CVE-2026-80905

Description

In the Linux kernel, the following vulnerability has been resolved: net: tap: fix wrong transport_header when sending VLAN-tagged frame In tap_get_user_xdp(), when processing a VLAN-tagged frame (e.g. ETH_P_8021Q), skb_set_network_header() is called first to advance network_header past the VLAN tag to the inner protocol header. skb_probe_transport_header() is then called with skb->protocol still set to ETH_P_8021Q, while nhoff (derived from skb_network_offset()) already points past the VLAN tag to the inner protocol header. In __skb_flow_dissect(), proto is initialized to ETH_P_8021Q and nhoff points past the VLAN tag. When the dissector hits case ETH_P_8021Q, it reads a struct vlan_hdr at the current nhoff via __skb_header_pointer(), but that offset contains the inner protocol header (e.g. an IP header). The bytes are misinterpreted as a VLAN header, yielding a garbage encapsulated EtherType that matches no known protocol. The dissector returns false, so skb_probe_transport_header() never calls skb_set_transport_header(), leaving transport_header at its uninitialized sentinel value (~0U). Move skb_set_network_header() to after skb_probe_transport_header(). At the time skb_probe_transport_header() is called, network_header still points to the VLAN header (offset ETH_HLEN), so nhoff is correct and the flow dissector can parse the VLAN header, extract the inner EtherType, and advance nhoff to the inner protocol header, allowing transport_header to be set correctly.

Predictions

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

References

• https://git.kernel.org/stable/c/5ffaa5d7f56ab24a8e23cf131eadfef31a3bbc4b
• https://git.kernel.org/stable/c/88b79ac89ecc04d7f2613f7e1c0b46f0c4ddb2f3
• https://git.kernel.org/stable/c/cbb35cbe8db268fefe34c23df15348cf99025298