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

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Date:   Mon, 24 Jan 2022 16:52:16 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, 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 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.

> 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.