linux-kernel - Re: [PATCH] KVM: x86: restrict maximal physical address

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Date:   Fri, 25 Nov 2016 17:10:45 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: restrict maximal physical address



On 25/11/2016 15:51, Radim Krčmář wrote:
> The guest could have configured a maximal physical address that exceeds
> the host.  Prevent that situation as it could easily lead to a bug.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  arch/x86/kvm/cpuid.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 25f0f15fab1a..aed910e9fbed 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -136,7 +136,13 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>  		((best->eax & 0xff00) >> 8) != 0)
>  		return -EINVAL;
>  
> -	/* Update physical-address width */
> +
> +	/*
> +	 * Update physical-address width.
> +	 * Make sure that it does not exceed hardware capabilities.
> +	 */
> +	if (cpuid_query_maxphyaddr(vcpu) > boot_cpu_data.x86_phys_bits)
> +		return -EINVAL;
>  	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
>  
>  	kvm_pmu_refresh(vcpu);
> 

Not possible unfortunately, this would break most versions of QEMU that
hard-code 40 for MAXPHYADDR.

Also, "wider" physical addresses in the guest are actually possible with
shadow paging.

Paolo