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 17:18:59 +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 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)

>>>  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