CVE-2021-47255

In the Linux kernel, the following vulnerability has been resolved: kvm:
LAPIC: Restore guard to prevent illegal APIC register access Per the SDM,
“any access that touches bytes 4 through 15 of an APIC register may cause
undefined behavior and must not be executed.” Worse, such an access in
kvm_lapic_reg_read can result in a leak of kernel stack contents. Prior to
commit 01402cf81051 (“kvm: LAPIC: write down valid APIC registers”), such
an access was explicitly disallowed. Restore the guard that was removed in
that commit.