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 17:36:58 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Nitesh Narayan Lal <nitesh@...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

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")

and with that and irq.level initialized to 'false' feel free to add

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

tag. Thanks!


>>
>>>>>  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);

-- 
Vitaly