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.