KVM: VMX: Fix stale PID-pointer table entry left after vCPU free

vCPU creation in kvm_vm_ioctl_create_vcpu() may fail after
kvm_arch_vcpu_create() -> vmx_vcpu_create() already succeeded. In such
case kvm_vm_ioctl_create_vcpu() destroys the newly created vCPU in the
failure path. However, that leaves a side effect: the IPIv pid_table
entry remains configured with this vCPU's pi_desc address. As a result,
when another vCPU sends an IPI to the APIC ID of this failed-to-create
vCPU, it will cause HW to write to this (freed!) pi_desc memory. [*]

Fix this by clearing the pid_table entry when destroying the vCPU.

Note that the same issue exists for SVM AVIC as well [1], to be fixed.

[*] Although, since this memory is freed into the kvm_vcpu_cache kmem
    cache which is only used for allocating kvm_vcpus, _maybe_ this
    memory will only be reused for pi_desc of another vCPU, not for
    anything else. So _maybe_ this will only result in delivering the
    IPI to a wrong vCPU (possibly of another VM) in the worst case, not
    in a random corruption of kernel memory.

Link: https://lore.kernel.org/kvm/al4rNqpBYy8FGKPw@blrnaveerao1 [1]
Signed-off-by: Dmytro Maluka <[email protected]>
Reviewed-by: Kai Huang <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

arch/x86/kvm/vmx/vmx.c

@@ -7681,6 +7681,9 @@ void vmx_vcpu_free(struct kvm_vcpu *vcpu)
 	nested_vmx_free_vcpu(vcpu);
 	free_loaded_vmcs(vmx->loaded_vmcs);
 	free_page((unsigned long)vmx->ve_info);
+
+	if (vmx_can_use_ipiv(vcpu))
+		WRITE_ONCE(to_kvm_vmx(vcpu->kvm)->pid_table[vcpu->vcpu_id], 0);
 }
 
 int vmx_vcpu_create(struct kvm_vcpu *vcpu)