Re: [PATCH HVM v2 1/1] hvm: refactor set param

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.02.2021 14:56, Norbert Manthey wrote:
> On 2/9/21 2:45 PM, Jan Beulich wrote:
>> On 09.02.2021 14:41, Norbert Manthey wrote:
>>> On 2/9/21 10:40 AM, Jan Beulich wrote:
>>>> On 08.02.2021 20:47, Norbert Manthey wrote:
>>>>> On 2/8/21 3:21 PM, Jan Beulich wrote:
>>>>>> On 05.02.2021 21:39, Norbert Manthey wrote:
>>>>>>> @@ -4108,6 +4108,13 @@ static int hvm_allow_set_param(struct domain *d,
>>>>>>>      if ( rc )
>>>>>>>          return rc;
>>>>>>>
>>>>>>> +    if ( index >= HVM_NR_PARAMS )
>>>>>>> +        return -EINVAL;
>>>>>>> +
>>>>>>> +    /* Make sure we evaluate permissions before loading data of 
>>>>>>> domains. */
>>>>>>> +    block_speculation();
>>>>>>> +
>>>>>>> +    value = d->arch.hvm.params[index];
>>>>>>>      switch ( index )
>>>>>>>      {
>>>>>>>      /* The following parameters should only be changed once. */
>>>>>> I don't see the need for the heavier block_speculation() here;
>>>>>> afaict array_access_nospec() should do fine. The switch() in
>>>>>> context above as well as the switch() further down in the
>>>>>> function don't have any speculation susceptible code.
>>>>> The reason to block speculation instead of just using the hardened index
>>>>> access is to not allow to speculatively load data from another domain.
>>>> Okay, looks like I got mislead by the added bounds check. Why
>>>> do you add that, when the sole caller already has one? It'll
>>>> suffice since you move the array access past the barrier,
>>>> won't it?
>>> I can drop that bound check again. This was added to make sure other
>>> callers would be save as well. Thinking about this a little more, the
>>> check could actually be moved into the hvm_allow_set_param function,
>>> above the first index access in that function. Are there good reasons to
>>> not move the index check into the allow function?
>> I guess I'm confused: We're talking about dropping the check
>> you add to hvm_allow_set_param() and you suggest to "move" it
>> right there?
> 
> Yes. I can either just no introduce that check in that function (which
> is what you suggested). As an alternative, to have all checks in one
> function, I proposed to instead move it into hvm_allow_set_param.

Oh, I see. What I'd like to be the result of your re-arrangement is
symmetry between "get" and "set" where possible in this regard, and
asymmetry having a clear reason. Seeing that hvm_{get,set}_param()
are the non-static functions here, I think it would be quite
desirable for the bounds checking to live there. Since
hvm_allow_{get,set}_param() are specifically helpers of the two
earlier named functions, checks consistently living there would as
well be fine with me.

Jan