From da23be77e1292cd611e736c3aa17da633d7ddce7 Mon Sep 17 00:00:00 2001 From: Paolo Abeni Date: Tue, 2 Jun 2026 22:14:11 +1000 Subject: [PATCH] mptcp: allow subflow rcv wnd to shrink In MPTCP connection, the `window` field in the TCP header refers to the MPTCP-level rcv_nxt and it's right edge should not move backward. Such constraint is enforced at DSS option generation time. At the same time, the TCP stack ensures independently that the TCP-level rcv wnd right's edge does not move backward. That in turn causes artificial inflating of the MPTCP rcv window when the incoming data is acked at the TCP level and is OoO in the MPTCP sequence space (or lands in the backlog). As a consequence, the incoming traffic can exceed the receiver rcvbuf size even when the sender is not misbehaving. Prevent such scenario forcibly allowing the TCP subflow to shrink the TCP-level rcv wnd regardless of the current netns setting. Fixes: f3589be0c420 ("mptcp: never shrink offered window") Cc: stable@vger.kernel.org Signed-off-by: Paolo Abeni Reviewed-by: Matthieu Baerts (NGI0) Signed-off-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260602-net-mptcp-misc-fixes-7-1-rc7-v2-4-856831229976@kernel.org Signed-off-by: Jakub Kicinski --- net/mptcp/options.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/net/mptcp/options.c b/net/mptcp/options.c index 2d25f319f328..51ca334678b4 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -566,6 +566,7 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb, { struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk); struct mptcp_sock *msk = mptcp_sk(subflow->conn); + struct tcp_sock *tp = tcp_sk(sk); unsigned int dss_size = 0; struct mptcp_ext *mpext; unsigned int ack_size; @@ -614,6 +615,12 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb, if (dss_size == 0) ack_size += TCPOLEN_MPTCP_DSS_BASE; + /* The caller is __tcp_transmit_skb(), and will compute the new rcv + * wnd soon: ensure that the window can shrink. + */ + if (skb) + tp->rcv_wnd = tp->rcv_nxt - tp->rcv_wup; + dss_size += ack_size; *size = ALIGN(dss_size, 4); -- 2.47.3