linux-kernel - Re: [PATCH v2 6/7] KVM: VMX: Check Intel PT related CPUID leaves

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Message-ID: <486f0075-494d-1d84-2d85-1d451496d1f0@redhat.com>
Date:   Mon, 18 Oct 2021 14:47:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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 v2 6/7] KVM: VMX: Check Intel PT related CPUID leaves

On 10/09/21 03:59, Xiaoyao Li wrote:
>> 
>> Ugh, looking at the rest of the code, even this isn't sufficient
>> because pt_desc.guest.addr_{a,b} are hardcoded at 4 entries, i.e.
>> running KVM on hardware with >4 entries will lead to buffer
>> overflows.
> 
> it's hardcoded to 4 because there is a note of "no processors support
>  more than 4 address ranges" in SDM vol.3 Chapter 31.3.1, table
> 31-11

True, but I agree with Sean that it's not pretty.

>> One option would be to bump that to the theoretical max of 15,
>> which doesn't seem too horrible, especially if pt_desc as a whole
>> is allocated on-demand, which it probably should be since it isn't
>> exactly tiny (nor ubiquitous)
>> 
>> A different option would be to let userspace define whatever it
>> wants for guest CPUID, and instead cap nr_addr_ranges at
>> min(host.cpuid, guest.cpuid, RTIT_ADDR_RANGE).

This is the safest option.

Paolo