netfilter: ip6t_eui64: reject invalid MAC header for all packets

jira VULN-183036
cve CVE-2026-31685
commit-author Zhengchuan Liang <zcliangcn@gmail.com>
commit fdce0b3

`eui64_mt6()` derives a modified EUI-64 from the Ethernet source address
and compares it with the low 64 bits of the IPv6 source address.

The existing guard only rejects an invalid MAC header when
`par->fragoff != 0`. For packets with `par->fragoff == 0`, `eui64_mt6()`
can still reach `eth_hdr(skb)` even when the MAC header is not valid.

Fix this by removing the `par->fragoff != 0` condition so that packets
with an invalid MAC header are rejected before accessing `eth_hdr(skb)`.

Fixes: 1da177e ("Linux-2.6.12-rc2")
	Reported-by: Yifan Wu <yifanwucs@gmail.com>
	Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Co-developed-by: Yuan Tan <yuantan098@gmail.com>
	Signed-off-by: Yuan Tan <yuantan098@gmail.com>
	Suggested-by: Xin Liu <bird@lzu.edu.cn>
	Tested-by: Ren Wei <enjou1224z@gmail.com>
	Signed-off-by: Zhengchuan Liang <zcliangcn@gmail.com>
	Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
	Signed-off-by: Florian Westphal <fw@strlen.de>
(cherry picked from commit fdce0b3)
	Signed-off-by: CIQ Kernel Automation <ciq_kernel_automation@ciq.com>

Author: CIQ Kernel Automation

net/ipv6/netfilter/ip6t_eui64.c

@@ -22,8 +22,7 @@ eui64_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 	unsigned char eui64[8];
 
 	if (!(skb_mac_header(skb) >= skb->head &&
-	      skb_mac_header(skb) + ETH_HLEN <= skb->data) &&
-	    par->fragoff != 0) {
+	      skb_mac_header(skb) + ETH_HLEN <= skb->data)) {
 		par->hotdrop = true;
 		return false;
 	}