linux-kernel - Re: [PATCH 2/5] KVM: VMX: Use cached vmx->pt_desc.addr

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Date:   Tue, 24 Aug 2021 23:42:44 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] KVM: VMX: Use cached vmx->pt_desc.addr_range

On 8/24/2021 11:24 PM, Sean Christopherson wrote:
> On Tue, Aug 24, 2021, Xiaoyao Li wrote:
>> The number of guest's valid PT ADDR MSRs is cached in
> 
> Can you do s/cached/precomputed in the shortlog and changelog?  Explanation below.

OK.

>> vmx->pt_desc.addr_range. Use it instead of calculating it again.
>>
>> Signed-off-by: Xiaoyao Li <xiaoyao.li@...el.com>
>> ---
>>   arch/x86/kvm/vmx/vmx.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index e0a9460e4dab..7ed96c460661 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -2202,8 +2202,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   		if (!pt_can_write_msr(vmx))
>>   			return 1;
>>   		index = msr_info->index - MSR_IA32_RTIT_ADDR0_A;
>> -		if (index >= 2 * intel_pt_validate_cap(vmx->pt_desc.caps,
>> -						       PT_CAP_num_address_ranges))
>> +		if (index >= 2 * vmx->pt_desc.addr_range)
> 
> Ugh, "validate" is a lie, a better name would be intel_pt_get_cap() or so.  There
> is no validation, the helper is simply extracting the requested cap from the
> passed in array of capabilities.
> 
> That matters in this case because the number of address ranges exposed to the
> guest is not bounded by the number of address ranges present in hardware, i.e.
> it's not "validated".  And that matters because KVM uses vmx->pt_desc.addr_range
> to pass through the ADDRn_{A,B} MSRs when tracing enabled.  In other words,
> userspace can expose MSRs to the guest that do not exist.

That's why I provided patch 5.

> The bug shouldn't be a security issue, so long as Intel CPUs are bug free and
> aren't doing silly things with MSR indexes.  The number of possible address ranges
> is encoded in three bits, thus the theoretical max is 8 ranges.  So userspace can't
> get access to arbitrary MSRs, just ADDR0_A -> ADDR7_B.
> 
> And since KVM would be modifying the "validated" value, it's more than just a
> cache, hence the request to use "precomputed".
> 
> Finally, vmx_get_msr() should use the precomputed value as well.

Argh, I missed it.

> P.S. If you want to introduce a bit of churn, s/addr_range/nr_addr_ranges would
>       be a welcome change as well.

In a separate patch?

>>   			return 1;
>>   		if (is_noncanonical_address(data, vcpu))
>>   			return 1;
>> -- 
>> 2.27.0
>>