CVE-2026-43483: KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

KVM: SVM: Set/clear CR8 write interception when AVIC is (de)activated

Explicitly set/clear CR8 write interception when AVIC is (de)activated to
fix a bug where KVM leaves the interception enabled after AVIC is
activated.  E.g. if KVM emulates INIT=>WFS while AVIC is deactivated, CR8
will remain intercepted in perpetuity.

On its own, the dangling CR8 intercept is "just" a performance issue, but
combined with the TPR sync bug fixed by commit d02e48830e3f ("KVM: SVM:
Sync TPR from LAPIC into VMCB::V_TPR even if AVIC is active"), the danging
intercept is fatal to Windows guests as the TPR seen by hardware gets
wildly out of sync with reality.

Note, VMX isn't affected by the bug as TPR_THRESHOLD is explicitly ignored
when Virtual Interrupt Delivery is enabled, i.e. when APICv is active in
KVM's world.  I.e. there's no need to trigger update_cr8_intercept(), this
is firmly an SVM implementation flaw/detail.

WARN if KVM gets a CR8 write #VMEXIT while AVIC is active, as KVM should
never enter the guest with AVIC enabled and CR8 writes intercepted.

[Squash fix to avic_deactivate_vmcb. - Paolo]

The Linux kernel CVE team has assigned CVE-2026-43483 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.7 with commit 3bbf3565f48ce3999b5a12cde946f81bd4475312 and fixed in 6.1.167 with commit a4123fe5d9122eef9852e4921f7cc463420f30d4
	Issue introduced in 4.7 with commit 3bbf3565f48ce3999b5a12cde946f81bd4475312 and fixed in 6.6.130 with commit 816fa1dfae4532e851b1fe6b2434c753ecbd86c7
	Issue introduced in 4.7 with commit 3bbf3565f48ce3999b5a12cde946f81bd4475312 and fixed in 6.12.78 with commit 01651e7751edbbc0fb4598f8367a3dabcfc8c182
	Issue introduced in 4.7 with commit 3bbf3565f48ce3999b5a12cde946f81bd4475312 and fixed in 6.18.19 with commit ba3bca40f9f25c053f69413e5f4a41dd0fd762bf
	Issue introduced in 4.7 with commit 3bbf3565f48ce3999b5a12cde946f81bd4475312 and fixed in 6.19.9 with commit 737410b32bd615b321da4fbeda490351b9af5e8b
	Issue introduced in 4.7 with commit 3bbf3565f48ce3999b5a12cde946f81bd4475312 and fixed in 7.0 with commit 87d0f901a9bd8ae6be57249c737f20ac0cace93d

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-43483
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/x86/kvm/svm/avic.c
	arch/x86/kvm/svm/svm.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a4123fe5d9122eef9852e4921f7cc463420f30d4
	https://git.kernel.org/stable/c/816fa1dfae4532e851b1fe6b2434c753ecbd86c7
	https://git.kernel.org/stable/c/01651e7751edbbc0fb4598f8367a3dabcfc8c182
	https://git.kernel.org/stable/c/ba3bca40f9f25c053f69413e5f4a41dd0fd762bf
	https://git.kernel.org/stable/c/737410b32bd615b321da4fbeda490351b9af5e8b
	https://git.kernel.org/stable/c/87d0f901a9bd8ae6be57249c737f20ac0cace93d