[PATCH 6.18.y 1/2] mptcp: cleanup fallback dummy mapping generation

[Sasha Levin <sashal@kernel.org>]

From: Paolo Abeni <pabeni@redhat.com>

[ Upstream commit 2834f8edd74d5dda368087a654c0e52b141e9893 ]

MPTCP currently access ack_seq outside the msk socket log scope to
generate the dummy mapping for fallback socket. Soon we are going
to introduce backlog usage and even for fallback socket the ack_seq
value will be significantly off outside of the msk socket lock scope.

Avoid relying on ack_seq for dummy mapping generation, using instead
the subflow sequence number. Note that in case of disconnect() and
(re)connect() we must ensure that any previous state is re-set.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Geliang Tang <geliang@kernel.org>
Tested-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251121-net-next-mptcp-memcg-backlog-imp-v1-6-1f34b6c1e0b1@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 0981f90e1a05 ("mptcp: reset rcv wnd on disconnect")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/protocol.c | 3 +++
 net/mptcp/subflow.c  | 8 +++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 1dbe6c8a8b8c5..16ade718b27c7 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3308,6 +3308,9 @@ static int mptcp_disconnect(struct sock *sk, int flags)
 	msk->rcvspace_init = 0;
 	msk->fastclosing = 0;
 
+	/* for fallback's sake */
+	WRITE_ONCE(msk->ack_seq, 0);
+
 	WRITE_ONCE(sk->sk_shutdown, 0);
 	sk_error_report(sk);
 	return 0;
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 3226fef7237ef..42fd759c51df4 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -491,6 +491,9 @@ static void subflow_set_remote_key(struct mptcp_sock *msk,
 	mptcp_crypto_key_sha(subflow->remote_key, NULL, &subflow->iasn);
 	subflow->iasn++;
 
+	/* for fallback's sake */
+	subflow->map_seq = subflow->iasn;
+
 	WRITE_ONCE(msk->remote_key, subflow->remote_key);
 	WRITE_ONCE(msk->ack_seq, subflow->iasn);
 	WRITE_ONCE(msk->can_ack, true);
@@ -1435,9 +1438,12 @@ static bool subflow_check_data_avail(struct sock *ssk)
 
 	skb = skb_peek(&ssk->sk_receive_queue);
 	subflow->map_valid = 1;
-	subflow->map_seq = READ_ONCE(msk->ack_seq);
 	subflow->map_data_len = skb->len;
 	subflow->map_subflow_seq = tcp_sk(ssk)->copied_seq - subflow->ssn_offset;
+	subflow->map_seq = __mptcp_expand_seq(subflow->map_seq,
+					      subflow->iasn +
+					      TCP_SKB_CB(skb)->seq -
+					      subflow->ssn_offset - 1);
 	WRITE_ONCE(subflow->data_avail, true);
 	return true;
 }
-- 
2.53.0