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

commit a8de7f100bb5989d9c3627d3a223ee1c863f3b69 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: 214ff83d ("KVM: x86: hyperv: implement PV IPI send hypercalls")
Fixes: 2bc39970 ("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>