KVM: x86: Reject Hyper-V's SEND_IPI hypercalls if local APIC isn't in-kernel

commit a8de7f1 upstream.

Advertise support for Hyper-V's SEND_IPI and SEND_IPI_EX hypercalls if and
only if the local API is emulated/virtualized by KVM, and explicitly reject
said hypercalls if the local APIC is emulated in userspace, i.e. don't rely
on userspace to opt-in to KVM_CAP_HYPERV_ENFORCE_CPUID.

Rejecting SEND_IPI and SEND_IPI_EX fixes a NULL-pointer dereference if
Hyper-V enlightenments are exposed to the guest without an in-kernel local
APIC:

  dump_stack+0xbe/0xfd
  __kasan_report.cold+0x34/0x84
  kasan_report+0x3a/0x50
  __apic_accept_irq+0x3a/0x5c0
  kvm_hv_send_ipi.isra.0+0x34e/0x820
  kvm_hv_hypercall+0x8d9/0x9d0
  kvm_emulate_hypercall+0x506/0x7e0
  __vmx_handle_exit+0x283/0xb60
  vmx_handle_exit+0x1d/0xd0
  vcpu_enter_guest+0x16b0/0x24c0
  vcpu_run+0xc0/0x550
  kvm_arch_vcpu_ioctl_run+0x170/0x6d0
  kvm_vcpu_ioctl+0x413/0xb20
  __se_sys_ioctl+0x111/0x160
  do_syscal1_64+0x30/0x40
  entry_SYSCALL_64_after_hwframe+0x67/0xd1

Note, checking the sending vCPU is sufficient, as the per-VM irqchip_mode
can't be modified after vCPUs are created, i.e. if one vCPU has an
in-kernel local APIC, then all vCPUs have an in-kernel local APIC.

Reported-by: Dongjie Zou <zoudongjie@huawei.com>
Fixes: 214ff83 ("KVM: x86: hyperv: implement PV IPI send hypercalls")
Fixes: 2bc3997 ("x86/kvm/hyper-v: Introduce KVM_GET_SUPPORTED_HV_CPUID")
Cc: stable@vger.kernel.org
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20250118003454.2619573-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit aca8be4403fb90db7adaf63830e27ebe787a76e8)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>

Author: sean-jc

arch/x86/kvm/hyperv.c

@@ -2232,6 +2232,9 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
 	u32 vector;
 	bool all_cpus;
 
+	if (!lapic_in_kernel(vcpu))
+		return HV_STATUS_INVALID_HYPERCALL_INPUT;
+
 	if (hc->code == HVCALL_SEND_IPI) {
 		if (!hc->fast) {
 			if (unlikely(kvm_read_guest(kvm, hc->ingpa, &send_ipi,
@@ -2858,7 +2861,8 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
 			ent->eax |= HV_X64_REMOTE_TLB_FLUSH_RECOMMENDED;
 			ent->eax |= HV_X64_APIC_ACCESS_RECOMMENDED;
 			ent->eax |= HV_X64_RELAXED_TIMING_RECOMMENDED;
-			ent->eax |= HV_X64_CLUSTER_IPI_RECOMMENDED;
+			if (!vcpu || lapic_in_kernel(vcpu))
+				ent->eax |= HV_X64_CLUSTER_IPI_RECOMMENDED;
 			ent->eax |= HV_X64_EX_PROCESSOR_MASKS_RECOMMENDED;
 			if (evmcs_ver)
 				ent->eax |= HV_X64_ENLIGHTENED_VMCS_RECOMMENDED;