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

Liran Alon
2018-08-29T10:43:21Z

> On 29 Aug 2018, at 13:29, Dan Carpenter <dan.carpenter@oracle.com> wrote:
>
> On Wed, Aug 29, 2018 at 06:23:08PM +0800, Wanpeng Li wrote:
>> On Wed, 29 Aug 2018 at 18:18, Dan Carpenter <dan.carpenter@oracle.com> wrote:
>>>
>>> On Wed, Aug 29, 2018 at 01:12:05PM +0300, Dan Carpenter wrote:
>>>> On Wed, Aug 29, 2018 at 12:05:06PM +0300, Liran Alon wrote:
>>>>>> 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 :)
>>>>
>>>> That's an integer overflow.
>>>>
>>>> But I do prefer to put the variable on the left. The truth is that some
>>>> Smatch checks just ignore code which is backwards written because
>>>> otherwise you have to write duplicate code and the most code is written
>>>> with the variable on the left.
>>>>
>>>> if (min > (s32)(map->max_apic_id - __fls(ipi_bitmap_low))
>>>
>>> Wait, the (s32) cast doesn't make sense. We want negative min values to
>>> be treated as invalid.
>>
>> In v2, how about:
>>
>> if (unlikely(min > map->max_apic_id || (min + __fls(ipi_bitmap_low)) >
>> map->max_apic_id))
>> goto out;
>
> That works, too. It still has the off by one and we should set
> "count = -KVM_EINVAL;".
>
> Is the unlikely() really required? I don't know what the fast paths are
> in KVM, so I don't know.
>
> regards,
> dan carpenter

Why is “min” defined as “int” instead of “unsigned int”?
It represents the lowest APIC ID in bitmap so it can’t be negative…

"if (unlikely(min > map->max_apic_id || (min + __fls(ipi_bitmap_low)) > map->max_apic_id))”
should indeed be ok.

-Liran