Re: [patch RFC] kvm, cpuid: silence a buffer overflow warning

[walter harms <wharms@bfs.de>]

Am 20.02.2014 14:07, schrieb Paolo Bonzini:
> Il 20/02/2014 13:34, Dan Carpenter ha scritto:
>> This seems like a harmless off by one overflow if "i" is the last
>> element in the vcpu->arch.cpuid_entries[] array.
>>
>> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
>> ---
>> Not tested.  I always wonder if it's worth fixing these or if it's worth
>> reporting them?  Either of those seem like a lot of work for something
>> harmless.
> 
> Could it oops if cpuid_nent is INT_MAX?  If so, it's not entirely harmless.
> In this case I'd rather take the occasion to cleanup the code like this
> (compile-tested):
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 9fed5bedaad6..2fd6e7169936 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -656,18 +656,19 @@ out:
>  static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
>  {
>  	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
> -	int j, nent = vcpu->arch.cpuid_nent;
> +	struct kvm_cpuid_entry2 *ej;
> +	int j = i, nent = vcpu->arch.cpuid_nent;
>  
>  	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
> +
>  	/* when no next entry is found, the current entry[i] is reselected */
> -	for (j = i + 1; ; j = (j + 1) % nent) {
> -		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
> -		if (ej->function == e->function) {
> -			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
> -			return j;
> -		}
> -	}
> -	return 0; /* silence gcc, even though control never reaches here */
> +	do {
> +		j = (j + 1) % nent;
> +		ej = &vcpu->arch.cpuid_entries[j];
> +	} while (ej->function != e->function);
> +
> +	ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
> +	return j;
>  }
>  
>  /* find an entry with matching function, matching index (if needed), and that
> 
> What do you think?
> 

Is there any guaranty that this will not loop forever ?

an if (i==j) return 0; would be on the save side. (I guess that
these was the idea behind the for).

re,
 wh

> Paolo
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