linux-kernel - Re: [Patch v2] KVM: x86: Initializing all kvm_lapic_irq fields in ioapic_write

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Date:   Fri, 13 Mar 2020 12:22:33 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mtosatti@...hat.com, sean.j.christopherson@...el.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        pbonzini@...hat.com, peterx@...hat.com
Subject: Re: [Patch v2] KVM: x86: Initializing all kvm_lapic_irq fields in
 ioapic_write_indirect


On 3/13/20 12:18 PM, Vitaly Kuznetsov wrote:
> Nitesh Narayan Lal <nitesh@...hat.com> writes:
>
>> On 3/13/20 9:38 AM, Nitesh Narayan Lal wrote:
>>> On 3/13/20 9:25 AM, Vitaly Kuznetsov wrote:
>>>> Nitesh Narayan Lal <nitesh@...hat.com> writes:
>>>>
>>>>> Previously all fields of structure kvm_lapic_irq were not initialized
>>>>> before it was passed to kvm_bitmap_or_dest_vcpus(). Which will cause
>>>>> an issue when any of those fields are used for processing a request.
>>>>> For example not initializing the msi_redir_hint field before passing
>>>>> to the kvm_bitmap_or_dest_vcpus(), may lead to a misbehavior of
>>>>> kvm_apic_map_get_dest_lapic(). This will specifically happen when the
>>>>> kvm_lowest_prio_delivery() returns TRUE due to a non-zero garbage
>>>>> value of msi_redir_hint, which should not happen as the request belongs
>>>>> to APIC fixed delivery mode and we do not want to deliver the
>>>>> interrupt only to the lowest priority candidate.
>>>>>
>>>>> This patch initializes all the fields of kvm_lapic_irq based on the
>>>>> values of ioapic redirect_entry object before passing it on to
>>>>> kvm_bitmap_or_dest_vcpus().
>>>>>
>>>>> Fixes: 7ee30bc132c6("KVM: x86: deliver KVM IOAPIC scan request to target vCPUs")
>>>>> Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
>>>>> ---
>>>>>  arch/x86/kvm/ioapic.c | 7 +++++--
>>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
>>>>> index 7668fed..3a8467d 100644
>>>>> --- a/arch/x86/kvm/ioapic.c
>>>>> +++ b/arch/x86/kvm/ioapic.c
>>>>> @@ -378,12 +378,15 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
>>>>>  		if (e->fields.delivery_mode == APIC_DM_FIXED) {
>>>>>  			struct kvm_lapic_irq irq;
>>>>>  
>>>>> -			irq.shorthand = APIC_DEST_NOSHORT;
>>>>>  			irq.vector = e->fields.vector;
>>>>>  			irq.delivery_mode = e->fields.delivery_mode << 8;
>>>>> -			irq.dest_id = e->fields.dest_id;
>>>>>  			irq.dest_mode =
>>>>>  			    kvm_lapic_irq_dest_mode(!!e->fields.dest_mode);
>>>>> +			irq.level = 1;
>>>> 'level' is bool in struct kvm_lapic_irq but other than that, is there a
>>>> reason we set it to 'true' here? I understand that any particular
>>>> setting is likely better than random
>>> Yes, that is the only reason which I had in my mind while doing this change.
>>> I was not particularly sure about the value, so I copied what ioapic_serivce()
>>> is doing.
>> Do you think I should skip setting this here?
>>
> Personally, i'd initialize it to 'false': usualy, if something is not
> properly initialized it's either 0 or garbage)

I think that's true, initializing it to 'false' might make more sense.
Any other concerns or comments that I can improve?

>
>>>>  and it should actually not be used
>>>> without setting it first but still?
>>>>
>>>>> +			irq.trig_mode = e->fields.trig_mode;
>>>>> +			irq.shorthand = APIC_DEST_NOSHORT;
>>>>> +			irq.dest_id = e->fields.dest_id;
>>>>> +			irq.msi_redir_hint = false;
>>>>>  			bitmap_zero(&vcpu_bitmap, 16);
>>>>>  			kvm_bitmap_or_dest_vcpus(ioapic->kvm, &irq,
>>>>>  						 &vcpu_bitmap);
-- 
Nitesh



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