Re: [PATCH] iommu/vtd: fix address translation for superpages

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.05.2023 17:19, Roger Pau Monné wrote:
> On Wed, May 10, 2023 at 03:30:21PM +0200, Jan Beulich wrote:
>> On 10.05.2023 12:22, Roger Pau Monné wrote:
>>> On Wed, May 10, 2023 at 12:00:51PM +0200, Jan Beulich wrote:
>>>> On 10.05.2023 10:27, Roger Pau Monné wrote:
>>>>> On Tue, May 09, 2023 at 06:06:45PM +0200, Jan Beulich wrote:
>>>>>> On 09.05.2023 12:41, Roger Pau Monne wrote:
>>>>>>> When translating an address that falls inside of a superpage in the
>>>>>>> IOMMU page tables the fetching of the PTE physical address field
>>>>>>> wasn't using dma_pte_addr(), which caused the returned data to be
>>>>>>> corrupt as it would contain bits not related to the address field.
>>>>>>
>>>>>> I'm afraid I don't understand:
>>>>>>
>>>>>>> --- a/xen/drivers/passthrough/vtd/iommu.c
>>>>>>> +++ b/xen/drivers/passthrough/vtd/iommu.c
>>>>>>> @@ -359,16 +359,18 @@ static uint64_t addr_to_dma_page_maddr(struct 
>>>>>>> domain *domain, daddr_t addr,
>>>>>>>  
>>>>>>>              if ( !alloc )
>>>>>>>              {
>>>>>>> -                pte_maddr = 0;
>>>>>>>                  if ( !dma_pte_present(*pte) )
>>>>>>> +                {
>>>>>>> +                    pte_maddr = 0;
>>>>>>>                      break;
>>>>>>> +                }
>>>>>>>  
>>>>>>>                  /*
>>>>>>>                   * When the leaf entry was requested, pass back the 
>>>>>>> full PTE,
>>>>>>>                   * with the address adjusted to account for the 
>>>>>>> residual of
>>>>>>>                   * the walk.
>>>>>>>                   */
>>>>>>> -                pte_maddr = pte->val +
>>>>>>> +                pte_maddr +=
>>>>>>>                      (addr & ((1UL << level_to_offset_bits(level)) - 1) 
>>>>>>> &
>>>>>>>                       PAGE_MASK);
>>>>>>
>>>>>> With this change you're now violating what the comment says (plus what
>>>>>> the comment ahead of the function says). And it says what it says for
>>>>>> a reason - see intel_iommu_lookup_page(), which I think your change is
>>>>>> breaking.
>>>>>
>>>>> Hm, but the code in intel_iommu_lookup_page() is now wrong as it takes
>>>>> the bits in DMA_PTE_CONTIG_MASK as part of the physical address when
>>>>> doing the conversion to mfn?  maddr_to_mfn() doesn't perform a any
>>>>> masking to remove the bits above PADDR_BITS.
>>>>
>>>> Oh, right. But that's a missing dma_pte_addr() in intel_iommu_lookup_page()
>>>> then. (It would likely be better anyway to switch "uint64_t val" to
>>>> "struct dma_pte pte" there, to make more visible that it's a PTE we're
>>>> dealing with.) I indeed overlooked this aspect when doing the earlier
>>>> change.
>>>
>>> I guess I'm still confused, as the other return value for target == 0
>>> (when the address is not part of a superpage) does return
>>> dma_pte_addr(pte).  I think that needs further fixing then.
>>
>> Hmm, indeed. But I think it's worse than this: addr_to_dma_page_maddr()
>> also does one too many iterations in that case. All "normal" callers
>> supply a positive "target". We need to terminate the walk at level 1
>> also when target == 0.
> 
> Don't we do that already due to the following check:
> 
> if ( --level == target )
>     break;
> 
> Which prevents mapping the PTE address as a page table directory?

I don't think this is enough - this code, afaict right now, is only
sufficient when target >= 1.

Jan