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@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>
Cc: Bruce Curtis <brucec@netflix.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name tcp-enforce-tcp_min_snd_mss-in-tcp_mtu_probing.patch

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index b1b5a648def6cd263bd4d8c927488243acfca255..17335a370e6452b58bd3490d6821e842d706db53 100644 (file)
--- a/net/ipv4/tcp_timer.c
+++ b/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);