Bug 2528795 (CVE-2026-80905)

Summary: CVE-2026-80905 kernel: net: tap: fix wrong transport_header when sending VLAN-tagged frame
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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Doc Text:
A flaw was found in the kernel. When processing VLAN-tagged frames, the network header is incorrectly advanced past the VLAN tag before the transport header is probed. This misinterprets the inner protocol header as a VLAN header, resulting in an uninitialized transport header. This issue can lead to incorrect packet dissection and potentially affect network functionality.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-04 17:35:31 UTC 
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.
Comment 1 Mauro Matteo Cascella 2026-09-07 08:25:19 UTC 
Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-80905.mbox