tcp: fix TCP_MAXSEG for established IPv6 passive sockets

Commit f5fff5d forgot to fix TCP_MAXSEG behavior IPv6 sockets, so IPv6
TCP server sockets that used TCP_MAXSEG would find that the advmss of
child sockets would be incorrect. This commit mirrors the advmss logic
from tcp_v4_syn_recv_sock in tcp_v6_syn_recv_sock. Eventually this
logic should probably be shared between IPv4 and IPv6, but this at
least fixes this issue.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 0 deletions

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 86cfe6005f40..98256cf72f9d 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1383,6 +1383,10 @@ static struct sock * tcp_v6_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
         tcp_mtup_init(newsk);
         tcp_sync_mss(newsk, dst_mtu(dst));
         newtp->advmss = dst_metric_advmss(dst);
+        if (tcp_sk(sk)->rx_opt.user_mss &&
+            tcp_sk(sk)->rx_opt.user_mss < newtp->advmss)
+                newtp->advmss = tcp_sk(sk)->rx_opt.user_mss;
+
         tcp_initialize_rcv_mss(newsk);
         if (tcp_rsk(req)->snt_synack)
                 tcp_valid_rtt_meas(newsk,