Re: [PATCH net v2] netfilter: ebtables: ebt_802_3: check the LLC fields before reading them

[Pablo Neira Ayuso]

On Thu, Oct 08, 2026 at 10:45:39AM +0800, Guo Zihao wrote:
> ebt_802_3_mt() derives the frame pointer from skb_mac_header() and reads
> the LLC fields through it without making sure that those bytes are
> available at all. ebtables guarantees that the Ethernet header is linear,
> but nothing between the receive path and this match pulls the LLC header,
> so a frame that carries nothing past the 14-byte Ethernet header is read
> past the linear area.
> 
> Also, ebt_802_3_mt() does not need to look at every LLC field for every
> rule. A SAP-only rule reads only dsap and ssap, and a rule without any
> SAP or TYPE constraint selects frames by their 802.3 length field alone,
> which ebt_basic_match() has already handled. Reading the type field there
> was wasted work and could not be checked against anything.
> 
> Go through skb_header_pointer() with the mac header offset, which handles
> frames whose LLC header is not in the linear area, and require only the
> bytes each rule actually examines:
> 
>  - no SAP or TYPE constraint: nothing to look at, match right away;
>  - SAP-only rule: dsap/ssap, at machdr + 14..15;
>  - TYPE rule: the whole union, at machdr + 14..23, which also covers the
>    control byte that selects between the ui and ni views of the union.
> 
> A frame too short to hold the fields a rule examines now fails to match
> that rule. The previous hotdrop approach was dropped because ebtables
> legacy only honors hotdrop when every match of the rule so far has
> matched, so returning false is the only behaviour both front-ends agree
> on. Users who want truncated 802.3 frames dropped can match 802.3 alone.
> 
> Discovered by a static-analysis scan of net/; confirmed by code
> inspection of the receive path and the ebtables core. Not triggered in
> testing; no runtime report is available.

If no reproducible crash, then this has to be targeted at nf-next.

And llc support has been removed from the kernel IIRC.

> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reviewed-by: Liu Chao <liuc63@xxxxxxxxxxxx>
> Signed-off-by: Guo Zihao <guozh23@xxxxxxxxxxxx>
> ---
>  net/bridge/netfilter/ebt_802_3.c | 47 +++++++++++++++++++++++++-------
>  1 file changed, 37 insertions(+), 10 deletions(-)
> 
> diff --git a/net/bridge/netfilter/ebt_802_3.c b/net/bridge/netfilter/ebt_802_3.c
> index 68c2519bd..1a67376d1 100644
> --- a/net/bridge/netfilter/ebt_802_3.c
> +++ b/net/bridge/netfilter/ebt_802_3.c
> @@ -9,32 +9,59 @@
>   *
>   */
>  #include <linux/module.h>
> +#include <linux/stddef.h>
>  #include <linux/netfilter/x_tables.h>
>  #include <linux/netfilter_bridge/ebtables.h>
>  #include <linux/skbuff.h>
>  #include <uapi/linux/netfilter_bridge/ebt_802_3.h>
>  
> -static struct ebt_802_3_hdr *ebt_802_3_hdr(const struct sk_buff *skb)
> -{
> -	return (struct ebt_802_3_hdr *)skb_mac_header(skb);
> -}
> -
>  static bool
>  ebt_802_3_mt(const struct sk_buff *skb, struct xt_action_param *par)
>  {
>  	const struct ebt_802_3_info *info = par->matchinfo;
> -	const struct ebt_802_3_hdr *hdr = ebt_802_3_hdr(skb);
> -	__be16 type = hdr->llc.ui.ctrl & IS_UI ? hdr->llc.ui.type : hdr->llc.ni.type;
> +	struct ebt_802_3_hdr _frame;
> +	const struct ebt_802_3_hdr *frame;
> +	unsigned int hlen;
> +	__be16 type;
> +
> +	/* Without SAP or TYPE constraints the match only selects 802.3
> +	 * frames, which ebt_basic_match() has already decided from the
> +	 * ethertype/length field. Nothing in the LLC header is read.
> +	 */
> +	if (!(info->bitmask & (EBT_802_3_SAP | EBT_802_3_TYPE)))
> +		return true;
> +
> +	/* Only the dsap/ssap pair is examined for a SAP-only rule; the
> +	 * type field (and the control byte that selects between the ui
> +	 * and ni views of the union) is needed for a TYPE rule alone.
> +	 */
> +	hlen = sizeof(_frame);
> +	if (!(info->bitmask & EBT_802_3_TYPE))
> +		hlen = offsetof(struct ebt_802_3_hdr, llc.ui.ctrl);
> +
> +	/* skb->data sits past the Ethernet header on the receive path, so
> +	 * the frame starts at a negative offset from skb->data. Pass the
> +	 * mac header offset so the check follows the frame wherever the
> +	 * hook reset the mac header, and covers paged frames too. A frame
> +	 * too short to hold the LLC fields simply does not match; users
> +	 * who want such frames dropped can match 802.3 alone.
> +	 */

I don't think it is worth to add so many comments in the code for
this, the skb_header_pointer() call plus what you can retrieve via
'git annotate' is sufficient to document why this
skb_header_pointer() is needed.

> +	frame = skb_header_pointer(skb, skb_mac_offset(skb), hlen, &_frame);
> +	if (!frame)
> +		return false;
>  
>  	if (info->bitmask & EBT_802_3_SAP) {
> -		if (NF_INVF(info, EBT_802_3_SAP, info->sap != hdr->llc.ui.ssap))
> +		if (NF_INVF(info, EBT_802_3_SAP, info->sap != frame->llc.ui.ssap))
>  			return false;
> -		if (NF_INVF(info, EBT_802_3_SAP, info->sap != hdr->llc.ui.dsap))
> +		if (NF_INVF(info, EBT_802_3_SAP, info->sap != frame->llc.ui.dsap))
>  			return false;
>  	}
>  
>  	if (info->bitmask & EBT_802_3_TYPE) {
> -		if (!(hdr->llc.ui.dsap == CHECK_TYPE && hdr->llc.ui.ssap == CHECK_TYPE))
> +		type = frame->llc.ui.ctrl & IS_UI ? frame->llc.ui.type :
> +						    frame->llc.ni.type;
> +		if (!(frame->llc.ui.dsap == CHECK_TYPE &&
> +		      frame->llc.ui.ssap == CHECK_TYPE))
>  			return false;
>  		if (NF_INVF(info, EBT_802_3_TYPE, info->type != type))
>  			return false;
> -- 
> 2.50.1
>