CVE-2026-90078net/sched: act_skbmod: fix length calculations and avoid invalid header warnings

Official description

In the Linux kernel, the following vulnerability has been resolved: net/sched: act_skbmod: fix length calculations and avoid invalid header warnings syzbot reported a warning in skb_network_header_len() triggered by tcf_skbmod_act(): !skb_transport_header_was_set(skb) WARNING: CPU: 0 PID: 14949 at include/linux/skbuff.h:3243 skb_network_header_len include/linux/skbuff.h:3243 [inline] WARNING: CPU: 0 PID: 14949 at net/sched/act_skbmod.c:55 tcf_skbmod_act+0xfe8/0x1810 net/sched/act_skbmod.c:55 There are a few issues in tcf_skbmod_act(): 1. Calling skb_network_header_len() assumes skb->transport_header is set, which is not guaranteed when tcf_skbmod_act() runs at TC ingress. 2. Unconditionally calling skb_mac_header_len() at the beginning of tcf_skbmod_act() triggers a warning on L3 devices (e.g. TUN) where the MAC header is unset, evaluating to an underflowed garbage length. 3. On TC ingress, skb->data points to the network header. Adding the MAC header length to the IP header length causes skb_ensure_writable() to request more bytes than the actual IP packet length, dropping valid short packets (e.g. 28-byte UDP/IPv4 packets). Fix these by: - Using skb_network_offset(skb) + sizeof(struct iphdr/ipv6hdr) for SKBMOD_F_ECN so that the required length is correctly calculated on both ingress (offset == 0) and egress (offset == mac_len). - Setting max_edit_len to ETH_HLEN for Ethernet header modifications after validating ARPHRD_ETHER.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/985a37781ade19061400100c2ba0f43979dd4d63
• https://git.kernel.org/stable/c/d896843d8d925f0adbba319020595cfb1a7bcd57
• https://git.kernel.org/stable/c/0675cff3c2924eb9b80d9dd250cf56a1140af157
• https://git.kernel.org/stable/c/e8a2027b7e686784a69b2dfcee841d779919f313
• https://git.kernel.org/stable/c/1719865b20b22c88d2a55e922eff5ca31b0841f6
• https://git.kernel.org/stable/c/62126464f3a6159c0a3dd89e196ba65bfeb0afc6
• https://git.kernel.org/stable/c/81d0d1e64f30d9989c829c0953cd6e6c68d9c5fb