Bug 2502399 (CVE-2026-63937)

Summary:	CVE-2026-63937 kernel: KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer
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:	rhel-process-autobot, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
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Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in the Linux kernel's Kernel-based Virtual Machine (KVM) with Secure Encrypted Virtualization (SEV). This time-of-check to time-of-use (TOCTOU) vulnerability exists when KVM processes the guest-accessible Page State Change (PSC) buffer. A misbehaving guest could exploit this timing issue by modifying the buffer after it has been checked but before it is used, potentially leading to an impact on the integrity of the virtualized environment.	Story Points:	---
Clone Of:		Environment:
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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 2026-07-19 16:06:06 UTC

In the Linux kernel, the following vulnerability has been resolved:

KVM: SEV: Use READ_ONCE() when reading entries/indices from PSC buffer

Use READ_ONCE() when reading entries/indices from the guest-accessible
Page State Change buffer to defend against TOCTOU bugs.

Don't bother with READ_ONCE()/WRITE_ONCE() for cases where KVM is writing
(and not consuming the result!), as the guest isn't supposed to touch the
buffer while it's being processed.  I.e. using READ_ONCE() is all about
protecting against misbehaving guests.

Comment 1 Keith Grant 2026-07-21 21:48:39 UTC

Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026071951-CVE-2026-63937-80da@gregkh/T