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

[Wanpeng Li <kernellwp@xxxxxxxxx>]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

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 
and also checking whether or not map->phys_map[min + i] is NULL since the max_apic_id 
is set according to the max apic id, however, some phys_map maybe NULL when apic id 
is sparse, in addition, kvm also unconditionally set max_apic_id to 255 to reserve 
enough space for any xAPIC ID.

Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
Cc: Liran Alon <liran.alon@xxxxxxxxxx>
Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
 arch/x86/kvm/lapic.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 0cefba2..86e933c 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -571,18 +571,27 @@ int kvm_pv_send_ipi(struct kvm *kvm, unsigned long ipi_bitmap_low,
 	rcu_read_lock();
 	map = rcu_dereference(kvm->arch.apic_map);
 
+	if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_low)) < min))
+		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);
+		if (map->phys_map[min + i]) {
+			vcpu = map->phys_map[min + i]->vcpu;
+			count += kvm_apic_set_irq(vcpu, &irq, NULL);
+		}
 	}
 
 	min += cluster_size;
+	if (unlikely((s32)(map->max_apic_id - __fls(ipi_bitmap_high)) < min))
+		goto out;
 	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 (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;
 }
-- 
2.7.4