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

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Message-ID: <95f63ed6-743b-3547-dda1-4fe83bc39070@redhat.com>
Date:   Mon, 24 Jan 2022 15:02:50 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        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 11:36, Vitaly Kuznetsov wrote:
> kvm_cpuid_check_equal() should also check .flags equality but instead
> of adding it to the existing check, just switch to using memcmp() for
> the whole 'struct kvm_cpuid_entry2'.
> 
> When .flags are not checked, kvm_cpuid_check_equal() may allow an update
> which it shouldn't but kvm_set_cpuid() does not actually update anything
> and just returns success.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>   arch/x86/kvm/cpuid.c | 13 ++-----------
>   1 file changed, 2 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 89d7822a8f5b..7dd9c8f4f46e 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -123,20 +123,11 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
>   static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>   				 int nent)
>   {
> -	struct kvm_cpuid_entry2 *orig;
> -	int i;
> -
>   	if (nent != vcpu->arch.cpuid_nent)
>   		return -EINVAL;
>   
> -	for (i = 0; i < nent; i++) {
> -		orig = &vcpu->arch.cpuid_entries[i];
> -		if (e2[i].function != orig->function ||
> -		    e2[i].index != orig->index ||
> -		    e2[i].eax != orig->eax || e2[i].ebx != orig->ebx ||
> -		    e2[i].ecx != orig->ecx || e2[i].edx != orig->edx)
> -			return -EINVAL;
> -	}
> +	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.

Queued patch 1 though.

Paolo