diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 7fa822b55c34..afa486912f5a 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -518,11 +518,11 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 
 	if (subflow->use_64bit_ack) {
 		ack_size = TCPOLEN_MPTCP_DSS_ACK64;
-		opts->ext_copy.data_ack = msk->ack_seq;
+		opts->ext_copy.data_ack = READ_ONCE(msk->ack_seq);
 		opts->ext_copy.ack64 = 1;
 	} else {
 		ack_size = TCPOLEN_MPTCP_DSS_ACK32;
-		opts->ext_copy.data_ack32 = (uint32_t)(msk->ack_seq);
+		opts->ext_copy.data_ack32 = (uint32_t)READ_ONCE(msk->ack_seq);
 		opts->ext_copy.ack64 = 0;
 	}
 	opts->ext_copy.use_ack = 1;
@@ -782,7 +782,7 @@ static void update_una(struct mptcp_sock *msk,
 	}
 }
 
-bool mptcp_update_rcv_data_fin(struct mptcp_sock *msk, u64 data_fin_seq)
+bool mptcp_update_rcv_data_fin(struct mptcp_sock *msk, u64 data_fin_seq, bool use_64bit)
 {
 	/* Skip if DATA_FIN was already received.
 	 * If updating simultaneously with the recvmsg loop, values
@@ -792,7 +792,8 @@ bool mptcp_update_rcv_data_fin(struct mptcp_sock *msk, u64 data_fin_seq)
 	if (READ_ONCE(msk->rcv_data_fin) || !READ_ONCE(msk->first))
 		return false;
 
-	WRITE_ONCE(msk->rcv_data_fin_seq, data_fin_seq);
+	WRITE_ONCE(msk->rcv_data_fin_seq,
+		   expand_ack(READ_ONCE(msk->ack_seq), data_fin_seq, use_64bit));
 	WRITE_ONCE(msk->rcv_data_fin, 1);
 
 	return true;
@@ -875,7 +876,7 @@ void mptcp_incoming_options(struct sock *sk, struct sk_buff *skb,
 	 */
 	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
 		if (mp_opt.data_fin && mp_opt.data_len == 1 &&
-		    mptcp_update_rcv_data_fin(msk, mp_opt.data_seq) &&
+		    mptcp_update_rcv_data_fin(msk, mp_opt.data_seq, mp_opt.dsn64) &&
 		    schedule_work(&msk->work))
 			sock_hold(subflow->conn);
 
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 365ba96c84b0..5d747c6a610e 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -123,7 +123,7 @@ static void __mptcp_move_skb(struct mptcp_sock *msk, struct sock *ssk,
 
 	skb_ext_reset(skb);
 	skb_orphan(skb);
-	msk->ack_seq += copy_len;
+	WRITE_ONCE(msk->ack_seq, msk->ack_seq + copy_len);
 
 	tail = skb_peek_tail(&sk->sk_receive_queue);
 	if (offset == 0 && tail) {
@@ -261,7 +261,7 @@ static void mptcp_check_data_fin(struct sock *sk)
 	if (mptcp_pending_data_fin(sk, &rcv_data_fin_seq)) {
 		struct mptcp_subflow_context *subflow;
 
-		msk->ack_seq++;
+		WRITE_ONCE(msk->ack_seq, msk->ack_seq + 1);
 		WRITE_ONCE(msk->rcv_data_fin, 0);
 
 		sk->sk_shutdown |= RCV_SHUTDOWN;
@@ -1720,7 +1720,7 @@ struct sock *mptcp_sk_clone(const struct sock *sk,
 		msk->remote_key = mp_opt->sndr_key;
 		mptcp_crypto_key_sha(msk->remote_key, NULL, &ack_seq);
 		ack_seq++;
-		msk->ack_seq = ack_seq;
+		WRITE_ONCE(msk->ack_seq, ack_seq);
 	}
 
 	sock_reset_flag(nsk, SOCK_RCU_FREE);
@@ -2072,7 +2072,7 @@ bool mptcp_finish_join(struct sock *sk)
 	parent_sock = READ_ONCE(parent->sk_socket);
 	if (parent_sock && !sk->sk_socket)
 		mptcp_sock_graft(sk, parent_sock);
-	subflow->map_seq = msk->ack_seq;
+	subflow->map_seq = READ_ONCE(msk->ack_seq);
 	return true;
 }
 
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 60b27d44c184..20f04ac85409 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -387,7 +387,7 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk);
 bool mptcp_finish_join(struct sock *sk);
 void mptcp_data_acked(struct sock *sk);
 void mptcp_subflow_eof(struct sock *sk);
-bool mptcp_update_rcv_data_fin(struct mptcp_sock *msk, u64 data_fin_seq);
+bool mptcp_update_rcv_data_fin(struct mptcp_sock *msk, u64 data_fin_seq, bool use_64bit);
 
 void __init mptcp_token_init(void);
 static inline void mptcp_token_init_request(struct request_sock *req)
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 8cbeb68f3775..5f2fa935022d 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -731,7 +731,8 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 
 	if (mpext->data_fin == 1) {
 		if (data_len == 1) {
-			bool updated = mptcp_update_rcv_data_fin(msk, mpext->data_seq);
+			bool updated = mptcp_update_rcv_data_fin(msk, mpext->data_seq,
+								 mpext->dsn64);
 			pr_debug("DATA_FIN with no payload seq=%llu", mpext->data_seq);
 			if (subflow->map_valid) {
 				/* A DATA_FIN might arrive in a DSS
@@ -748,8 +749,17 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 				return MAPPING_DATA_FIN;
 			}
 		} else {
-			mptcp_update_rcv_data_fin(msk, mpext->data_seq + data_len);
-			pr_debug("DATA_FIN with mapping seq=%llu", mpext->data_seq + data_len);
+			u64 data_fin_seq = mpext->data_seq + data_len;
+
+			/* If mpext->data_seq is a 32-bit value, data_fin_seq
+			 * must also be limited to 32 bits.
+			 */
+			if (!mpext->dsn64)
+				data_fin_seq &= GENMASK_ULL(31, 0);
+
+			mptcp_update_rcv_data_fin(msk, data_fin_seq, mpext->dsn64);
+			pr_debug("DATA_FIN with mapping seq=%llu dsn64=%d",
+				 data_fin_seq, mpext->dsn64);
 		}
 
 		/* Adjust for DATA_FIN using 1 byte of sequence space */