[PATCH 5.10] netfilter: nf_log: validate MAC header was set before dumping it

[PATCH 5.10] netfilter: nf_log: validate MAC header was set before dumping it

[Alexander Martyniuk]

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>
---
Backport fix for CVE-2026-52942
 net/ipv4/netfilter/nf_log_ipv4.c | 4 ++--
 1 file changed, 2 insertions(+), 2 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;
 
-- 
2.43.0

Re: [PATCH 5.10] netfilter: nf_log: validate MAC header was set before dumping it

[Pablo Neira Ayuso]

Hi,

Thanks but why only 5.10?

On Wed, Jun 24, 2026 at 02:01:15PM +0000, 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>
> ---
> Backport fix for CVE-2026-52942
>  net/ipv4/netfilter/nf_log_ipv4.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 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;
>  
> -- 
> 2.43.0
> 

Re: [PATCH 5.10] netfilter: nf_log: validate MAC header was set before dumping it

[Pablo Neira Ayuso]

BTW, fixing Cc: to netfilter-devel@vger.kernel.org

On Wed, Jun 24, 2026 at 01:48:31PM +0200, Pablo Neira Ayuso wrote:
> Hi,
> 
> Thanks but why only 5.10?
> 
> On Wed, Jun 24, 2026 at 02:01:15PM +0000, 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>
> > ---
> > Backport fix for CVE-2026-52942
> >  net/ipv4/netfilter/nf_log_ipv4.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 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;
> >  
> > -- 
> > 2.43.0
> > 

Re: [PATCH 5.10] netfilter: nf_log: validate MAC header was set before dumping it

[Greg Kroah-Hartman]

On Wed, Jun 24, 2026 at 01:48:28PM +0200, Pablo Neira Ayuso wrote:
> Hi,
> 
> Thanks but why only 5.10?

It's already in the following releases:
	5.15.210 6.1.176 6.6.143 6.12.94 6.18.36 7.0.13 7.1

so 5.10.y seems like the only one missing it at the moment.

thanks,

greg k-h