The patch below does not apply to the 5.15-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y git checkout FETCH_HEAD git cherry-pick -x 2ca06a2f65310aeef30bb69b7405437a14766e4d # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025012037-siesta-sulfite-8b05@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 2ca06a2f65310aeef30bb69b7405437a14766e4d Mon Sep 17 00:00:00 2001 From: Paolo Abeni pabeni@redhat.com Date: Mon, 13 Jan 2025 16:44:56 +0100 Subject: [PATCH] mptcp: be sure to send ack when mptcp-level window re-opens

mptcp_cleanup_rbuf() is responsible to send acks when the user-space reads enough data to update the receive windows significantly.

It tries hard to avoid acquiring the subflow sockets locks by checking conditions similar to the ones implemented at the TCP level.

To avoid too much code duplication - the MPTCP protocol can't reuse the TCP helpers as part of the relevant status is maintained into the msk socket - and multiple costly window size computation, mptcp_cleanup_rbuf uses a rough estimate for the most recently advertised window size: the MPTCP receive free space, as recorded as at last-ack time.

Unfortunately the above does not allow mptcp_cleanup_rbuf() to detect a zero to non-zero win change in some corner cases, skipping the tcp_cleanup_rbuf call and leaving the peer stuck.

After commit ea66758c1795 ("tcp: allow MPTCP to update the announced window"), MPTCP has actually cheap access to the announced window value. Use it in mptcp_cleanup_rbuf() for a more accurate ack generation.

Fixes: e3859603ba13 ("mptcp: better msk receive window updates") Cc: stable@vger.kernel.org Reported-by: Jakub Kicinski kuba@kernel.org Closes: https://lore.kernel.org/20250107131845.5e5de3c5@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/20250113-net-mptcp-connect-st-flakes-v1-1-0d986ee7b... Signed-off-by: Jakub Kicinski kuba@kernel.org

diff --git a/net/mptcp/options.c b/net/mptcp/options.c index a62bc874bf1e..123f3f297284 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -607,7 +607,6 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb, } opts->ext_copy.use_ack = 1; opts->suboptions = OPTION_MPTCP_DSS; - WRITE_ONCE(msk->old_wspace, __mptcp_space((struct sock *)msk));

/* Add kind/length/subtype/flag overhead if mapping is not populated */ if (dss_size == 0) @@ -1288,7 +1287,7 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th) } MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDCONFLICT); } - return; + goto update_wspace; }

if (rcv_wnd_new != rcv_wnd_old) { @@ -1313,6 +1312,9 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th) th->window = htons(new_win); MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDSHARED); } + +update_wspace: + WRITE_ONCE(msk->old_wspace, tp->rcv_wnd); }

__sum16 __mptcp_make_csum(u64 data_seq, u32 subflow_seq, u16 data_len, __wsum sum)