Linux EUVD-2026-81939

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.

1. 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.

1. 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.