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Message-ID: <c0f9e1b6-e751-a06f-e753-6603f17a6bce@redhat.com>
Date:   Thu, 1 Apr 2021 11:28:33 +0200
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>,
        Alexander Graf <graf@...zon.com>,
        Andrew Jones <drjones@...hat.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 1/4] kvm: cpuid: adjust the returned nent field of
 kvm_cpuid2 for KVM_GET_SUPPORTED_CPUID and KVM_GET_EMULATED_CPUID



On 31/03/2021 20:31, Sean Christopherson wrote:
> On Wed, Mar 31, 2021, Emanuele Giuseppe Esposito wrote:
>> Calling the kvm KVM_GET_[SUPPORTED/EMULATED]_CPUID ioctl requires
>> a nent field inside the kvm_cpuid2 struct to be big enough to contain
>> all entries that will be set by kvm.
>> Therefore if the nent field is too high, kvm will adjust it to the
>> right value. If too low, -E2BIG is returned.
>>
>> However, when filling the entries do_cpuid_func() requires an
>> additional entry, so if the right nent is known in advance,
>> giving the exact number of entries won't work because it has to be
>> increased by one.
> 
> I'd strong prefer to reword the shortlog and changelog.  It's not immediately
> obvious what this is changing without the context from the v1 thread.  E.g.
> 
>    KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID
> 
>    When retrieving emulated CPUID entries, check for an insufficient array size
>    if and only if KVM is actually inserting an entry.  If userspace has a priori
>    knowledge of the exact array size, KVM_GET_EMULATED_CPUID will incorrectly
>    fail due to effectively requiring an extra, unused entry.

I will update it with v3, thanks.

> 
>> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@...hat.com>
>> ---
>>   arch/x86/kvm/cpuid.c | 35 ++++++++++++++++++-----------------
>>   1 file changed, 18 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 6bd2f8b830e4..02a51f921548 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -567,34 +567,34 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
>>   
>>   static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>>   {
>> -	struct kvm_cpuid_entry2 *entry;
>> -
>> -	if (array->nent >= array->maxnent)
>> -		return -E2BIG;
>> +	struct kvm_cpuid_entry2 entry;
>>   
>> -	entry = &array->entries[array->nent];
>> -	entry->function = func;
>> -	entry->index = 0;
>> -	entry->flags = 0;
>> +	entry.function = func;
>> +	entry.index = 0;
>> +	entry.flags = 0;
> 
> Depending on the leaf, eax/ebx/ecx/edx will be left uninitialized.  This wasn't
> a bug before since @array is zeroed on allocation.
> 
> What about pre-checking @func?  I don't particular like the duplicate checks,
> but none of the solutions are particularly elegant.  E.g.
You're right, I should have zeroed it. I agree that memsetting and 
memcopying is not elegant either, but unless I am missing something and 
it changes the intended behavior, IMHO this avoids duplicate checks and 
makes it simpler to add a new 'func'.

Emanuele
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6bd2f8b830e4..9824947bd5ad 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -565,14 +565,18 @@ static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
>          return entry;
>   }
> 
> -static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> +static noinline int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>   {
>          struct kvm_cpuid_entry2 *entry;
> 
> +       if (func != 0 && func != 1 && func != 7)
> +               return 0;
> +
>          if (array->nent >= array->maxnent)
>                  return -E2BIG;
> 
> -       entry = &array->entries[array->nent];
> +       entry = &array->entries[array->nent++];
> +
>          entry->function = func;
>          entry->index = 0;
>          entry->flags = 0;
> @@ -580,19 +584,17 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
>          switch (func) {
>          case 0:
>                  entry->eax = 7;
> -               ++array->nent;
>                  break;
>          case 1:
>                  entry->ecx = F(MOVBE);
> -               ++array->nent;
>                  break;
>          case 7:
>                  entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>                  entry->eax = 0;
>                  entry->ecx = F(RDPID);
> -               ++array->nent;
> -       default:
>                  break;
> +       default:
> +               BUG();
>          }
> 
>          return 0
> 
> 
>>   
>>   	switch (func) {
>>   	case 0:
>> -		entry->eax = 7;
>> -		++array->nent;
>> +		entry.eax = 7;
>>   		break;
>>   	case 1:
>> -		entry->ecx = F(MOVBE);
>> -		++array->nent;
>> +		entry.ecx = F(MOVBE);
>>   		break;
>>   	case 7:
>> -		entry->flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> -		entry->eax = 0;
>> -		entry->ecx = F(RDPID);
>> -		++array->nent;
>> -	default:
>> +		entry.flags |= KVM_CPUID_FLAG_SIGNIFCANT_INDEX;
>> +		entry.eax = 0;
>> +		entry.ecx = F(RDPID);
>>   		break;
>> +	default:
>> +		goto out;
>>   	}
>>   
>> +	if (array->nent >= array->maxnent)
>> +		return -E2BIG;
>> +
>> +	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
>> +
>> +out:
>>   	return 0;
>>   }
>>   
>> @@ -975,6 +975,7 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
>>   
>>   	if (cpuid->nent < 1)
>>   		return -E2BIG;
>> +
>>   	if (cpuid->nent > KVM_MAX_CPUID_ENTRIES)
>>   		cpuid->nent = KVM_MAX_CPUID_ENTRIES;
>>   
>> -- 
>> 2.30.2
>>
> 

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