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>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 0 deletions

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 5bad937ce779..c801cd37cc2a 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);