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@xxxxxxxxxx>
---
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
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