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.
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.
Metrics
Affected Vendors & Products
Advisories
No advisories yet.
Fixes
Solution
No solution given by the vendor.
Workaround
No workaround given by the vendor.
References
History
Thu, 17 Sep 2026 16:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| 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. | |
| Title | net/sched: act_skbmod: fix length calculations and avoid invalid header warnings | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
|
Projects
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MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-09-17T16:05:56.878Z
Reserved: 2026-09-11T19:38:34.785Z
Link: CVE-2026-90078
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-09-17T17:16:57.057
Modified: 2026-09-17T17:16:57.057
Link: CVE-2026-90078
Redhat
No data.
OpenCVE Enrichment
No data.
Weaknesses
No weakness.