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Date: Wed, 1 Apr 2026 21:14:57 +0200
From: Florian Westphal <fw@strlen.de>
To: Ren Wei <n05ec@lzu.edu.cn>
Cc: netfilter-devel@vger.kernel.org, pablo@netfilter.org, phil@nwl.cc,
	yifanwucs@gmail.com, tomapufckgml@gmail.com, yuantan098@gmail.com,
	bird@lzu.edu.cn, enjou1224z@gmail.com, zcliangcn@gmail.com
Subject: Re: [PATCH 1/1] netfilter: ip6t_eui64: validate MAC header before
 using it
Message-ID: <ac1uqcDxsmmQ-oxH@strlen.de>
References: <cover.1774859629.git.zcliangcn@gmail.com>
 <5267bb5b37997fa793c28c4b928a828cfb3a3927.1774859629.git.zcliangcn@gmail.com>
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 <acus9gpd-oKl_6dg@strlen.de>
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In-Reply-To: <acus9gpd-oKl_6dg@strlen.de>

[ Trimming Cc list ]

Florian Westphal <fw@strlen.de> wrote:
> Florian Westphal <fw@strlen.de> wrote:
> > 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.
> 
> There are other suspicious spots, e.g. in nf_log_syslog.c and in ipset.
> 
> Will you make a patch to cover all of these?

Ping.  Will you followup or do you expect us to take over?