Re: [PATCH v2] netfilter: nf_log: validate MAC header was set before dumping it

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Jun 25, 2026 at 07:47:55PM +0300, Alexander Martyniuk wrote:
> From: Xiang Mei <xmei5@asu.edu>
> 
> commit a84b6fedbc97078788be78dbdd7517d143ad1a77 upstream.
> 
> 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, and replace the open-coded MAC header length test with
> skb_mac_header_len(). 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>
> Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
> Signed-off-by: Alexander Martyniuk <alexevgmart@gmail.com>
> ---
>  net/ipv4/netfilter/nf_log_ipv4.c | 4 ++--
>  net/ipv6/netfilter/nf_log_ipv6.c | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/netfilter/nf_log_ipv4.c b/net/ipv4/netfilter/nf_log_ipv4.c
> index d07583fac8f8..d6164e8e2c73 100644
> --- a/net/ipv4/netfilter/nf_log_ipv4.c
> +++ b/net/ipv4/netfilter/nf_log_ipv4.c
> @@ -296,8 +296,8 @@ static void dump_ipv4_mac_header(struct nf_log_buf *m,
>  
>  fallback:
>  	nf_log_buf_add(m, "MAC=");
> -	if (dev->hard_header_len &&
> -	    skb->mac_header != skb->network_header) {
> +	if (dev->hard_header_len && skb_mac_header_was_set(skb) &&
> +	    skb_mac_header_len(skb) != 0) {
>  		const unsigned char *p = skb_mac_header(skb);
>  		unsigned int i;
>  
> diff --git a/net/ipv6/netfilter/nf_log_ipv6.c b/net/ipv6/netfilter/nf_log_ipv6.c
> index 8210ff34ed9b..cc724870a467 100644
> --- a/net/ipv6/netfilter/nf_log_ipv6.c
> +++ b/net/ipv6/netfilter/nf_log_ipv6.c
> @@ -309,8 +309,8 @@ static void dump_ipv6_mac_header(struct nf_log_buf *m,
>  
>  fallback:
>  	nf_log_buf_add(m, "MAC=");
> -	if (dev->hard_header_len &&
> -	    skb->mac_header != skb->network_header) {
> +	if (dev->hard_header_len && skb_mac_header_was_set(skb) &&
> +	    skb_mac_header_len(skb) != 0) {
>  		const unsigned char *p = skb_mac_header(skb);
>  		unsigned int len = dev->hard_header_len;
>  		unsigned int i;
> -- 
> 2.43.0
> 
> 

What kernel(s) is this for?