CVE-2026-89920 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: s390: Fix memory corruption by not reinjecting CK machine checks
Channel-subsystem damage machine checks are for the host channel subsystem. The guest channel subsystem is emulated in the userspace VMM. There is no point in forwarding such machine checks into the guest.
This also simplifies the machine check reinjection and avoids kfree of a stack variable as reported by sashiko. There might be still machine checks that have the ck bit set with another bit (like instruction damage), mask out the CK bit in s390_backup_mcck_info(), like the CP and ED bits already are.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel vulnerability identified within the KVM subsystem for IBM System z architecture involves a critical flaw in how channel-subsystem machine checks are handled during virtualization operations. Specifically, the issue centers on the incorrect reinjection of Channel-Subsystem (CK) damage machine checks into guest operating systems. In this architectural context, hardware-level events such as channel-subsystem damage machine checks are intended to be processed by the host kernel and its associated hypervisor components because they pertain directly to the physical infrastructure managing I/O operations. However, due to a logic error in the interrupt handling routine, these specific machine check exceptions were being forwarded or reinjected into the virtualized guest environment where they hold no valid meaning. This misdirection occurs because the guest channel subsystem is entirely emulated by the userspace Virtual Machine Monitor rather than existing as physical hardware within the guest's view of the system. Consequently, delivering a real hardware damage signal to an emulator that does not natively support or expect such low-level host-side integrity failures creates a fundamental mismatch in state management and exception handling protocols.
The operational impact of this flaw is severe, primarily manifesting as memory corruption within the kernel space. When the guest receives these inappropriate machine checks, it may attempt to process them using logic designed for different types of exceptions or ignore them incorrectly, leading to unpredictable behavior that can cascade into host-side instability. Furthermore, the code path responsible for reinjecting these interrupts contained a secondary flaw involving improper memory management. The implementation attempted to free a stack variable using kfree, which is strictly intended for dynamically allocated heap memory. Freeing stack-allocated data results in undefined behavior, potential kernel panics, and further memory corruption because the stack pointer remains unchanged while the memory content may be overwritten or accessed incorrectly by subsequent operations. This combination of logical misrouting and improper resource deallocation creates a high-severity vulnerability that can lead to denial of service through system crashes or potentially privilege escalation if an attacker can influence the state leading up to these checks.
From a technical classification perspective, this vulnerability aligns with CWE-416 Use After Free, given the incorrect invocation of kfree on stack memory, and CWE-787 Out-of-bounds Write, as the resulting corruption affects kernel data structures beyond their intended boundaries. In terms of adversary tactics, this flaw could be leveraged within an ATT&CK framework context under T1059 Command Scripting or T1203 Exploitation for Client Execution if a malicious guest can trigger specific conditions that force the host to process these malformed interrupts repeatedly. The vulnerability highlights the complexity of maintaining isolation boundaries between host and guest in hardware-assisted virtualization environments, particularly when dealing with legacy architecture-specific interrupt mechanisms like those found on s390 platforms where channel subsystems play a central role in I/O performance and reliability.
To mitigate this risk, the primary remediation involves modifying the KVM s390 code to explicitly filter out CK machine checks before they are considered for reinjection into the guest domain. By masking out the CK bit during the backup of machine check information, similar to how CP (Control Program) and ED (External Device) bits are already handled, the kernel ensures that only relevant and actionable interrupts reach the virtualized environment. This change not only prevents the corruption caused by misdirected exceptions but also resolves the memory management error by simplifying the reinjection logic and removing the erroneous stack variable deallocation. System administrators should apply the latest available kernel patches for s390 architectures to ensure that this logical flaw is corrected, thereby restoring proper isolation between host hardware events and guest operating system expectations. Regular auditing of virtualization configurations and keeping hypervisor software up to date are essential practices to maintain integrity in environments relying on IBM Z systems with KVM-based virtualization.