CVE-2026-64513

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 handletprbelowthreshold(). Commit eb90f3417a0c ("KVM: vmx: speed up TPR below threshold vmexits") optimized this function by calling apicupdateppr() instead of raising KVMREQEVENT. apicupdateppr() then raises KVMREQ_EVENT if there is a pending, deliverable interrupt.

However, if there are no new interrupts pending, apicupdateppr() does not issue the request. Thus, kvmlapicupdatecr8intercept() and vmxupdatecr8intercept() are not called before VM entry, which results in a high, stale TPRTHRESHOLD. 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 kvmlapicupdatecr8intercept() if apicupdateppr() does not find a deliverable interrupt (and thus does not raise KVMREQEVENT). Remove calls to kvmlapicupdatecr8intercept() on paths that end up in apicupdateppr(), 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).