linux-kernel - Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check

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Date:   Mon, 24 Jan 2022 18:07:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check_equal()

On 1/24/22 17:52, Sean Christopherson wrote:
> On Mon, Jan 24, 2022, Vitaly Kuznetsov wrote:
>> Paolo Bonzini <pbonzini@...hat.com> writes:
>>>> +	if (memcmp(e2, vcpu->arch.cpuid_entries, nent * sizeof(*e2)))
>>>> +		return -EINVAL;
>>>
>>> Hmm, not sure about that due to the padding in struct kvm_cpuid_entry2.
>>>    It might break userspace that isn't too careful about zeroing it.
> 
> Given that we already are fully committed to potentially breaking userspace by
> disallowing KVM_SET_CPUID{2} after KVM_RUN, we might as well get greedy.

Hmm, I thought this series was because we were _not_ fully committed. :)

>> FWIW, QEMU zeroes the whole thing before setting individual CPUID
>> entries. Legacy KVM_SET_CPUID call is also not afffected as it copies
>> entries to a newly allocated "struct kvm_cpuid_entry2[]" and explicitly
>> zeroes padding.
>>
>> Do we need to at least add a check for ".flags"?
> 
> Yes.

Yes, we do.  Alternatively, we can replace memdup with a copy in the 
style of KVM_CPUID.

Paolo