KVM: x86: Add LAPIC guard in kvm_apic_write_nodecode()

KVM: x86: Add LAPIC guard in kvm_apic_write_nodecode()

kvm_apic_write_nodecode() dereferences vcpu->arch.apic without first
checking whether the in-kernel LAPIC has been initialized.  If it has
not (e.g. the vCPU was created without an in-kernel LAPIC), the
dereference results in a NULL pointer access.

While APIC-write VM-Exits are not expected to occur on a vCPU without
an in-kernel LAPIC, kvm_apic_write_nodecode() should be robust against
such a scenario as a defense-in-depth measure, e.g. to guard against
KVM bugs or CPU errata that could generate a spurious APIC-write
VM-Exit.

Use KVM_BUG_ON() with lapic_in_kernel() instead of a simple
WARN_ON_ONCE(), as suggested by Sean Christopherson, so that KVM
kills the VM outright rather than letting it continue in a broken
state.

Found by a VMCS-targeted fuzzer based on syzkaller.

Signed-off-by: xuanqingshi <1356292400@qq.com>
Link: https://patch.msgid.link/tencent_7A9F1B4D75468C0CF5DE1B6902038C948B07@qq.com
Signed-off-by: Sean Christopherson <seanjc@google.com>