linux-kernel - Re: [PATCH 29/61] KVM: x86: Add Kconfig-controlled auditing of reverse CPUID lookups

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Date:   Tue, 25 Feb 2020 16:00:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     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 29/61] KVM: x86: Add Kconfig-controlled auditing of
 reverse CPUID lookups

On 24/02/20 14:54, Vitaly Kuznetsov wrote:
>> --- a/arch/x86/kvm/cpuid.h
>> +++ b/arch/x86/kvm/cpuid.h
>> @@ -98,6 +98,11 @@ static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature)
>>  static __always_inline u32 *__cpuid_entry_get_reg(struct kvm_cpuid_entry2 *entry,
>>  						  const struct cpuid_reg *cpuid)
>>  {
>> +#ifdef CONFIG_KVM_CPUID_AUDIT
>> +	WARN_ON_ONCE(entry->function != cpuid->function);
>> +	WARN_ON_ONCE(entry->index != cpuid->index);
>> +#endif
>> +
>>  	switch (cpuid->reg) {
>>  	case CPUID_EAX:
>>  		return &entry->eax;
> Honestly, I was thinking we should BUG_ON() and even in production builds
> but not everyone around is so rebellious I guess, so

BUG_ON is too much, but I agree the cost is so small that unconditional
WARN_ON makes sense.

Paolo