Bug 2535256 (CVE-2026-89920)

Summary:	CVE-2026-89920 kernel: KVM: s390: Fix memory corruption by not reinjecting CK machine checks
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security <prodsec-ir-bot>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in the Linux kernel's KVM (Kernel-based Virtual Machine) s390 component. This vulnerability arises from the incorrect reinjection of channel-subsystem damage machine checks into the guest virtual machine, which are intended for the host system. This misdirection can lead to memory corruption by attempting to free a stack variable. The issue is resolved by properly masking out the CK bit during machine check information backup.	Story Points:	---
Clone Of:		Environment:
Last Closed:		Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-16 11:39:37 UTC

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.

Comment 1 Keith Grant 2026-09-16 15:21:55 UTC

Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-89920.mbox