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

[Sean Christopherson]

On Tue, Apr 06, 2021, Emanuele Giuseppe Esposito wrote:
> 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@xxxxxxxxxx>

Don't think it needs stable@, but I think it's worthwhile to add:

Fixes: 433f4ba19041 ("KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)")


> ---
>  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;

Deep into nitpick territory... I think it makes sense to set entry.function only
after the switch statement, that way it'll be a bit more obvious that the default
case doesn't actually consume "entry".

>  
>  	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;
> +		entry.ecx = F(RDPID);
>  		break;
> +	default:
> +		goto out;
>  	}

Maybe add a comment here to call out that the check is done if and only if there
is a valid entry?

> +	if (array->nent >= array->maxnent)
> +		return -E2BIG;
> +
> +	memcpy(&array->entries[array->nent++], &entry, sizeof(entry));
> +
> +out:
>  	return 0;
>  }
>  
> -- 
> 2.30.2
>