net: mptcp: make DACK4/DACK8 usage consistent among all subflows

[ Upstream commit 37198e9 ]

using packetdrill it's possible to observe the same MPTCP DSN being acked
by different subflows with DACK4 and DACK8. This is in contrast with what
specified in RFC8684 §3.3.2: if an MPTCP endpoint transmits a 64-bit wide
DSN, it MUST be acknowledged with a 64-bit wide DACK. Fix 'use_64bit_ack'
variable to make it a property of MPTCP sockets, not TCP subflows.

Fixes: a0c1d0e ("mptcp: Use 32-bit DATA_ACK when possible")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dcaratti

net/mptcp/options.c

@@ -516,7 +516,7 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
 		return ret;
 	}
 
-	if (subflow->use_64bit_ack) {
+	if (READ_ONCE(msk->use_64bit_ack)) {
 		ack_size = TCPOLEN_MPTCP_DSS_ACK64;
 		opts->ext_copy.data_ack = READ_ONCE(msk->ack_seq);
 		opts->ext_copy.ack64 = 1;

net/mptcp/protocol.h

@@ -203,6 +203,7 @@ struct mptcp_sock {
 	bool		fully_established;
 	bool		rcv_data_fin;
 	bool		snd_data_fin_enable;
+	bool		use_64bit_ack; /* Set when we received a 64-bit DSN */
 	spinlock_t	join_list_lock;
 	struct work_struct work;
 	struct list_head conn_list;
@@ -295,7 +296,6 @@ struct mptcp_subflow_context {
 		backup : 1,
 		data_avail : 1,
 		rx_eof : 1,
-		use_64bit_ack : 1, /* Set when we received a 64-bit DSN */
 		can_ack : 1;	    /* only after processing the remote a key */
 	u32	remote_nonce;
 	u64	thmac;

net/mptcp/subflow.c

@@ -781,12 +781,11 @@ static enum mapping_status get_mapping_status(struct sock *ssk,
 	if (!mpext->dsn64) {
 		map_seq = expand_seq(subflow->map_seq, subflow->map_data_len,
 				     mpext->data_seq);
-		subflow->use_64bit_ack = 0;
 		pr_debug("expanded seq=%llu", subflow->map_seq);
 	} else {
 		map_seq = mpext->data_seq;
-		subflow->use_64bit_ack = 1;
 	}
+	WRITE_ONCE(mptcp_sk(subflow->conn)->use_64bit_ack, !!mpext->dsn64);
 
 	if (subflow->map_valid) {
 		/* Allow replacing only with an identical map */