Bug 2481992 (CVE-2026-46076)

Summary:	CVE-2026-46076 kernel: KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1
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
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	A flaw was found in the Kernel-based Virtual Machine (KVM) nSVM module of the Linux kernel. This vulnerability occurs when an unhandled VMMCALL is not properly intercepted by the Level 1 (L1) hypervisor. A malicious Level 2 (L2) guest operating system could exploit this by making specific hypercalls, leading to an Undefined Opcode exception (#UD) in the host. This could potentially result in a denial of service for the host system.	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 2026-05-27 15:06:00 UTC

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

KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1

Explicitly synthesize a #UD for VMMCALL if L2 is active, L1 does NOT want
to intercept VMMCALL, nested_svm_l2_tlb_flush_enabled() is true, and the
hypercall is something other than one of the supported Hyper-V hypercalls.
When all of the above conditions are met, KVM will intercept VMMCALL but
never forward it to L1, i.e. will let L2 make hypercalls as if it were L1.

The TLFS says a whole lot of nothing about this scenario, so go with the
architectural behavior, which says that VMMCALL #UDs if it's not
intercepted.

Opportunistically do a 2-for-1 stub trade by stub-ifying the new API
instead of the helpers it uses.  The last remaining "single" stub will
soon be dropped as well.

[sean: rewrite changelog and comment, tag for stable, remove defunct stubs]

Comment 1 Keith Grant 2026-05-27 17:38:31 UTC

Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026052759-CVE-2026-46076-699a@gregkh/T

Comment 5 Jon Orris 2026-09-28 17:45:04 UTC

This issue has been addressed in the following products:

  Red Hat Enterprise Linux 10

Via RHSA-2026:72624 https://access.redhat.com/errata/RHSA-2026:72624