mptcp: fix rx timestamp corruption on fastopen

commit 6254a16 upstream.

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

Fixes: 36b122b ("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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Paolo Abeni

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()
@@ -40,12 +41,13 @@ 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);