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.
- 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
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.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-64513 being exploited?
There is no known exploitation of CVE-2026-64513 in the wild and no public exploit code.
Sources
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.