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From:   Matthieu Baerts <matthieu.baerts@tessares.net>
Date:   Sat, 16 Sep 2023 12:52:45 +0200
Subject: [PATCH net 1/5] mptcp: fix bogus receive window shrinkage with
 multiple subflows
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Message-Id: <20230916-upstream-net-20230915-mptcp-hanging-conn-v1-1-05d1a8b851a8@tessares.net>
References: <20230916-upstream-net-20230915-mptcp-hanging-conn-v1-0-05d1a8b851a8@tessares.net>
In-Reply-To: <20230916-upstream-net-20230915-mptcp-hanging-conn-v1-0-05d1a8b851a8@tessares.net>
To:     mptcp@lists.linux.dev, Mat Martineau <martineau@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Davide Caratti <dcaratti@redhat.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Matthieu Baerts <matthieu.baerts@tessares.net>,
        stable@vger.kernel.org
Precedence: bulk

From: Paolo Abeni <pabeni@redhat.com>

In case multiple subflows race to update the mptcp-level receive
window, the subflow losing the race should use the window value
provided by the "winning" subflow to update it's own tcp-level
rcv_wnd.

To such goal, the current code bogusly uses the mptcp-level rcv_wnd
value as observed before the update attempt. On unlucky circumstances
that may lead to TCP-level window shrinkage, and stall the other end.

Address the issue feeding to the rcv wnd update the correct value.

Fixes: f3589be0c420 ("mptcp: never shrink offered window")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/427
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
---
 net/mptcp/options.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index c254accb14de..cd15ec73073e 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1269,12 +1269,13 @@ static void mptcp_set_rwin(struct tcp_sock *tp, struct tcphdr *th)
 
 			if (rcv_wnd == rcv_wnd_old)
 				break;
-			if (before64(rcv_wnd_new, rcv_wnd)) {
+
+			rcv_wnd_old = rcv_wnd;
+			if (before64(rcv_wnd_new, rcv_wnd_old)) {
 				MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDCONFLICTUPDATE);
 				goto raise_win;
 			}
 			MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_RCVWNDCONFLICT);
-			rcv_wnd_old = rcv_wnd;
 		}
 		return;
 	}

-- 
2.40.1