[PATCH net] macsec: fix replay protection at XPN lower-PN wrap

[PATCH net] macsec: fix replay protection at XPN lower-PN wrap

[Junrui Luo]

In macsec_post_decrypt(), when pn is U32_MAX, pn + 1 overflows u32 to 0
and the first branch never fires. If next_pn_halves.lower is also in the
upper half, pn_same_half(pn, lower) is true and the XPN else-if does not
fire either, leaving next_pn_halves unchanged. An attacker that captures
the legitimate frame carrying pn == 0xFFFFFFFF on an XPN association
can then replay it indefinitely, since lowest_pn never rises above
the captured pn and macsec_decrypt() reconstructs the same IV.

Extend the XPN else-if to also fire when pn + 1 wraps to 0, so receipt
of pn == U32_MAX advances next_pn_halves to (upper + 1, 0).

Fixes: a21ecf0e0338 ("macsec: Support XPN frame handling - IEEE 802.1AEbw")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
---
 drivers/net/macsec.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
index f6cad0746a02..cad95b7ec631 100644
--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -804,7 +804,8 @@ static bool macsec_post_decrypt(struct sk_buff *skb, struct macsec_secy *secy, u
 		if (pn + 1 > rx_sa->next_pn_halves.lower) {
 			rx_sa->next_pn_halves.lower = pn + 1;
 		} else if (secy->xpn &&
-			   !pn_same_half(pn, rx_sa->next_pn_halves.lower)) {
+			   (pn + 1 == 0 ||
+			    !pn_same_half(pn, rx_sa->next_pn_halves.lower))) {
 			rx_sa->next_pn_halves.upper++;
 			rx_sa->next_pn_halves.lower = pn + 1;
 		}

---
base-commit: 7aaa8047eafd0bd628065b15757d9b48c5f9c07d
change-id: 20260520-fixes-b8b72f2ec0d8

Best regards,
-- 
Junrui Luo <moonafterrain@outlook.com>

Re: [PATCH net] macsec: fix replay protection at XPN lower-PN wrap

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed, 20 May 2026 11:47:55 +0800 you wrote:
> In macsec_post_decrypt(), when pn is U32_MAX, pn + 1 overflows u32 to 0
> and the first branch never fires. If next_pn_halves.lower is also in the
> upper half, pn_same_half(pn, lower) is true and the XPN else-if does not
> fire either, leaving next_pn_halves unchanged. An attacker that captures
> the legitimate frame carrying pn == 0xFFFFFFFF on an XPN association
> can then replay it indefinitely, since lowest_pn never rises above
> the captured pn and macsec_decrypt() reconstructs the same IV.
> 
> [...]

Here is the summary with links:
  - [net] macsec: fix replay protection at XPN lower-PN wrap
    https://git.kernel.org/netdev/net/c/e68842b33564

You are awesome, thank you!
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