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

[Norbert Manthey <nmanthey@amazon.de>]

On 2/9/21 3:21 PM, Jan Beulich wrote:
> 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.

I agree with the symmetry for get and set. This is what I'd aim for:

 1. hvmop_set_param and hvmop_get_param (static) both check for the
index, and afterwards use the is_hvm_domain(d) function with its barrier
 2. hvm_set_param (static) and hvm_get_param both call their allow
helper function, evaluate the return code, and afterwards block speculation.
 2.1. hvm_get_param is declared in a public header, and cannot be turned
into a static function, hence needs the index check
 2.2. hvm_set_param is only called from hvmop_set_param, and index is
already checked there, hence, do not add check
 3. hvm_allow_set_param (static) and hvm_allow_get_param (static) do not
validate the index parameter
 3.1. hvm_allow_set_param blocks speculative execution with a barrier
after domain permissions have been evaluated, before accessing the
parameters of the domain. hvm_allow_get_param does not access the params
member of the domain, and hence does not require additional protection.

To simplify the code, I propose to furthermore make the hvmop_set_param
function static as well.

Please let me know whether the above would is acceptable.

Best,
Norbert

>
> Jan




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