Re: [PATCH] linux: layer7: Don't linearize the skb in the match

[Michael Tremer <michael.tremer@ipfire.org>]

Hello Antonio,

Thank you very much for tracking this one down. This bug has been open for a long time and nobody could reproduce it reliably, so the detailed reproducer is very much appreciated.

The analysis is spot on: when a match returns false, ipt_do_table() goes straight to the next rule with the cached IP header pointer, and a match should never modify the skb in the first place. Reading the data with skb_header_pointer() is the right way to do this.

I have merged the patch into next, so it will be part of Core Update 207, which ships a new kernel anyway. I have also removed the old layer7 patch for 6.12 which was not used any more.

Best,
-Michael

> On 3 Nov 2026, at 22:10, Antonio Quartulli <antonio@mandelbit.com> wrote:
> 
> skb_linearize() can free the old skb head while ipt_do_table() keeps
> using its cached IP header pointer for the next rule, which KFENCE
> reports as a use-after-free when QoS is enabled. Read the application
> data without modifying the skb instead.
> 
> Fixes: #13953
> Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
> ---
> How to reproduce (no QoS configuration needed):
> 
> 1. The match must see received, non-cloned, non-linear skbs with a small
>   kmalloc'ed head, e.g. on Intel I226-V (igc), or on a VM whose red virtio
>   NIC has mergeable rx buffers disabled and receives GSO packets:
>   <driver name='vhost'><host mrg_rxbuf='off'/></driver>
> 
> 2. Add the same rules QoS creates (a layer7 match that is false for
>   unclassified connections, followed by another rule):
> 
>   iptables -t mangle -N L7T
>   iptables -t mangle -A PREROUTING -i red0 -j L7T
>   iptables -t mangle -A L7T -m layer7 --l7dir /etc/l7-protocols/protocols \
>            ! --l7proto unset -j MARK --set-xmark 0x1/0xf
>   iptables -t mangle -A L7T -j CONNMARK --save-mark --mask 0xf
> 
> 3. Make KFENCE sample often:
> 
>   echo 1 > /sys/module/kfence/parameters/sample_interval
> 
> 4. Open many short TCP connections from the firewall to a host on red that
>   sends data back (e.g. 5000 connections downloading 256 KiB each).
> 
> Without this patch, "BUG: KFENCE: use-after-free read in ipt_do_table"
> shows up within seconds (31 reports for 5000 connections on a Core 205
> VM). With it, there were none after 50000 connections, and layer7 still
> classified all connections correctly.
> 
> src/patches/linux/linux-6.18-layer7.patch | 80 ++++++++++++++++-------
> 1 file changed, 55 insertions(+), 25 deletions(-)
> 
> diff --git a/src/patches/linux/linux-6.18-layer7.patch b/src/patches/linux/linux-6.18-layer7.patch
> index a99800d62..8ecf69b57 100644
> --- a/src/patches/linux/linux-6.18-layer7.patch
> +++ b/src/patches/linux/linux-6.18-layer7.patch
> @@ -1501,7 +1501,7 @@ new file mode 100644
> index 000000000000..a7eb760123bf
> --- /dev/null
> +++ b/net/netfilter/xt_layer7.c
> -@@ -0,0 +1,666 @@
> +@@ -0,0 +1,696 @@
> +/*
> +  Kernel module to match application layer (OSI layer 7) data in connections.
> +
> @@ -1572,6 +1572,10 @@ index 000000000000..a7eb760123bf
> +
> +DEFINE_SPINLOCK(l7_lock);
> +
> ++/* Copy of the application data of the packet being matched (maxdatalen
> ++bytes), protected by l7_lock */
> ++static char *l7_scratch;
> ++
> +#ifdef CONFIG_IP_NF_MATCH_LAYER7_DEBUG
> +/* Converts an unfriendly string into a friendly one by
> +replacing unprintables with periods and all whitespace with " ". */
> @@ -1763,7 +1767,8 @@ index 000000000000..a7eb760123bf
> + return 1;
> +}
> +
> -+/* Returns offset the into the skb->data that the application data starts */
> ++/* Returns offset the into the skb->data that the application data starts,
> ++or -1 if it can't be determined */
> +static int app_data_offset(const struct sk_buff *skb)
> +{
> + /* In case we are ported somewhere (ebtables?) where ip_hdr(skb)
> @@ -1774,10 +1779,15 @@ index 000000000000..a7eb760123bf
> + /* 12 == offset into TCP header for the header length field.
