KVM: x86: fix out-of-bounds access in lapic

Cluster xAPIC delivery incorrectly assumed that dest_id <= 0xff.
With enabled KVM_X2APIC_API_USE_32BIT_IDS in KVM_CAP_X2APIC_API, a
userspace can send an interrupt with dest_id that results in
out-of-bounds access.

Found by syzkaller:

  BUG: KASAN: slab-out-of-bounds in kvm_irq_delivery_to_apic_fast+0x11fa/0x1210 at addr ffff88003d9ca750
  Read of size 8 by task syz-executor/22923
  CPU: 0 PID: 22923 Comm: syz-executor Not tainted 4.9.0-rc4+ #49
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
   [...]
  Call Trace:
   [...] __dump_stack lib/dump_stack.c:15
   [...] dump_stack+0xb3/0x118 lib/dump_stack.c:51
   [...] kasan_object_err+0x1c/0x70 mm/kasan/report.c:156
   [...] print_address_description mm/kasan/report.c:194
   [...] kasan_report_error mm/kasan/report.c:283
   [...] kasan_report+0x231/0x500 mm/kasan/report.c:303
   [...] __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:329
   [...] kvm_irq_delivery_to_apic_fast+0x11fa/0x1210 arch/x86/kvm/lapic.c:824
   [...] kvm_irq_delivery_to_apic+0x132/0x9a0 arch/x86/kvm/irq_comm.c:72
   [...] kvm_set_msi+0x111/0x160 arch/x86/kvm/irq_comm.c:157
   [...] kvm_send_userspace_msi+0x201/0x280 arch/x86/kvm/../../../virt/kvm/irqchip.c:74
   [...] kvm_vm_ioctl+0xba5/0x1670 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3015
   [...] vfs_ioctl fs/ioctl.c:43
   [...] do_vfs_ioctl+0x18c/0x1040 fs/ioctl.c:679
   [...] SYSC_ioctl fs/ioctl.c:694
   [...] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
   [...] entry_SYSCALL_64_fastpath+0x1f/0xc2
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Fixes: e45115b6 ("KVM: x86: use physical LAPIC array for logical x2APIC")
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/lapic.c

@@ -138,7 +138,7 @@ static inline bool kvm_apic_map_get_logical_dest(struct kvm_apic_map *map,
 		*mask = dest_id & 0xff;
 		return true;
 	case KVM_APIC_MODE_XAPIC_CLUSTER:
-		*cluster = map->xapic_cluster_map[dest_id >> 4];
+		*cluster = map->xapic_cluster_map[(dest_id >> 4) & 0xf];
 		*mask = dest_id & 0xf;
 		return true;
 	default: