Re: [PATCH v4 1/4] KVM: x86: Fix a spurious -E2BIG in KVM_GET_EMULATED_CPUID

[Sean Christopherson]

On Thu, Apr 08, 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.
> 
> Fixes: 433f4ba19041 ("KVM: x86: fix out-of-bounds write in KVM_GET_EMULATED_CPUID (CVE-2019-19332)")
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@xxxxxxxxxx>
> ---
>  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..d30194081892 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));
>  
>  	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.ecx = F(RDPID);
>  		break;
> +	default:
> +		goto out;
>  	}
>  
> +	/* This check is performed only when func is valid */

Sorry to keep nitpicking and bikeshedding.  Funcs aren't really "invalid", KVM
just doesn't have any features it emulates in other leafs.  Maybe be more literal
in describing what triggers the check?

	/* Check the array capacity iff the entry is being copied over. */

Not a sticking point, so either way:

Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>

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