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

[Norbert Manthey <nmanthey@xxxxxxxxx>]

On 2/9/21 10:40 AM, Jan Beulich wrote:
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> 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?
>
>>>> @@ -4141,6 +4148,9 @@ static int hvm_set_param(struct domain *d, uint32_t 
>>>> index, uint64_t value)
>>>>      if ( rc )
>>>>          return rc;
>>>>
>>>> +    /* Make sure we evaluate permissions before loading data of domains. 
>>>> */
>>>> +    block_speculation();
>>>> +
>>>>      switch ( index )
>>>>      {
>>>>      case HVM_PARAM_CALLBACK_IRQ:
>>> Like you do for the "get" path I think this similarly renders
>>> pointless the use in hvmop_set_param() (and - see below - the
>>> same consideration wrt is_hvm_domain() applies).
>> Can you please be more specific why this is pointless? I understand that
>> the is_hvm_domain check comes with a barrier that can be used to not add
>> another barrier. However, I did not find such a barrier here, which
>> comes between the 'if (rc)' just above, and the potential next access
>> based on the value of 'index'. At least the access behind the switch
>> statement cannot be optimized and replaced with a constant value easily.
> I'm suspecting a misunderstanding (the more that further down
> you did agree to what I've said for hvmop_get_param()): I'm
> not saying your addition is pointless. Instead I'm saying that
> your addition should be accompanied by removal of the barrier
> from hvmop_set_param(), paralleling what you do to
> hvmop_get_param(). And additionally I'm saying that just like
> in hvmop_get_param() the barrier there was already previously
> redundant with that inside is_hvm_domain().

I now understand, thank you. I agree, the already existing barrier in
the hvmop_set_param function can be dropped as well. I will update the
diff accordingly, after we concluded where to put the index check.

Best,
Norbert




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