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

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Message-Id: <485CA660-9423-45B7-848F-49E3D13D5CBD@oracle.com>
Date:   Wed, 29 Aug 2018 13:43:21 +0300
From:   Liran Alon <liran.alon@...cle.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Wanpeng Li <kernellwp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: LAPIC: Fix pv ipis out-of-bounds access



> On 29 Aug 2018, at 13:29, Dan Carpenter <dan.carpenter@...cle.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@...cle.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