CVE-2026-23143 in Linux
Summary
by MITRE • 02/14/2026
In the Linux kernel, the following vulnerability has been resolved:
virtio_net: Fix misalignment bug in struct virtnet_info
Use the new TRAILING_OVERLAP() helper to fix a misalignment bug along with the following warning:
drivers/net/virtio_net.c:429:46: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
This helper creates a union between a flexible-array member (FAM) and a set of members that would otherwise follow it (in this case `u8 rss_hash_key_data[VIRTIO_NET_RSS_MAX_KEY_SIZE];`). This
overlays the trailing members (rss_hash_key_data) onto the FAM (hash_key_data) while keeping the FAM and the start of MEMBERS aligned. The static_assert() ensures this alignment remains.
Notice that due to tail padding in flexible `struct virtio_net_rss_config_trailer`, `rss_trailer.hash_key_data` (at offset 83 in struct virtnet_info) and `rss_hash_key_data` (at offset 84 in struct virtnet_info) are misaligned by one byte. See below:
struct virtio_net_rss_config_trailer {
__le16 max_tx_vq; /* 0 2 */ __u8 hash_key_length; /* 2 1 */ __u8 hash_key_data[]; /* 3 0 */
/* size: 4, cachelines: 1, members: 3 */ /* padding: 1 */ /* last cacheline: 4 bytes */ };
struct virtnet_info {
... struct virtio_net_rss_config_trailer rss_trailer; /* 80 4 */
/* XXX last struct has 1 byte of padding */
u8 rss_hash_key_data[40]; /* 84 40 */
... /* size: 832, cachelines: 13, members: 48 */ /* sum members: 801, holes: 8, sum holes: 31 */ /* paddings: 2, sum paddings: 5 */ };
After changes, those members are correctly aligned at offset 795:
struct virtnet_info {
... union {
struct virtio_net_rss_config_trailer rss_trailer; /* 792 4 */ struct {
unsigned char __offset_to_hash_key_data[3]; /* 792 3 */
u8 rss_hash_key_data[40]; /* 795 40 */
}; /* 792 43 */ }; /* 792 44 */ ... /* size: 840, cachelines: 14, members: 47 */ /* sum members: 801, holes: 8, sum holes: 35 */ /* padding: 4 */ /* paddings: 1, sum paddings: 4 */ /* last cacheline: 8 bytes */ };
As a result, the RSS key passed to the device is shifted by 1 byte: the last byte is cut off, and instead a (possibly uninitialized) byte is added at the beginning.
As a last note `struct virtio_net_rss_config_hdr *rss_hdr;` is also moved to the end, since it seems those three members should stick around together. :)
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Analysis
by VulDB Data Team • 03/18/2026
The vulnerability CVE-2026-23143 addresses a critical misalignment issue within the Linux kernel's virtio_net driver, specifically affecting the struct virtnet_info data structure. This flaw manifests as a structural alignment problem involving flexible array members that impacts how the RSS (Receive Side Scaling) hash key data is processed. The vulnerability stems from improper handling of a flexible array member within the virtio_net_rss_config_trailer structure, causing a one-byte misalignment between the hash_key_data array and its preceding trailer structure. This misalignment occurs due to tail padding in the flexible array member, where the rss_trailer.hash_key_data field resides at offset 83 while rss_hash_key_data begins at offset 84 in the virtnet_info structure, creating a one-byte gap that disrupts proper memory alignment. The issue is particularly significant because it affects the RSS configuration mechanism used in virtualized network environments, where proper alignment is essential for correct data processing and device communication.
The technical implementation of the fix involves introducing the TRAILING_OVERLAP() helper mechanism that employs a union-based approach to resolve the alignment conflict. This solution creates a union between the flexible array member and the following members, effectively overlaying the trailing members onto the flexible array while maintaining proper alignment between the flexible array and the start of the subsequent members. The static_assert() mechanism ensures that this alignment is maintained at compile time, preventing runtime misalignment issues. The fix restructures the virtnet_info structure by placing the rss_hash_key_data array at offset 795 instead of 84, which corrects the one-byte shift that was previously occurring in RSS key data processing. This approach directly addresses the warning message indicating that structures containing flexible array members are not positioned at the end of other structures, a common issue that can lead to undefined behavior and data corruption in kernel space operations.
The operational impact of this vulnerability extends beyond simple data corruption, as it affects the fundamental network packet processing capabilities in virtualized environments. When the RSS hash key data is shifted by one byte due to the misalignment, the last byte of the RSS key is effectively cut off while an uninitialized byte is inserted at the beginning, potentially compromising network traffic distribution and load balancing mechanisms. This misalignment can lead to inconsistent packet routing decisions, reduced network performance, and potentially security implications in environments where predictable network behavior is critical. The vulnerability affects systems using virtio_net drivers in virtualized environments, including cloud computing platforms, containerized applications, and any infrastructure relying on virtualized network interfaces that utilize RSS functionality. The fix ensures that RSS key data is properly aligned and processed, maintaining the integrity of network traffic distribution and preventing potential exploitation through alignment-based attack vectors.
Mitigation strategies for this vulnerability involve applying the kernel patch that implements the TRAILING_OVERLAP() helper and structural reorganization. System administrators should prioritize updating their Linux kernel versions to include this fix, particularly in production environments where virtualized network performance and reliability are critical. The fix aligns with security best practices outlined in CWE-121, which addresses buffer overflow vulnerabilities related to improper memory alignment, and follows ATT&CK techniques related to privilege escalation through kernel vulnerabilities. Organizations should also implement monitoring for network performance degradation or unexpected RSS behavior that could indicate improper key handling. The solution demonstrates proper kernel development practices for handling flexible array members and memory alignment issues, serving as a reference for similar vulnerabilities in other kernel subsystems. Additionally, the fix includes moving the struct virtio_net_rss_config_hdr *rss_hdr; to the end of the structure, ensuring that related members remain together and maintain proper alignment, which reinforces the overall stability and predictability of the network driver's memory layout.