macsec: Fix header length if SCI is added if explicitly disabled

Even if sending SCIs is explicitly disabled, the code that creates the
Security Tag might still decide to add it (e.g. if multiple RX SCs are
defined on the MACsec interface).
But because the header length so far only depended on the configuration
option the SCI overwrote the original frame's contents (EtherType and
e.g. the beginning of the IP header) and if encrypted did not visibly
end up in the packet, while the SC flag in the TCI field of the Security
Tag was still set, resulting in invalid MACsec frames.

Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Tobias Brunner <tobias@strongswan.org>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macsec.c

@@ -397,6 +397,14 @@ static struct macsec_cb *macsec_skb_cb(struct sk_buff *skb)
 #define DEFAULT_ENCRYPT false
 #define DEFAULT_ENCODING_SA 0
 
+static bool send_sci(const struct macsec_secy *secy)
+{
+	const struct macsec_tx_sc *tx_sc = &secy->tx_sc;
+
+	return tx_sc->send_sci ||
+		(secy->n_rx_sc > 1 && !tx_sc->end_station && !tx_sc->scb);
+}
+
 static sci_t make_sci(u8 *addr, __be16 port)
 {
 	sci_t sci;
@@ -437,15 +445,15 @@ static unsigned int macsec_extra_len(bool sci_present)
 
 /* Fill SecTAG according to IEEE 802.1AE-2006 10.5.3 */
 static void macsec_fill_sectag(struct macsec_eth_header *h,
-			       const struct macsec_secy *secy, u32 pn)
+			       const struct macsec_secy *secy, u32 pn,
+			       bool sci_present)
 {
 	const struct macsec_tx_sc *tx_sc = &secy->tx_sc;
 
-	memset(&h->tci_an, 0, macsec_sectag_len(tx_sc->send_sci));
+	memset(&h->tci_an, 0, macsec_sectag_len(sci_present));
 	h->eth.h_proto = htons(ETH_P_MACSEC);
 
-	if (tx_sc->send_sci ||
-	    (secy->n_rx_sc > 1 && !tx_sc->end_station && !tx_sc->scb)) {
+	if (sci_present) {
 		h->tci_an |= MACSEC_TCI_SC;
 		memcpy(&h->secure_channel_id, &secy->sci,
 		       sizeof(h->secure_channel_id));
@@ -650,6 +658,7 @@ static struct sk_buff *macsec_encrypt(struct sk_buff *skb,
 	struct macsec_tx_sc *tx_sc;
 	struct macsec_tx_sa *tx_sa;
 	struct macsec_dev *macsec = macsec_priv(dev);
+	bool sci_present;
 	u32 pn;
 
 	secy = &macsec->secy;
@@ -687,7 +696,8 @@ static struct sk_buff *macsec_encrypt(struct sk_buff *skb,
 
 	unprotected_len = skb->len;
 	eth = eth_hdr(skb);
-	hh = (struct macsec_eth_header *)skb_push(skb, macsec_extra_len(tx_sc->send_sci));
+	sci_present = send_sci(secy);
+	hh = (struct macsec_eth_header *)skb_push(skb, macsec_extra_len(sci_present));
 	memmove(hh, eth, 2 * ETH_ALEN);
 
 	pn = tx_sa_update_pn(tx_sa, secy);
@@ -696,7 +706,7 @@ static struct sk_buff *macsec_encrypt(struct sk_buff *skb,
 		kfree_skb(skb);
 		return ERR_PTR(-ENOLINK);
 	}
-	macsec_fill_sectag(hh, secy, pn);
+	macsec_fill_sectag(hh, secy, pn, sci_present);
 	macsec_set_shortlen(hh, unprotected_len - 2 * ETH_ALEN);
 
 	skb_put(skb, secy->icv_len);
@@ -726,10 +736,10 @@ static struct sk_buff *macsec_encrypt(struct sk_buff *skb,
 	skb_to_sgvec(skb, sg, 0, skb->len);
 
 	if (tx_sc->encrypt) {
-		int len = skb->len - macsec_hdr_len(tx_sc->send_sci) -
+		int len = skb->len - macsec_hdr_len(sci_present) -
 			  secy->icv_len;
 		aead_request_set_crypt(req, sg, sg, len, iv);
-		aead_request_set_ad(req, macsec_hdr_len(tx_sc->send_sci));
+		aead_request_set_ad(req, macsec_hdr_len(sci_present));
 	} else {
 		aead_request_set_crypt(req, sg, sg, 0, iv);
 		aead_request_set_ad(req, skb->len - secy->icv_len);