linux-kernel - Re: [PATCH] KVM: x86: always allow writing '0' to MSR_KVM_ASYNC_PF

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Date:   Fri, 11 Sep 2020 19:25:33 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: always allow writing '0' to MSR_KVM_ASYNC_PF_EN

On 11/09/20 11:31, Vitaly Kuznetsov wrote:
> Even without in-kernel LAPIC we should allow writing '0' to
> MSR_KVM_ASYNC_PF_EN as we're not enabling the mechanism. In
> particular, QEMU with 'kernel-irqchip=off' fails to start
> a guest with
> 
> qemu-system-x86_64: error: failed to set MSR 0x4b564d02 to 0x0
> 
> Fixes: 9d3c447c72fb2 ("KVM: X86: Fix async pf caused null-ptr-deref")
> Reported-by: Dr. David Alan Gilbert <dgilbert@...hat.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d39d6cf1d473..44a86f7f2397 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2730,9 +2730,6 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
>  	if (data & 0x30)
>  		return 1;
>  
> -	if (!lapic_in_kernel(vcpu))
> -		return 1;
> -
>  	vcpu->arch.apf.msr_en_val = data;
>  
>  	if (!kvm_pv_async_pf_enabled(vcpu)) {
> @@ -2741,6 +2738,9 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
>  		return 0;
>  	}
>  
> +	if (!lapic_in_kernel(vcpu))
> +		return 1;
> +
>  	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
>  					sizeof(u64)))
>  		return 1;
> 

Queued, thanks.

Paolo