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

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Date:   Mon, 24 Jan 2022 16:10:22 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...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()

Paolo Bonzini <pbonzini@...hat.com> writes:

> 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.

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"?

>
> Queued patch 1 though.
>
> Paolo
>

-- 
Vitaly