PT-2026-64734 · Linux · Linux
CVE-2026-64513
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Published
2026-07-25
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Updated
2026-07-25
None
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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).
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Related Identifiers
Affected Products
Linux