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

> A flaw in Linux KVM allows a guest to cause a VM entry failure and denial of service on older or nested virtualization platforms lacking APICv support.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Jul 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-93563, BELL-CVE-2026-64513, DEBIAN-CVE-2026-64513, GHSA-2vpm-g563-9g48, ROOT-OS-UBUNTU-2204-CVE-2026-64513, UBUNTU-CVE-2026-64513

## What is CVE-2026-64513?

CVE-2026-64513 is a denial of service vulnerability in Linux, published on 25 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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).

## Which versions are affected by CVE-2026-64513?


## How do I fix CVE-2026-64513?

Apply the vendor's patch.

## Is CVE-2026-64513 being exploited?

There is no known exploitation of CVE-2026-64513 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-64513 and fix it if it is affected.

What it is: CVE-2026-64513 is a denial of service vulnerability in Linux, published on 25 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-64513 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-64513/ (Markdown: https://insights.sbomify.com/CVE-2026-64513/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-64513
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-64513
- OSV: https://osv.dev/vulnerability/CVE-2026-64513

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-64513/
Summaries are generated and may contain mistakes; the official description above is the record.