CVE-2026-64004

Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source

AV:LAttack vector, local

The flaw is reached through the getsockopt(2) syscall on an AF_IUCV HIPER socket; the generic socket layer does not acquire the socket lock before calling iucv_sock_getsockopt(), so exploitation requires local access to the socket file descriptor.

AC:LAttack complexity, low

The attacker controls both sides of the race by running concurrent getsockopt(SO_MSGSIZE) and recvmsg()/close() threads on the same socket, making the window reliably winnable without external timing luck.

PR:LPrivileges required, low

The fix commit explicitly states any AF_IUCV HIPER user can trigger the crash, and iucv_sock_create imposes no capability or root requirement beyond creating and binding a HiperSockets AF_IUCV socket on s390.

UI:NUser interaction, none

Exploitation requires no action from any other user; the attacker races their own syscalls on a socket they control.

S:UScope, unchanged

The impact is a kernel oops or crash within the same kernel security boundary, with no cross-VM, cross-container, or IOMMU boundary crossing.

C:HConfidentiality impact, high

The unlocked TOCTOU between the hs_dev NULL check and dereference races with dev_put() freeing the net_device, creating a use-after-free read whose mtu field is copied to userspace via copy_to_user.

I:HIntegrity impact, high

Use-after-free on a freed net_device structure in kernel heap memory is a memory corruption primitive that could be further leveraged for control-flow hijacking, and per kernel CVSS guidance UAF bugs warrant High integrity impact.

A:HAvailability impact, high

Successful exploitation produces a NULL pointer dereference oops or kernel panic, causing complete loss of availability on the affected IBM Z LPAR or guest.