KVM: LAPIC: Fix pv ipis out-of-bounds access

Dan Carpenter reported that the untrusted data returns from kvm_register_read()
results in the following static checker warning:
  arch/x86/kvm/lapic.c:576 kvm_pv_send_ipi()
  error: buffer underflow 'map->phys_map' 's32min-s32max'

KVM guest can easily trigger this by executing the following assembly sequence
in Ring0:

mov $10, %rax
mov $0xFFFFFFFF, %rbx
mov $0xFFFFFFFF, %rdx
mov $0, %rsi
vmcall

As this will cause KVM to execute the following code-path:
vmx_handle_exit() -> handle_vmcall() -> kvm_emulate_hypercall() -> kvm_pv_send_ipi()
which will reach out-of-bounds access.

This patch fixes it by adding a check to kvm_pv_send_ipi() against map->max_apic_id,
ignoring destinations that are not present and delivering the rest. We also check
whether or not map->phys_map[min + i] is NULL since the max_apic_id is set to the
max apic id, some phys_map maybe NULL when apic id is sparse, especially kvm
unconditionally set max_apic_id to 255 to reserve enough space for any xAPIC ID.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Liran Alon <liran.alon@oracle.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
[Add second "if (min > map->max_apic_id)" to complete the fix. -Radim]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1455,7 +1455,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event);
 void kvm_vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu);
 
 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);
 
 u64 kvm_get_arch_capabilities(void);

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;
@@ -571,18 +571,31 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
 	rcu_read_lock();
 	map = rcu_dereference(kvm->arch.apic_map);
 
+	if (min > map->max_apic_id)
+		goto out;
 	/* 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);
+	for_each_set_bit(i, &ipi_bitmap_low,
+		min((u32)BITS_PER_LONG, (map->max_apic_id - min + 1))) {
+		if (map->phys_map[min + i]) {
+			vcpu = map->phys_map[min + i]->vcpu;
+			count += kvm_apic_set_irq(vcpu, &irq, NULL);
+		}
 	}
 
 	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);
+
+	if (min > map->max_apic_id)
+		goto out;
+
+	for_each_set_bit(i, &ipi_bitmap_high,
+		min((u32)BITS_PER_LONG, (map->max_apic_id - min + 1))) {
+		if (map->phys_map[min + i]) {
+			vcpu = map->phys_map[min + i]->vcpu;
+			count += kvm_apic_set_irq(vcpu, &irq, NULL);
+		}
 	}
 
+out:
 	rcu_read_unlock();
 	return count;
 }