CVE-2026-98929 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
macsec: initialize SecY before registering the netdevice
Creating a MACsec device with MAC offload over an LRO-capable lower device triggers a warning in rtmsg_ifinfo_build_skb() when IPv4 forwarding is enabled by default.
register_netdevice() invokes inetdev_init(), which disables LRO and emits a NETDEV_FEAT_CHANGE notification. This reaches macsec_fill_info() before macsec_add_dev() initializes the SecY. key_len is still zero, so macsec_fill_info() returns -EMSGSIZE and trips the WARN_ON in rtmsg_ifinfo_build_skb(), even though the skb has enough space.
Even without the warning, notifications during registration can report uninitialized SecY attributes, including the SCI. This ordering has existed since the driver was introduced.
Initialize the SecY and apply the new-link attributes before registration. Move MAC address inheritance into macsec_newlink() so the SCI can also be initialized before registration-time notifications report it. Move the per-CPU statistics and metadata destination allocation into ndo_init(), and release partial allocations on failure.
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability in question resides within the Linux kernel's implementation of MAC Security, commonly referred to as macsec, specifically affecting devices that utilize hardware offloading capabilities over lower-layer network interfaces capable of Large Receive Offload or LRO. The core technical flaw is a race condition and improper initialization ordering during the creation of a new macsec device interface. When a user attempts to create such a device with MAC offload enabled on an LRO-capable underlying device, the kernel's netdevice registration process triggers a sequence of events that exposes uninitialized data structures. Specifically, the function register_netdevice is invoked, which in turn calls inetdev_init. This initialization routine has the side effect of disabling LRO and subsequently emitting a NETDEV_FEAT_CHANGE notification to notify subsystems about feature changes.
This premature notification reaches the macsec_fill_info callback before the critical macsec_add_dev function has had the opportunity to initialize the SecY structure, which represents the security association context for the device. Because this initialization is deferred until after the registration process begins but not completed, key parameters such as key_len remain at their default zero value when macsec_fill_info attempts to populate information about the new interface. Consequently, macsec_fill_info returns an error code indicating message size mismatch, specifically -EMSGSIZE. This return value triggers a WARN_ON macro within rtmsg_ifinfo_build_skb, resulting in a kernel warning being logged even though the socket buffer actually possesses sufficient space for the data. While this primarily manifests as a noisy warning under specific conditions where IPv4 forwarding is enabled by default, it indicates a deeper issue with state consistency during interface lifecycle management.
Beyond the immediate trigger of the kernel warning, there are significant operational impacts related to information integrity and potential security implications. Even if the warning condition were bypassed or ignored, notifications generated during the registration phase would report uninitialized attributes from the SecY structure. Most notably, this includes the SCI, or Security Channel Identifier, which is a critical component for identifying secure channels in macsec configurations. Reporting uninitialized or zero-valued SCIs to network management tools and monitoring systems can lead to incorrect state tracking, potential misconfiguration of security policies, and confusion for administrators attempting to audit or manage macsec tunnels. This ordering issue has persisted since the driver was originally introduced into the kernel codebase, representing a long-standing architectural flaw in how initialization dependencies are handled during device creation.
The resolution involves restructuring the initialization sequence to ensure that all necessary data structures are fully prepared before any network notifications are emitted. The fix mandates initializing the SecY structure and applying new-link attributes prior to calling register_netdevice. This ensures that when inetdev_init triggers feature change notifications, the macsec_fill_info callback can access valid, initialized data rather than uninitialized memory or default zero values. Additionally, the logic for MAC address inheritance is moved into the macsec_newlink function, which allows the SCI to be properly calculated and initialized before registration-time notifications occur. This reordering eliminates the window where incomplete state information could be exposed to the rest of the kernel networking stack.
Furthermore, the remediation addresses resource management hygiene by moving per-CPU statistics allocation and metadata destination initialization into the ndo_init callback. This change ensures that these resources are allocated in a controlled manner consistent with other network device drivers. Crucially, the fix also implements proper error handling to release partial allocations if any step during this new initialization sequence fails. This prevents potential memory leaks or resource exhaustion scenarios that could arise from incomplete teardown procedures during failed setup attempts. By aligning the macsec driver's behavior with standard kernel expectations for netdevice registration and notification timing, the vulnerability is mitigated, ensuring stable operation without spurious warnings and accurate reporting of security parameters to management interfaces.
From a classification perspective, this issue relates to CWE-362, Concurrent Execution using Shared Resource with Improper Synchronization, as it involves accessing shared data structures during an initialization phase before they are fully prepared for concurrent access by notification handlers. It also touches upon CWE-908, Use of Uninitialized Resource, given that uninitialized SecY attributes were being reported to external observers via kernel notifications. In terms of the MITRE ATT&CK framework, while this is primarily a stability and correctness issue rather than an exploitable attack vector for privilege escalation or data exfiltration in most contexts, it falls under T1078, Valid Accounts, if one considers system-level interfaces as accounts, but more accurately aligns with techniques related to Defense Evasion through log manipulation via kernel warnings that could obscure other activities. However, its primary categorization remains within software quality and reliability defects rather than direct security exploits.
Mitigation strategies for organizations running affected Linux kernels involve applying the upstream patch or updating to a patched distribution release as soon as available. For environments where immediate patching is not feasible, administrators should monitor kernel logs for WARN_ON messages related to rtmsg_ifinfo_build_skb when creating macsec interfaces with hardware offloading enabled on LRO-capable devices. Disabling IPv4 forwarding temporarily during the creation of such interfaces may prevent the specific trigger condition from activating the warning path, although this is a workaround rather than a fix. Long-term mitigation requires ensuring that all network interface configurations adhere to proper initialization sequences and that monitoring systems are configured to filter or ignore these benign but noisy kernel warnings if they cannot be patched immediately.