CVE-2026-98554: vlan: ensure sufficient headroom in vlan_dev_hard_header()
In the Linux kernel, the following vulnerability has been resolved:
vlan: ensure sufficient headroom in vlandevhardheader()
Callers that only reserve ETHHLEN or less (such as llcallocframe()), or skbs allocated before dynamic device/headroom changes (e.g. toggling VLANFLAGREORDERHDR or bonding/team switching slaves), can reach vlandevhardheader() with insufficient headroom and trigger skbunderpanic().
Use skbcowhead() in vlandevhardheader() when VLANFLAGREORDERHDR is not set to ensure sufficient headroom for the VLAN header(s) and the underlying device hard header.
Use READONCE() to read dev->hardheaderlen and dev->neededheadroom as they can be updated concurrently under RTNL (e.g. in vlantransferfeatures()) while vlandevhardheader() runs locklessly on the transmit path. Also avoid LLRESERVEDSPACE(dev) here so that the extra HHDATAMOD alignment padding does not trigger unnecessary pskbexpandhead() reallocations on inner stacked VLAN devices after the outer VLAN header has been pushed.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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Fixed in 6.6.159.1-1 - Upgrade
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Event History
Frequently Asked Questions
What conditions can trigger the failure?
The transmit path can reach vlan_dev_hard_header() with too little skb headroom when callers reserve only ETH_HLEN or less, or when an skb was allocated before device headroom settings changed. Examples include llc_alloc_frame(), toggling VLAN_FLAG_REORDER_HDR, and bonding or team slave changes.
What is the observable impact?
Insufficient headroom can trigger skb_under_panic(). The provided information identifies this as a kernel-side failure during VLAN header construction.
Can concurrent network-device configuration changes contribute to exposure?
Yes. dev->hard_header_len and dev->needed_headroom can be updated under RTNL while vlan_dev_hard_header() runs locklessly on the transmit path, including during vlan_transfer_features().
What mitigation is described when applying the fix?
The fix uses skb_cow_head() when VLAN_FLAG_REORDER_HDR is not set, ensuring room for VLAN headers and the underlying device hard header. It also reads headroom-related device fields with READ_ONCE() to account for concurrent updates.