Re: [Xen-devel] [PATCH SpectreV1+L1TF v7 8/9] x86/hvm: add nospec to hvmop param

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.02.19 at 09:16, <nmanthey@xxxxxxxxx> wrote:
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -4109,6 +4109,13 @@ static int hvmop_set_param(
>      if ( a.index >= HVM_NR_PARAMS )
>          return -EINVAL;
>  
> +    /*
> +     * Make sure the guest controlled value a.index is bounded even during
> +     * speculative execution.
> +     */
> +    a.index = array_index_nospec(a.index, HVM_NR_PARAMS);
> +    block_speculation();
> +
>      d = rcu_lock_domain_by_any_id(a.domid);
>      if ( d == NULL )
>          return -ESRCH;
> @@ -4375,6 +4382,13 @@ static int hvmop_get_param(
>      if ( a.index >= HVM_NR_PARAMS )
>          return -EINVAL;
>  
> +    /*
> +     * Make sure the guest controlled value a.index is bounded even during
> +     * speculative execution.
> +     */
> +    a.index = array_index_nospec(a.index, HVM_NR_PARAMS);
> +    block_speculation();

Please can the comments briefly explain the otherwise apparently
pointless redundancy of both constructs?

Jan



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