Re: [Patch v1] KVM: x86: Initializing all kvm_lapic_irq fields

[Nitesh Narayan Lal <nitesh@redhat.com>]

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On 3/11/20 2:49 PM, Paolo Bonzini wrote:
> On 11/03/20 19:34, Nitesh Narayan Lal wrote:
>> 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.
>> This patch initializes all the fields of kvm_lapic_irq based on the
>> values which are passed through the ioapic redirect_entry object.
> Can you explain better how the bug manifests itself?

For example not initializing the irq.msi_redir_hint field, could lead to a
situation where it carries garbage (non-zero) value.
This will lead to misbehavior of kvm_apic_map_get_dest_lapic() when it invokes
the kvm_lowest_prio_delivery(), that will return true because of non-zero
msi_redir_hint field.
To be on the safe side, I thought of initializing other struct fields as well.

If the above explanation makes sense, I can include it in the patch
subject and send a second version of this patch?

>
> Thanks,
>
> Paolo
>
>> Fixes: 7ee30bc132c6("KVM: x86: deliver KVM IOAPIC scan request to target vCPUs")
>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.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;
>> +			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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