CVE-2026-64513: KVM: x86: Unconditionally recompute CR8 intercept on PPR update

From: Greg Kroah-Hartman <[email protected]>

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

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

KVM: x86: Unconditionally recompute CR8 intercept on PPR update

The TPR_THRESHOLD field in the VMCS is used by VMX to induce VM exits
when the guest's virtual TPR falls under the specified threshold,
allowing KVM to inject previously masked interrupts.

KVM handles these VM exits in handle_tpr_below_threshold().
Commit eb90f3417a0c ("KVM: vmx: speed up TPR below threshold vmexits")
optimized this function by calling apic_update_ppr() instead of raising
KVM_REQ_EVENT. apic_update_ppr() then raises KVM_REQ_EVENT if there is
a pending, deliverable interrupt.

However, if there are no new interrupts pending, apic_update_ppr() does
not issue the request. Thus, kvm_lapic_update_cr8_intercept() and
vmx_update_cr8_intercept() are not called before VM entry, which results
in a high, stale TPR_THRESHOLD. This is problematic due to the following
sentence in 28.2.1.1 "VM-Execution Control Fields" in the SDM:

  The following check is performed if the “use TPR shadow” VM-execution
  control is 1 and the “virtualize APIC accesses” and “virtual-interrupt
  delivery” VM-execution controls are both 0: the value of bits 3:0 of
  the TPR threshold VM-execution control field should not be greater
  than the value of bits 7:4 of VTPR.

This error condition is typically not observed when KVM runs on a bare
metal system because modern processors support APICv, which enables
virtual-interrupt delivery, and which KVM uses when possible. This
causes the processor to no longer generate TPR-below-threshold exits
and to no longer check TPR_THRESHOLD on entry. However, when running
on older platforms, or under nested virtualization on a hypervisor that
does not support virtual-interrupt delivery and enforces this check
(like Hyper-V) this can cause a VM entry failure with hardware error
0x7, as seen in [1].

Call kvm_lapic_update_cr8_intercept() if apic_update_ppr() does not
find a deliverable interrupt (and thus does not raise KVM_REQ_EVENT).
Remove calls to kvm_lapic_update_cr8_intercept() on paths that end up in
apic_update_ppr(), as they now become redundant. This ensures that any
path that updates the guest's PPR also figures out if KVM needs to wait
for a TPR change (using TPR_THRESHOLD on VMX or CR8 intercepts on SVM).

The Linux kernel CVE team has assigned CVE-2026-64513 to this issue.


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

	Issue introduced in 4.11 with commit eb90f3417a0cc4880e979ccc84e41890d410ea5b and fixed in 6.18.39 with commit ff9c4c6428883182960cfe5c78928f0896d80ebc
	Issue introduced in 4.11 with commit eb90f3417a0cc4880e979ccc84e41890d410ea5b and fixed in 7.1.4 with commit 8c8e8ac22ee17d52f9eb2bc814bca7fab90fb8df
	Issue introduced in 4.11 with commit eb90f3417a0cc4880e979ccc84e41890d410ea5b and fixed in 7.2-rc1 with commit bb365a506b1e6fb050c0fceaad354fe395385ef0

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-2026-64513
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/lapic.c
	arch/x86/kvm/x86.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/ff9c4c6428883182960cfe5c78928f0896d80ebc
	https://git.kernel.org/stable/c/8c8e8ac22ee17d52f9eb2bc814bca7fab90fb8df
	https://git.kernel.org/stable/c/bb365a506b1e6fb050c0fceaad354fe395385ef0