[PATCH 6.6.y 2/2] mptcp: fix rx timestamp corruption on fastopen

[Sasha Levin <sashal@kernel.org>]

From: Paolo Abeni <pabeni@redhat.com>

[ Upstream commit 6254a16d6f0c672e3809ca5d7c9a28a55d71f764 ]

The skb cb offset containing the timestamp presence flag is cleared
before loading such information. Cache such value before MPTCP CB
initialization.

Fixes: 36b122baf6a8 ("mptcp: add subflow_v(4,6)_send_synack()")
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/20260501-net-mptcp-misc-fixes-7-1-rc3-v1-3-b70118df778e@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/fastopen.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
index 7777f5a2d1437..d4dbdd3d5679d 100644
--- a/net/mptcp/fastopen.c
+++ b/net/mptcp/fastopen.c
@@ -12,6 +12,7 @@ void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subf
 	struct sock *sk, *ssk;
 	struct sk_buff *skb;
 	struct tcp_sock *tp;
+	bool has_rxtstamp;
 
 	/* on early fallback the subflow context is deleted by
 	 * subflow_syn_recv_sock()
@@ -39,12 +40,13 @@ void mptcp_fastopen_subflow_synack_set_params(struct mptcp_subflow_context *subf
 	 */
 	tp->copied_seq += skb->len;
 	subflow->ssn_offset += skb->len;
+	has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
 
 	/* Only the sequence delta is relevant */
 	MPTCP_SKB_CB(skb)->map_seq = -skb->len;
 	MPTCP_SKB_CB(skb)->end_seq = 0;
 	MPTCP_SKB_CB(skb)->offset = 0;
-	MPTCP_SKB_CB(skb)->has_rxtstamp = TCP_SKB_CB(skb)->has_rxtstamp;
+	MPTCP_SKB_CB(skb)->has_rxtstamp = has_rxtstamp;
 	MPTCP_SKB_CB(skb)->cant_coalesce = 1;
 
 	mptcp_data_lock(sk);
-- 
2.53.0