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 12:38:13 -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 12:36 PM, Vitaly Kuznetsov wrote:
> Nitesh Narayan Lal <nitesh@...hat.com> writes:
>
>> 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?
>>
> Please add the missing space to the 'Fixes' tag:
>
> Fixes: 7ee30bc132c6 ("KVM: x86: deliver KVM IOAPIC scan request to target vCPUs")

My bad.

>
> and with that and irq.level initialized to 'false' feel free to add
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>
> tag. Thanks!

Sure, thank you.

>
>
>>>>>>  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
My



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