linux-kernel - Re: [PATCH v3 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED

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Message-ID: <87k0pfcx4b.fsf@vitty.brq.redhat.com>
Date:   Tue, 06 Apr 2021 16:21:08 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <seanjc@...gle.com>,
        Emanuele Giuseppe Esposito <eesposit@...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 v3 1/4] KVM: x86: Fix a spurious -E2BIG in
 KVM_GET_EMULATED_CPUID

Emanuele Giuseppe Esposito <eesposit@...hat.com> writes:

> 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.
>
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@...hat.com>
> ---
>  arch/x86/kvm/cpuid.c | 33 ++++++++++++++++-----------------
>  1 file changed, 16 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6bd2f8b830e4..27059ddf9f0a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -567,34 +567,33 @@ 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;
> +	memset(&entry, 0, sizeof(entry));
> +	entry.function = func;
>  
>  	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;

Nitpick: there's no need to set entry.eax = 0 as the whole structure was
zeroed. Also, '|=' for flags could be just '='.

> +		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;
>  }

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly