Linux kernel (arch/x86/kvm): Guest-supplied array indices in the host's local-APIC emulation (an IPI destination id and

Impact

Guest-supplied array indices in the host's local-APIC emulation (an IPI destination id and a lowest-priority search index) were bounds-checked but never clamped, leaving a Spectre-v1 gadget that the guest controls. A tenant can train the bounds check and have the host speculatively read memory past those arrays, then recover it through a cache side channel - host kernel data disclosed into the VM.

Who can reach it

Guest-driven and unprivileged inside the VM: the tenant issues IPIs and APIC/MSR writes with out-of-range destination values from any vCPU. No host privilege, no device node, no VMM cooperation, and it works on both Intel and AMD hosts with in-kernel APIC emulation (the default).

What to do

Update to a stable kernel with the linked fix (no fixed release enumerated; take the branch carrying commit d51e381beed5). No practical interim control - moving APIC emulation to userspace is not a realistic production option.

References