macsec: don't read an unset MAC header in macsec_encrypt()

macsec_encrypt() reads the Ethernet header via eth_hdr(skb)
(skb->head + skb->mac_header) to memmove() the 12 source/destination MAC
bytes forward and make room for the SecTAG.

On the AF_PACKET SOCK_RAW + PACKET_QDISC_BYPASS transmit path the skb
reaches the macsec 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: a 12-byte heap over-read that is also emitted on the wire as the
frame's outer source/destination MAC. KASAN reports a slab-out-of-bounds
read in macsec_start_xmit() on 6.0; on current mainline a CONFIG_DEBUG_NET
build flags it as an unset mac header in skb_mac_header().

On the TX path the L2 header is at skb->data, so use skb_eth_hdr(), added
by commit 96cc4b6958 ("macvlan: do not assume mac_header is set in
macvlan_broadcast()") for exactly this purpose.

Fixes: c09440f7dc ("macsec: introduce IEEE 802.1AE driver")
Cc: stable@vger.kernel.org
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20260703083634.2035145-1-4ncienth@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/macsec.c

@@ -646,7 +646,7 @@ static struct sk_buff *macsec_encrypt(struct sk_buff *skb,
 	}
 
 	unprotected_len = skb->len;
-	eth = eth_hdr(skb);
+	eth = skb_eth_hdr(skb);
 	sci_present = macsec_send_sci(secy);
 	hh = skb_push(skb, macsec_extra_len(sci_present));
 	memmove(hh, eth, 2 * ETH_ALEN);