Re: [Xen-devel] [PATCH V4 1/4] x86/mm: Add array_index_nospec to guest provided index values

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.12.2019 09:06, Alexandru Stefan ISAILA wrote:
> On 17.12.2019 18:50, Jan Beulich wrote:
>> On 17.12.2019 16:12, Alexandru Stefan ISAILA wrote:
>>> --- a/xen/arch/x86/mm/mem_access.c
>>> +++ b/xen/arch/x86/mm/mem_access.c
>>> @@ -367,10 +367,11 @@ long p2m_set_mem_access(struct domain *d, gfn_t gfn, 
>>> uint32_t nr,
>>>       if ( altp2m_idx )
>>>       {
>>>           if ( altp2m_idx >= MAX_ALTP2M ||
>>> -             d->arch.altp2m_eptp[altp2m_idx] == mfn_x(INVALID_MFN) )
>>> +             d->arch.altp2m_eptp[array_index_nospec(altp2m_idx, MAX_EPTP)] 
>>> ==
>>
>> The bounds check is against MAX_ALTP2M. Both MAX_ values look to be
>> independent, which means bounds check and value passed to the
>> helper need to match up (not just here).
> 
> I will have both checks against MAX_ALTP2M.

An alternative would be something along the lines of

           if ( altp2m_idx >= min(MAX_ALTP2M, MAX_EPTP) ||

Jan

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