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

Liran Alon
2018-08-29T09:05:06Z

> On 29 Aug 2018, at 8:52, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> From: Wanpeng Li <wanpengli@tencent.com>
>
> 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@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>
> ---
> 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;

I personally think “if ((min + __fls(ipi_bitmap_low)) > map->max_apic_id)” is more readable.
But that’s just a matter of taste :)

> /* 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
>

Reviewed-By: Liran Alon <liran.alon@oracle.com>