> + Can't get this with skb->h.th->doff because the tcphdr
> + struct doesn't get set when routing (this is confirmed to be
> -+ true in Netfilter as well as QoS.) */
> -+ int tcp_hl = 4*(skb->data[ip_hl + 12] >> 4);
> ++ true in Netfilter as well as QoS.) The TCP header might not be
> ++ in the linear part, so don't read it from skb->data directly. */
> ++ u8 _doff, *doff;
> ++
> ++ doff = skb_header_pointer(skb, ip_hl + 12, sizeof(_doff), &_doff);
> ++ if (!doff)
> ++ return -1;
> +
> -+ return ip_hl + tcp_hl;
> ++ return ip_hl + 4 * (*doff >> 4);
> + } else if( ip_hdr(skb)->protocol == IPPROTO_UDP  ) {
> + return ip_hl + 8; /* UDP header is always 8 bytes */
> + } else if( ip_hdr(skb)->protocol == IPPROTO_ICMP ) {
> @@ -1945,6 +1955,7 @@ index 000000000000..a7eb760123bf
> + struct nf_conn *master_conntrack, *conntrack;
> + unsigned char * app_data;
> + unsigned int pattern_result, appdatalen;
> ++ int appdataoff;
> + regexp * comppattern;
> +
> + /* Be paranoid/incompetent - lock the entire match function. */
> @@ -1992,20 +2003,6 @@ index 000000000000..a7eb760123bf
> + return (pattern_result ^ info->invert);
> + }
> +
> -+ if(skb_is_nonlinear(skb)){
> -+ if(skb_linearize(skb) != 0){
> -+ if (net_ratelimit())
> -+ printk(KERN_ERR "layer7: failed to linearize "
> -+ "packet, bailing.\n");
> -+ spin_unlock_bh(&l7_lock);
> -+ return info->invert;
> -+ }
> -+ }
> -+
> -+ /* now that the skb is linearized, it's safe to set these. */
> -+ app_data = skb->data + app_data_offset(skb);
> -+ appdatalen = skb_tail_pointer(skb) - app_data;
> -+
> + /* the return value gets checked later, when we're ready to use it */
> + comppattern = compile_and_cache(info->pattern, info->protocol);
> +
> @@ -2028,6 +2025,25 @@ index 000000000000..a7eb760123bf
> + if(!skb->layer7_flags[0]){
> + int newbytes;
> + master_conntrack->layer7.packets++;
> ++
> ++ /* Don't linearize the skb to get at the application data: a
> ++ match must not modify the skb, because ipt_do_table() keeps
> ++ using pointers into its old head, which skb_linearize() might
> ++ free. Copy the data into l7_scratch instead if it is not in
> ++ the linear part. */
> ++ appdataoff = app_data_offset(skb);
> ++ if(appdataoff < 0 || appdataoff > skb->len){
> ++ app_data = NULL;
> ++ appdatalen = 0;
> ++ } else {
> ++ appdatalen = min_t(unsigned int, skb->len - appdataoff,
> ++   maxdatalen);
> ++ app_data = skb_header_pointer(skb, appdataoff,
> ++      appdatalen, l7_scratch);
> ++ if(!app_data)
> ++ appdatalen = 0;
> ++ }
> ++
> + newbytes = add_data(master_conntrack, app_data, appdatalen);
> + if(newbytes == 0) { /* didn't add any data */
> + skb->layer7_flags[0] = 1;
> @@ -2137,11 +2153,9 @@ index 000000000000..a7eb760123bf
> +
> +static int __init xt_layer7_init(void)
> +{
> -+// need_conntrack();
> ++ int ret;
> +
> -+ // Register proc interface
> -+ proc_create_data("layer7_numpackets", 0644,
> -+ init_net.proc_net, &layer7_numpackets_proc_fops, NULL);
> ++// need_conntrack();
> +
> + if(maxdatalen < 1) {
> + printk(KERN_WARNING "layer7: maxdatalen can't be < 1, "
> @@ -2155,14 +2169,30 @@ index 000000000000..a7eb760123bf
> + "using 65536\n");
> + maxdatalen = 65536;
> + }
> -+ return xt_register_matches(xt_layer7_match,
> -+   ARRAY_SIZE(xt_layer7_match));
> ++
> ++ l7_scratch = kmalloc(maxdatalen, GFP_KERNEL);
> ++ if(!l7_scratch)
> ++ return -ENOMEM;
> ++
> ++ // Register proc interface
> ++ proc_create_data("layer7_numpackets", 0644,
> ++ init_net.proc_net, &layer7_numpackets_proc_fops, NULL);
> ++
> ++ ret = xt_register_matches(xt_layer7_match,
> ++  ARRAY_SIZE(xt_layer7_match));
> ++ if(ret) {
> ++ remove_proc_entry("layer7_numpackets", init_net.proc_net);
> ++ kfree(l7_scratch);
> ++ }
> ++
> ++ return ret;
> +}
> +
> +static void __exit xt_layer7_fini(void)
> +{
> + remove_proc_entry("layer7_numpackets", init_net.proc_net);
> + xt_unregister_matches(xt_layer7_match, ARRAY_SIZE(xt_layer7_match));
> ++ kfree(l7_scratch);
> +}
> +
> +module_init(xt_layer7_init);
> -- 
> 2.56.0
> 
>