[PATCH net 2/3] net: dsa: tag_ksz: don't read an unset MAC header in lan937x_xmit()

[Doruk Tan Ozturk <doruk@0sec.ai>]

lan937x_xmit() reads the Ethernet header via eth_hdr(skb) to test the
destination address. The sibling xmit paths in this file (ksz8795_xmit,
ksz9477_xmit, ksz9893_xmit) already use skb_eth_hdr(); lan937x_xmit() is
the lone hold-out.

On the AF_PACKET SOCK_RAW + PACKET_QDISC_BYPASS transmit path the skb
reaches ndo_start_xmit() with the MAC header unset, so eth_hdr(skb)
resolves to skb->head + (u16)~0 and the read is out of bounds.

On the TX path the L2 header is at skb->data, so use skb_eth_hdr(), as
done for the same class by
commit f5089008f90c ("macsec: don't read an unset MAC header in macsec_encrypt()")
and commit 96cc4b69581d ("macvlan: do not assume mac_header is set in macvlan_broadcast()").

Fixes: 092f875131dc ("net: dsa: tag_ksz: add tag handling for Microchip LAN937x")
Cc: stable@vger.kernel.org
Found by 0sec automated security-research tooling (https://0sec.ai).
Assisted-by: 0sec:claude-opus-4-8
Signed-off-by: Doruk Tan Ozturk <doruk@0sec.ai>
---
 net/dsa/tag_ksz.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/dsa/tag_ksz.c b/net/dsa/tag_ksz.c
index 67fa89f102e0..4f74336ae396 100644
--- a/net/dsa/tag_ksz.c
+++ b/net/dsa/tag_ksz.c
@@ -430,7 +430,7 @@ static struct sk_buff *lan937x_xmit(struct sk_buff *skb,
 	u16 queue_mapping = skb_get_queue_mapping(skb);
 	u8 prio = netdev_txq_to_tc(dev, queue_mapping);
 	struct dsa_port *dp = dsa_user_to_port(dev);
-	const struct ethhdr *hdr = eth_hdr(skb);
+	const struct ethhdr *hdr = skb_eth_hdr(skb);
 	__be16 *tag;
 	u16 val;
 
-- 
2.43.0