Re: [PATCH 3/4] KVM: x86: allow 256 logical x2APICs again

[Nadav Amit <nadav.amit@gmail.com>]

Radim Krčmář <rkrcmar@redhat.com> wrote:

> While fixing an x2apic bug,
> 17d68b7 KVM: x86: fix guest-initiated crash with x2apic (CVE-2013-6376)
> we've made only one cluster available.  This means that the amount of
> logically addressible x2APICs was reduced to 16 and VCPUs kept
> overwriting themselves in that region, so even the first cluster wasn't
> set up correctly.
> 
> This patch extends x2APIC support back to the logical_map's limit, and
> keeps the CVE fixed as messages for non-present APICs are dropped.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
> arch/x86/kvm/lapic.c | 11 ++++++-----
> arch/x86/kvm/lapic.h |  2 --
> 2 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 049d30f..f6e3369 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -132,8 +132,6 @@ static inline int kvm_apic_id(struct kvm_lapic *apic)
> 	return (kvm_apic_get_reg(apic, APIC_ID) >> 24) & 0xff;
> }
> 
> -#define KVM_X2APIC_CID_BITS 0
> -
> static void recalculate_apic_map(struct kvm *kvm)
> {
> 	struct kvm_apic_map *new, *old = NULL;
> @@ -163,8 +161,7 @@ static void recalculate_apic_map(struct kvm *kvm)
> 		if (apic_x2apic_mode(apic)) {
> 			new->ldr_bits = 32;
> 			new->cid_shift = 16;
> -			new->cid_mask = (1 << KVM_X2APIC_CID_BITS) - 1;
> -			new->lid_mask = 0xffff;
> +			new->cid_mask = new->lid_mask = 0xffff;
You set cid_mask to 0xffff, while there are only 16 clusters. I think it is
risky (if you twist my hand would come with a scenario). Yet, why not to set
cid_mask to (ARRAY_SIZE(map->logical_map) - 1) ?


Nadav