Re: [PATCH] KVM: X86: eliminate some meaningless code

[Sean Christopherson]

On Fri, Feb 21, 2020 at 10:05:26PM +0800, linmiaohe wrote:
> From: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> 
> When kvm_vcpu_ioctl_get_cpuid2() fails, we set cpuid->nent to the value of
> vcpu->arch.cpuid_nent. But this is in vain as cpuid->nent is not copied to
> userspace by copy_to_user() from call site. Get rid of this meaningless
> assignment and further cleanup the var r and out jump label.

Ha, took me a while to see that.
 
> On the other hand, when kvm_vcpu_ioctl_get_cpuid2() succeeds, we do not
> change the content of struct cpuid. We can avoid copy_to_user() from call
> site as struct cpuid remain unchanged.
> 
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 14 ++++----------
>  arch/x86/kvm/x86.c   |  6 ------
>  2 files changed, 4 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index b1c469446b07..b83cedc63328 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -265,20 +265,14 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
>  			      struct kvm_cpuid2 *cpuid,
>  			      struct kvm_cpuid_entry2 __user *entries)
>  {
> -	int r;
> -
> -	r = -E2BIG;
>  	if (cpuid->nent < vcpu->arch.cpuid_nent)
> -		goto out;
> -	r = -EFAULT;
> +		return -E2BIG;
> +
>  	if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
>  			 vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
> -		goto out;
> -	return 0;

Hmm, so this ioctl() is straight up broken.  cpuid->nent should be updated
on success so that userspace knows how many entries were retrieved, i.e.
the code should look something like below, with kvm_arch_vcpu_ioctl()
unchanged.

I'm guessing no VMM actually uses this ioctl(), e.g. neither Qemu or CrosVM
use it, which is why the broken behavior has gone unnoticed.  Don't suppose
you'd want to write a selftest to hammer KVM_{SET,GET}_CPUID2?

int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
                              struct kvm_cpuid2 *cpuid,
                              struct kvm_cpuid_entry2 __user *entries)
{
        if (cpuid->nent < vcpu->arch.cpuid_nent)
                return -E2BIG;

        if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
                         vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
                return -EFAULT;

	cpuid->nent = vcpu->arch.cpuid_nent;

        return 0;
}

> +		return -EFAULT;
>  
> -out:
> -	cpuid->nent = vcpu->arch.cpuid_nent;
> -	return r;
> +	return 0;
>  }
>  
>  static __always_inline void cpuid_mask(u32 *word, int wordnum)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fbabb2f06273..683c54e7be36 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4295,12 +4295,6 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  			goto out;
>  		r = kvm_vcpu_ioctl_get_cpuid2(vcpu, &cpuid,
>  					      cpuid_arg->entries);
> -		if (r)
> -			goto out;
> -		r = -EFAULT;
> -		if (copy_to_user(cpuid_arg, &cpuid, sizeof(cpuid)))
> -			goto out;
> -		r = 0;
>  		break;
>  	}
>  	case KVM_GET_MSRS: {
> -- 
> 2.19.1
>