[PATCH net] macsec: prevent AES-GCM nonce reuse after packet number wrap

["Jérémy Jean" <Jeremy.Jean@oss.cyber.gouv.fr>]

When MACsec uses 32-bit packet numbers, 0xffffffff is the last valid
packet number, and after allocating it, MACsec deactivates the
transmit SA and wraps the next packet number to zero. Packets already
in flight can still be processed after that.

The first late packet gets packet number zero and is dropped, but
tx_sa_update_pn() has already advanced the stored counter to one before
macsec_encrypt() drops it. A second late packet can then be sent with
packet number one again, reusing the AES-GCM nonce from the start of the
SA.

Keep next_pn at zero after wrap so all late packets are dropped.

Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
Cc: stable@vger.kernel.org
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---
 drivers/net/macsec.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
index 78a19b134632..233391acebb0 100644
--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -486,6 +486,9 @@ static pn_t tx_sa_update_pn(struct macsec_tx_sa *tx_sa,
 	spin_lock_bh(&tx_sa->lock);
 
 	pn = tx_sa->next_pn_halves;
+	if (unlikely(pn.full64 == 0))
+		goto out;
+
 	if (secy->xpn)
 		tx_sa->next_pn++;
 	else
@@ -493,6 +496,8 @@ static pn_t tx_sa_update_pn(struct macsec_tx_sa *tx_sa,
 
 	if (tx_sa->next_pn == 0)
 		__macsec_pn_wrapped(secy, tx_sa);
+
+out:
 	spin_unlock_bh(&tx_sa->lock);
 
 	return pn;
-- 
2.47.3