
Cc: Yuchung Cheng <[email protected]>
Cc: Tyler Hicks <[email protected]>
Cc: Bruce Curtis <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Jason Xing <[email protected]>
Signed-off-by: Dust Li <[email protected]>

net/ipv4/tcp_timer.c

@@ -166,6 +166,7 @@ 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);