Re: [PATCH nf] netfilter: nf_log: validate MAC header was set before dumping it

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Sun, Jun 07, 2026 at 05:11:24PM -0700, Xiang Mei wrote:
> The fallback path of dump_mac_header() guards the MAC header access
> only with "skb->mac_header != skb->network_header", without checking
> skb_mac_header_was_set().  When the MAC header is unset, mac_header is
> 0xffff, so the test passes and skb_mac_header(skb) returns
> skb->head + 0xffff, ~64 KiB past the buffer; the loop then reads
> dev->hard_header_len bytes out of bounds into the kernel log.
> 
> This is reachable via the netdev logger: nf_log_unknown_packet() calls
> dump_mac_header() unconditionally, and an skb sent through AF_PACKET
> with PACKET_QDISC_BYPASS reaches the egress hook with mac_header still
> unset (__dev_queue_xmit(), which would reset it, is bypassed).
> 
> Add the skb_mac_header_was_set() check the ARPHRD_ETHER path already
> uses.  Only skbs with an unset MAC header are affected; valid ones are
> dumped as before.
> 
>  BUG: KASAN: slab-out-of-bounds in dump_mac_header (net/netfilter/nf_log_syslog.c:831)
>  Read of size 1 at addr ffff88800ea49d3f by task exploit/148
>  Call Trace:
>   kasan_report (mm/kasan/report.c:595)
>   dump_mac_header (net/netfilter/nf_log_syslog.c:831)
>   nf_log_netdev_packet (net/netfilter/nf_log_syslog.c:938 net/netfilter/nf_log_syslog.c:963)
>   nf_log_packet (net/netfilter/nf_log.c:260)
>   nft_log_eval (net/netfilter/nft_log.c:60)
>   nft_do_chain (net/netfilter/nf_tables_core.c:285)
>   nft_do_chain_netdev (net/netfilter/nft_chain_filter.c:307)
>   nf_hook_slow (net/netfilter/core.c:619)
>   nf_hook_direct_egress (net/packet/af_packet.c:257)
>   packet_xmit (net/packet/af_packet.c:280)
>   packet_sendmsg (net/packet/af_packet.c:3114)
>   __sys_sendto (net/socket.c:2265)
> 
> Fixes: 7eb9282cd0ef ("netfilter: ipt_LOG/ip6t_LOG: add option to print decoded MAC header")
> Reported-by: Weiming Shi <bestswngs@gmail.com>
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Xiang Mei <xmei5@asu.edu>
> ---
>  net/netfilter/nf_log_syslog.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/nf_log_syslog.c b/net/netfilter/nf_log_syslog.c
> index 7a8952b049d1..ed5283fb6b67 100644
> --- a/net/netfilter/nf_log_syslog.c
> +++ b/net/netfilter/nf_log_syslog.c
> @@ -815,7 +815,7 @@ static void dump_mac_header(struct nf_log_buf *m,
>  
>  fallback:
>  	nf_log_buf_add(m, "MAC=");
> -	if (dev->hard_header_len &&
> +	if (dev->hard_header_len && skb_mac_header_was_set(skb) &&
>  	    skb->mac_header != skb->network_header) {

Maybe this instead?

+           skb_mac_header_was_set(skb) &&
+           skb_mac_header_len(skb) != 0) {

>  		const unsigned char *p = skb_mac_header(skb);
>  		unsigned int i;
> -- 
> 2.43.0
>