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

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Message-ID: <20161125165750.GA10792@potion>
Date:   Fri, 25 Nov 2016 17:57:50 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: restrict maximal physical address

2016-11-25 17:10+0100, Paolo Bonzini:
> 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.

We don't disable EPT in that case, though.  I guess that situations
where QEMU configures mem slot into high physical addresses are not hit
in production ...

Is any solution better than ignoring this situation?