Re: [PATCH v2 04/13] KVM: x86: dynamic kvm_apic_map

Yang Zhang
2016-07-11T06:07:07Z

On 2016/7/8 1:15, Radim Krčmář wrote:
> x2APIC supports up to 2^32-1 LAPICs, but most guest in coming years will
> have slighly less VCPUs. Dynamic size saves memory at the cost of
> turning one constant into a variable.
>
> apic_map mutex had to be moved before allocation to avoid races with cpu
> hotplug.
>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
> v2:
> * replaced size with max_apic_id to minimize chances of overflow [Andrew]
> * fixed allocation size [Paolo]
>
> arch/x86/include/asm/kvm_host.h | 3 ++-
> arch/x86/kvm/lapic.c | 18 +++++++++++++-----
> arch/x86/kvm/lapic.h | 2 +-
> 3 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 3194b19b9c7b..643e3dffcd85 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -682,11 +682,12 @@ struct kvm_arch_memory_slot {
> struct kvm_apic_map {
> struct rcu_head rcu;
> u8 mode;
> - struct kvm_lapic *phys_map[256];
> + u32 max_apic_id;
> union {
> struct kvm_lapic *xapic_flat_map[8];
> struct kvm_lapic *xapic_cluster_map[16][4];
> };
> + struct kvm_lapic *phys_map[];
> };
>
> /* Hyper-V emulation context */
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9880d03f533d..224fc1c5fcc6 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -120,7 +120,7 @@ static inline bool kvm_apic_map_get_logical_dest(struct kvm_apic_map *map,
> switch (map->mode) {
> case KVM_APIC_MODE_X2APIC: {
> u32 offset = (dest_id >> 16) * 16;
> - u32 max_apic_id = ARRAY_SIZE(map->phys_map) - 1;
> + u32 max_apic_id = map->max_apic_id;
>
> if (offset <= max_apic_id) {
> u8 cluster_size = min(max_apic_id - offset + 1, 16U);
> @@ -152,14 +152,22 @@ static void recalculate_apic_map(struct kvm *kvm)
> struct kvm_apic_map *new, *old = NULL;
> struct kvm_vcpu *vcpu;
> int i;
> -
> - new = kzalloc(sizeof(struct kvm_apic_map), GFP_KERNEL);
> + u32 max_id = 255;
>
> mutex_lock(&kvm->arch.apic_map_lock);
>
> + kvm_for_each_vcpu(i, vcpu, kvm)
> + if (kvm_apic_present(vcpu))
> + max_id = max(max_id, kvm_apic_id(vcpu->arch.apic));
> +
> + new = kzalloc(sizeof(struct kvm_apic_map) +
> + sizeof(struct kvm_lapic *) * (max_id + 1), GFP_KERNEL);
> +

I think this may cause the host runs out of memory if a malicious guest
did follow thing:
1. vcpu a is doing apic map recalculation.
2. vcpu b write the apic id with 0xff
3. then vcpu b enable the x2apic: in kvm_lapic_set_base(), we will set
apic_base to new value before reset the apic id.
4. vcpu a may see the x2apic enabled in vcpu b plus an old apic
id(0xff), and max_id will become (0xff >> 24).

> if (!new)
> goto out;
>
> + new->max_apic_id = max_id;
> +
> kvm_for_each_vcpu(i, vcpu, kvm) {
> struct kvm_lapic *apic = vcpu->arch.apic;
> struct kvm_lapic **cluster;
> @@ -172,7 +180,7 @@ static void recalculate_apic_map(struct kvm *kvm)
> aid = kvm_apic_id(apic);
> ldr = kvm_lapic_get_reg(apic, APIC_LDR);
>
> - if (aid < ARRAY_SIZE(new->phys_map))
> + if (aid <= new->max_apic_id)
> new->phys_map[aid] = apic;
>
> if (apic_x2apic_mode(apic)) {
> @@ -710,7 +718,7 @@ static inline bool kvm_apic_map_get_dest_lapic(struct kvm *kvm,
> return false;
>
> if (irq->dest_mode == APIC_DEST_PHYSICAL) {
> - if (irq->dest_id >= ARRAY_SIZE(map->phys_map)) {
> + if (irq->dest_id > map->max_apic_id) {
> *bitmap = 0;
> } else {
> *dst = &map->phys_map[irq->dest_id];
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 336ba51bb16e..8d811139d2b3 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -200,7 +200,7 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
> return lapic_in_kernel(vcpu) && test_bit(KVM_APIC_INIT, &vcpu->arch.apic->pending_events);
> }
>
> -static inline int kvm_apic_id(struct kvm_lapic *apic)
> +static inline u32 kvm_apic_id(struct kvm_lapic *apic)
> {
> return (kvm_lapic_get_reg(apic, APIC_ID) >> 24) & 0xff;
> }
>

--
best regards
yang