Re: [PATCH] KVM: X86: Fix async pf caused null-ptr-deref

[Paolo Bonzini]

On 29/06/20 15:46, Vitaly Kuznetsov wrote:
>> +	if (!lapic_in_kernel(vcpu))
>> +		return 1;
>> +
> I'm not sure how much we care about !lapic_in_kernel() case but this
> change should be accompanied with userspace changes to not expose
> KVM_FEATURE_ASYNC_PF_INT or how would the guest know that writing a
> legitimate value will result in #GP?

Almost any pv feature is broken with QEMU if kernel_irqchip=off.  I
wouldn't bother and I am seriously thinking of dropping all support for
that, including:

- just injecting #UD for MOV from/to CR8 unless lapic_in_kernel()

- make KVM_INTERRUPT fail unless irqchip_in_kernel(), so that
KVM_INTERRUPT is only used to inject EXTINT with kernel_irqchip=split

Paolo

> Alternatively, we may just return '0' here: guest will be able to check
> what's in the MSR to see if the feature was enabled. Normally, guests
> shouldn't care about this but maybe there are cases when they do?
>