Re: [PATCH 1/1] netfilter: ip6t_eui64: validate MAC header before using it

[Florian Westphal <fw@strlen.de>]

Ren Wei <n05ec@lzu.edu.cn> wrote:
> From: Zhengchuan Liang <zcliangcn@gmail.com>
> 
> `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 match unconditionally reaches `skb_mac_header()` and `eth_hdr(skb)`
> after a guard that only rejects an invalid MAC header when
> `par->fragoff != 0`. As a result, non-fragment packets can still reach
> `eth_hdr(skb)` even when the skb has no MAC header set, or when the MAC
> header does not cover a full Ethernet header.
> 
> Fix this by first checking that the MAC header is set and spans a full
> Ethernet header before accessing it, then using that validated header
> directly for the EUI-64 comparison. Preserve the existing hotdrop
> behavior for non-first fragments with an invalid MAC header.

I find this rather confusing.  E.g. why is net/netfilter/xt_mac.c safe?
I get the feeling that this patch is not sufficient?

> +	if (!skb_mac_header_was_set(skb)) {

Makes sense to me.

> +	mac = skb_mac_header(skb);
> +	if (mac < skb->head || mac + ETH_HLEN > skb->data) {
> +		if (par->fragoff != 0)
> +			par->hotdrop = true;
> +		return false;

Why do we still need this stunt?  Why not:

mac_len = skb_mac_header_len(skb);
if (mac_len < ETH_HLEN) {
	par->hotdrop = true;
	return false;
}

?

I also feel there should be a check for ethernet, i.e.
        if (skb->dev == NULL || skb->dev->type != ARPHRD_ETHER)

... like in xt_mac.c.