KVM was using arrays of size KVM_MAX_VCPUS with vcpu_id, but ID can be
bigger that the maximal number of VCPUs, resulting in out-of-bounds
access.

Found by syzkaller:

  BUG: KASAN: slab-out-of-bounds in __apic_accept_irq+0xb33/0xb50 at addr [...]
  Write of size 1 by task a.out/27101
  CPU: 1 PID: 27101 Comm: a.out Not tainted 4.9.0-rc5+ #49
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
   [...]
  Call Trace:
   [...] __apic_accept_irq+0xb33/0xb50 arch/x86/kvm/lapic.c:905
   [...] kvm_apic_set_irq+0x10e/0x180 arch/x86/kvm/lapic.c:495
   [...] kvm_irq_delivery_to_apic+0x732/0xc10 arch/x86/kvm/irq_comm.c:86
   [...] ioapic_service+0x41d/0x760 arch/x86/kvm/ioapic.c:360
   [...] ioapic_set_irq+0x275/0x6c0 arch/x86/kvm/ioapic.c:222
   [...] kvm_ioapic_inject_all arch/x86/kvm/ioapic.c:235
   [...] kvm_set_ioapic+0x223/0x310 arch/x86/kvm/ioapic.c:670
   [...] kvm_vm_ioctl_set_irqchip arch/x86/kvm/x86.c:3668
   [...] kvm_arch_vm_ioctl+0x1a08/0x23c0 arch/x86/kvm/x86.c:3999
   [...] kvm_vm_ioctl+0x1fa/0x1a70 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3099

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Fixes: af1bae54

 ("KVM: x86: bump KVM_MAX_VCPU_ID to 1023")
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>