Re: [Xen-devel] [PATCH SpectreV1+L1TF v6 3/9] x86/hvm: block speculative out-of-bound accesses

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.02.19 at 15:05, <nmanthey@xxxxxxxxx> wrote:
> On 2/12/19 14:25, Jan Beulich wrote:
>>>>> On 08.02.19 at 14:44, <nmanthey@xxxxxxxxx> wrote:
>>> @@ -4104,6 +4108,12 @@ 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);
>>> +
>>>      d = rcu_lock_domain_by_any_id(a.domid);
>>>      if ( d == NULL )
>>>          return -ESRCH;
>>> @@ -4370,6 +4380,12 @@ 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);
>> ... the usefulness of these two. To make forward progress it may
>> be worthwhile to split off these two changes into a separate patch.
>> If you're fine with this, I could strip these two before committing,
>> in which case the remaining change is
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Taking apart the commit is fine with me. I will submit a follow up
> change that does not update the values but fixes the reads.

As pointed out during the v5 discussion, I'm unconvinced that if
you do so the compiler can't re-introduce the issue via CSE. I'd
really like a reliable solution to be determined first.

Jan



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