CVE-2026-23143

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. :)