linux-kernel - Re: [PATCH] KVM: Use previously computed array

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Message-ID: <6088fa0f-668a-f221-515b-413ca8c0c363@linux.com>
Date:   Sat, 30 May 2020 20:28:34 +0300
From:   Denis Efremov <efremov@...ux.com>
To:     Joe Perches <joe@...ches.com>, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Use previously computed array_size()



On 5/30/20 6:58 PM, Joe Perches wrote:
> On Sat, 2020-05-30 at 17:35 +0300, Denis Efremov wrote:
>> array_size() is used in alloc calls to compute the allocation
>> size. Next, "raw" multiplication is used to compute the size
>> for copy_from_user(). The patch removes duplicated computation
>> by saving the size in a var. No security concerns, just a small
>> optimization.
>>
>> Signed-off-by: Denis Efremov <efremov@...ux.com>
> 
> Perhaps use vmemdup_user?

vmemdup_user() uses kvmalloc internally. I think it will also require
changing vfree to kvfree.

> 
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> []
>> @@ -184,14 +184,13 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
>>  		goto out;
>>  	r = -ENOMEM;
>>  	if (cpuid->nent) {
>> -		cpuid_entries =
>> -			vmalloc(array_size(sizeof(struct kvm_cpuid_entry),
>> -					   cpuid->nent));
>> +		const size_t size = array_size(sizeof(struct kvm_cpuid_entry),
>> +					       cpuid->nent);
>> +		cpuid_entries = vmalloc(size);
>>  		if (!cpuid_entries)
>>  			goto out;
>>  		r = -EFAULT;
>> -		if (copy_from_user(cpuid_entries, entries,
>> -				   cpuid->nent * sizeof(struct kvm_cpuid_entry)))
>> +		if (copy_from_user(cpuid_entries, entries, size))
> 
> 		cpuid_entries = vmemdup_user(entries,
> 					     array_size(sizeof(struct kvm_cpuid_entry), cpuid->nent));
> 		if (IS_ERR(cpuid_entries))
> 			...
> 
> etc...
>