From: Paolo Abeni pabeni@redhat.com
commit 4f102d747cadd8f595f2b25882eed9bec1675fb1 upstream.
The rcv window is shared among all the subflows. Currently, MPTCP sync the TCP-level rcv window with the MPTCP one at tcp_transmit_skb() time.
The above means that incoming data may sporadically observe outdated TCP-level rcv window and being wrongly dropped by TCP.
Address the issue checking for the edge condition before queuing the data at TCP level, and eventually syncing the rcv window as needed.
Note that the issue is actually present from the very first MPTCP implementation, but backports older than the blamed commit below will range from impossible to useless.
Before:
$ nstat -n; sleep 1; nstat -z TcpExtBeyondWindow TcpExtBeyondWindow 14 0.0
After:
$ nstat -n; sleep 1; nstat -z TcpExtBeyondWindow TcpExtBeyondWindow 0 0.0
Fixes: fa3fe2b15031 ("mptcp: track window announced to peer") Cc: stable@vger.kernel.org Signed-off-by: Paolo Abeni pabeni@redhat.com Reviewed-by: Matthieu Baerts (NGI0) matttbe@kernel.org Signed-off-by: Matthieu Baerts (NGI0) matttbe@kernel.org Link: https://patch.msgid.link/20251118-net-mptcp-misc-fixes-6-18-rc6-v1-2-806d378... Signed-off-by: Jakub Kicinski kuba@kernel.org [ Conflicts in options.c, because the new rwin_update() helper has been added after __mptcp_snd_una_update() which is not in this version -- see commit -- and then causing conflicts in the context. There were also some conflicts in mptcp_set_rwin(), because commit f3589be0c420 ("mptcp: never shrink offered window") is not in this version. Only the update of subflow->rcv_wnd_sent has been added. Also msk->rcv_wnd_sent is a u64 before this commit: in rwin_update(), READ_ONCE() is used instead of atomic64_read(&). ] Signed-off-by: Matthieu Baerts (NGI0) matttbe@kernel.org --- net/mptcp/options.c | 32 ++++++++++++++++++++++++++++++-- net/mptcp/protocol.h | 1 + 2 files changed, 31 insertions(+), 2 deletions(-)
diff --git a/net/mptcp/options.c b/net/mptcp/options.c index b4237e50e23f..85fcc378c711 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -1001,6 +1001,31 @@ u64 __mptcp_expand_seq(u64 old_seq, u64 cur_seq) return cur_seq; }
+static void rwin_update(struct mptcp_sock *msk, struct sock *ssk, + struct sk_buff *skb) +{ + struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk); + struct tcp_sock *tp = tcp_sk(ssk); + u64 mptcp_rcv_wnd; + + /* Avoid touching extra cachelines if TCP is going to accept this + * skb without filling the TCP-level window even with a possibly + * outdated mptcp-level rwin. + */ + if (!skb->len || skb->len < tcp_receive_window(tp)) + return; + + mptcp_rcv_wnd = READ_ONCE(msk->rcv_wnd_sent); + if (!after64(mptcp_rcv_wnd, subflow->rcv_wnd_sent)) + return; + + /* Some other subflow grew the mptcp-level rwin since rcv_wup, + * resync. + */ + tp->rcv_wnd += mptcp_rcv_wnd - subflow->rcv_wnd_sent; + subflow->rcv_wnd_sent = mptcp_rcv_wnd; +} + static void ack_update_msk(struct mptcp_sock *msk, struct sock *ssk, struct mptcp_options_received *mp_opt) @@ -1160,6 +1185,7 @@ bool mptcp_incoming_options(struct sock *sk, struct sk_buff *skb) */ if (mp_opt.use_ack) ack_update_msk(msk, sk, &mp_opt); + rwin_update(msk, sk, skb);
/* Zero-data-length packets are dropped by the caller and not * propagated to the MPTCP layer, so the skb extension does not @@ -1212,7 +1238,7 @@ bool mptcp_incoming_options(struct sock *sk, struct sk_buff *skb) static void mptcp_set_rwin(const struct tcp_sock *tp) { const struct sock *ssk = (const struct sock *)tp; - const struct mptcp_subflow_context *subflow; + struct mptcp_subflow_context *subflow; struct mptcp_sock *msk; u64 ack_seq;
@@ -1221,8 +1247,10 @@ static void mptcp_set_rwin(const struct tcp_sock *tp)
ack_seq = READ_ONCE(msk->ack_seq) + tp->rcv_wnd;
- if (after64(ack_seq, READ_ONCE(msk->rcv_wnd_sent))) + if (after64(ack_seq, READ_ONCE(msk->rcv_wnd_sent))) { WRITE_ONCE(msk->rcv_wnd_sent, ack_seq); + subflow->rcv_wnd_sent = ack_seq; + } }
static void mptcp_track_rwin(const struct tcp_sock *tp) diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h index c93399d11650..3450c3cd015a 100644 --- a/net/mptcp/protocol.h +++ b/net/mptcp/protocol.h @@ -417,6 +417,7 @@ struct mptcp_subflow_context { u64 remote_key; u64 idsn; u64 map_seq; + u64 rcv_wnd_sent; u32 snd_isn; u32 token; u32 rel_write_seq;