Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access

[Radim Krcmar]

2018-08-29 15:55+0200, Radim Krcmar:
> 2018-08-29 13:43+0300, Liran Alon:
> > Why is “min” defined as “int” instead of “unsigned int”?
> > It represents the lowest APIC ID in bitmap so it can’t be negative…
> 
> Right,
> 
> I think the code would look better as something like (untested):
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 0cefba28c864..24fc84eb97d2 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -548,7 +548,7 @@ int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct 
> kvm_lapic_irq *irq,
>  }
>  
>  int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
> -                 unsigned long ipi_bitmap_high, int min,
> +                 unsigned long ipi_bitmap_high, u32 min,
>                   unsigned long icr, int op_64_bit)
>  {
>       int i;
> @@ -557,6 +557,7 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long 
> ipi_bitmap_low,
>       struct kvm_lapic_irq irq = {0};
>       int cluster_size = op_64_bit ? 64 : 32;
>       int count = 0;
> +     unsigned long ipi_bitmap[2] = {ipi_bitmap_low, ipi_bitmap_high};

The patch is wrong, I missed the 32/64 bit cluster size.

It's salvageable with something like

  if (op_64_bit) {
        ipi_bitmap[0] = ipi_bitmap_low;
        ipi_bitmap[1] = ipi_bitmap_high;
        ipi_bitmap_size = 128;
  } else {
        ipi_bitmap[0] = (u32)ipi_bitmap_low | ipi_bitmap_high << 32;
        ipi_bitmap_size = 64;
  }

>  
>       irq.vector = icr & APIC_VECTOR_MASK;
>       irq.delivery_mode = icr & APIC_MODE_MASK;
> @@ -571,16 +572,14 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long 
> ipi_bitmap_low,
>       rcu_read_lock();
>       map = rcu_dereference(kvm->arch.apic_map);
>  
> -     /* Bits above cluster_size are masked in the caller.  */
> -     for_each_set_bit(i, &ipi_bitmap_low, BITS_PER_LONG) {
> -             vcpu = map->phys_map[min + i]->vcpu;
> -             count += kvm_apic_set_irq(vcpu, &irq, NULL);
> -     }
> +     if (min <= map->max_apic_id) {
> +             size_t ipi_bitmap_size = MIN(sizeof(ipi_bitmap) * 8,
> +                                          map->max_apic_id - min + 1);
> -     min += cluster_size;
> -     for_each_set_bit(i, &ipi_bitmap_high, BITS_PER_LONG) {
> -             vcpu = map->phys_map[min + i]->vcpu;
> -             count += kvm_apic_set_irq(vcpu, &irq, NULL);
> +             for_each_set_bit(i, ipi_bitmap, ipi_bitmap_size) {

and

  ... MIN(ipi_bitmap_size, map->max_apic_id - min + 1) ...

Not good, but could still be nicer than the alternatives.

> +                     vcpu = map->phys_map[min + i]->vcpu;
> +                     count += kvm_apic_set_irq(vcpu, &irq, NULL);
> +             }
>       }
>  
>       rcu_read_unlock();