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

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Date:   Sat, 30 May 2020 08:58:26 -0700
From:   Joe Perches <joe@...ches.com>
To:     Denis Efremov <efremov@...ux.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 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?

> 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...