[PATCH net v3] macsec: MACsec SCI assignment for ES = 0

[carlos.fernandez@technica-engineering.de]

From: Carlos Fernandez <carlos.fernandez@technica-engineering.de>

According to 802.1AE standard, when ES and SC flags in TCI are zero,
used SCI should be the current active SC_RX. Current code uses the
header MAC address. Without this patch, when ES flag is 0 (using a
bridge or switch), header MAC will not fit the SCI and MACSec frames
will be discarted.

Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
Co-developed-by: Andreu Montiel <Andreu.Montiel@technica-engineering.de>
Signed-off-by: Andreu Montiel <Andreu.Montiel@technica-engineering.de>
Signed-off-by: Carlos Fernandez <carlos.fernandez@technica-engineering.de>
---
v3:
* Wrong drop frame afer macsec_frame_sci
* Wrong Fixes tag in message 

v2: https://patchwork.kernel.org/project/netdevbpf/patch/20250604113213.2595524-1-carlos.fernandez@technica-engineering.de/
* Active sci lookup logic in a separate helper.
* Unnecessary loops avoided. 
* Check RXSC is exactly one for lower device.
* Drops frame in case of error.


v1: https://patchwork.kernel.org/project/netdevbpf/patch/20250529124455.2761783-1-carlos.fernandez@technica-engineering.de/

 drivers/net/macsec.c | 40 ++++++++++++++++++++++++++++++++++------
 1 file changed, 34 insertions(+), 6 deletions(-)

diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
index 3d315e30ee47..7edbe76b5455 100644
--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -247,15 +247,39 @@ static sci_t make_sci(const u8 *addr, __be16 port)
 	return sci;
 }
 
-static sci_t macsec_frame_sci(struct macsec_eth_header *hdr, bool sci_present)
+static sci_t macsec_active_sci(struct macsec_secy *secy)
 {
-	sci_t sci;
+	struct macsec_rx_sc *rx_sc = rcu_dereference_bh(secy->rx_sc);
+
+	/* Case single RX SC */
+	if (rx_sc && !rcu_dereference_bh(rx_sc->next))
+		return (rx_sc->active) ? rx_sc->sci : 0;
+	/* Case no RX SC or multiple */
+	else
+		return 0;
+}
+
+static sci_t macsec_frame_sci(struct macsec_eth_header *hdr, bool sci_present,
+			      struct macsec_rxh_data *rxd)
+{
+	struct macsec_dev *macsec;
+	sci_t sci = 0;
 
-	if (sci_present)
+	/* SC = 1 */
+	if (sci_present) {
 		memcpy(&sci, hdr->secure_channel_id,
 		       sizeof(hdr->secure_channel_id));
-	else
+	/* SC = 0; ES = 0 */
+	} else if ((!(hdr->tci_an & (MACSEC_TCI_ES | MACSEC_TCI_SC))) &&
+		   (list_is_singular(&rxd->secys))) {
+		/* Only one SECY should exist on this scenario */
+		macsec = list_first_or_null_rcu(&rxd->secys, struct macsec_dev,
+						secys);
+		if (macsec)
+			return macsec_active_sci(&macsec->secy);
+	} else {
 		sci = make_sci(hdr->eth.h_source, MACSEC_PORT_ES);
+	}
 
 	return sci;
 }
@@ -1109,7 +1133,7 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
 	struct macsec_rxh_data *rxd;
 	struct macsec_dev *macsec;
 	unsigned int len;
-	sci_t sci;
+	sci_t sci = 0;
 	u32 hdr_pn;
 	bool cbit;
 	struct pcpu_rx_sc_stats *rxsc_stats;
@@ -1156,11 +1180,14 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
 
 	macsec_skb_cb(skb)->has_sci = !!(hdr->tci_an & MACSEC_TCI_SC);
 	macsec_skb_cb(skb)->assoc_num = hdr->tci_an & MACSEC_AN_MASK;
-	sci = macsec_frame_sci(hdr, macsec_skb_cb(skb)->has_sci);
 
 	rcu_read_lock();
 	rxd = macsec_data_rcu(skb->dev);
 
+	sci = macsec_frame_sci(hdr, macsec_skb_cb(skb)->has_sci, rxd);
+	if (!sci)
+		goto drop_nosc;
+
 	list_for_each_entry_rcu(macsec, &rxd->secys, secys) {
 		struct macsec_rx_sc *sc = find_rx_sc(&macsec->secy, sci);
 
@@ -1283,6 +1310,7 @@ static rx_handler_result_t macsec_handle_frame(struct sk_buff **pskb)
 	macsec_rxsa_put(rx_sa);
 drop_nosa:
 	macsec_rxsc_put(rx_sc);
+drop_nosc:
 	rcu_read_unlock();
 drop_direct:
 	kfree_skb(skb);
-- 
2.43.0