Re: [PATCH] iommu/vt-d: Force to flush iotlb before creating superpage

[Lu Baolu <baolu.lu@linux.intel.com>]

On 4/2/21 11:41 AM, Longpeng (Mike, Cloud Infrastructure Service Product 
Dept.) wrote:
> Hi Baolu,
> 
> 在 2021/4/2 11:06, Lu Baolu 写道:
>> Hi Longpeng,
>>
>> On 4/1/21 3:18 PM, Longpeng(Mike) wrote:
>>> The translation caches may preserve obsolete data when the
>>> mapping size is changed, suppose the following sequence which
>>> can reveal the problem with high probability.
>>>
>>> 1.mmap(4GB,MAP_HUGETLB)
>>> 2.
>>>     while (1) {
>>>      (a)    DMA MAP   0,0xa0000
>>>      (b)    DMA UNMAP 0,0xa0000
>>>      (c)    DMA MAP   0,0xc0000000
>>>                * DMA read IOVA 0 may failure here (Not present)
>>>                * if the problem occurs.
>>>      (d)    DMA UNMAP 0,0xc0000000
>>>     }
>>>
>>> The page table(only focus on IOVA 0) after (a) is:
>>>    PML4: 0x19db5c1003   entry:0xffff899bdcd2f000
>>>     PDPE: 0x1a1cacb003  entry:0xffff89b35b5c1000
>>>      PDE: 0x1a30a72003  entry:0xffff89b39cacb000
>>>       PTE: 0x21d200803  entry:0xffff89b3b0a72000
>>>
>>> The page table after (b) is:
>>>    PML4: 0x19db5c1003   entry:0xffff899bdcd2f000
>>>     PDPE: 0x1a1cacb003  entry:0xffff89b35b5c1000
>>>      PDE: 0x1a30a72003  entry:0xffff89b39cacb000
>>>       PTE: 0x0          entry:0xffff89b3b0a72000
>>>
>>> The page table after (c) is:
>>>    PML4: 0x19db5c1003   entry:0xffff899bdcd2f000
>>>     PDPE: 0x1a1cacb003  entry:0xffff89b35b5c1000
>>>      PDE: 0x21d200883   entry:0xffff89b39cacb000 (*)
>>>
>>> Because the PDE entry after (b) is present, it won't be
>>> flushed even if the iommu driver flush cache when unmap,
>>> so the obsolete data may be preserved in cache, which
>>> would cause the wrong translation at end.
>>>
>>> However, we can see the PDE entry is finally switch to
>>> 2M-superpage mapping, but it does not transform
>>> to 0x21d200883 directly:
>>>
>>> 1. PDE: 0x1a30a72003
>>> 2. __domain_mapping
>>>        dma_pte_free_pagetable
>>>          Set the PDE entry to ZERO
>>>        Set the PDE entry to 0x21d200883
>>>
>>> So we must flush the cache after the entry switch to ZERO
>>> to avoid the obsolete info be preserved.
>>>
>>> Cc: David Woodhouse <dwmw2@infradead.org>
>>> Cc: Lu Baolu <baolu.lu@linux.intel.com>
>>> Cc: Nadav Amit <nadav.amit@gmail.com>
>>> Cc: Alex Williamson <alex.williamson@redhat.com>
>>> Cc: Kevin Tian <kevin.tian@intel.com>
>>> Cc: Gonglei (Arei) <arei.gonglei@huawei.com>
>>>
>>> Fixes: 6491d4d02893 ("intel-iommu: Free old page tables before creating
>>> superpage")
>>> Cc: <stable@vger.kernel.org> # v3.0+
>>> Link:
>>> https://lore.kernel.org/linux-iommu/670baaf8-4ff8-4e84-4be3-030b95ab5a5e@huawei.com/
>>>
>>> Suggested-by: Lu Baolu <baolu.lu@linux.intel.com>
>>> Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
>>> ---
>>>    drivers/iommu/intel/iommu.c | 15 +++++++++++++--
>>>    1 file changed, 13 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>> index ee09323..cbcb434 100644
>>> --- a/drivers/iommu/intel/iommu.c
>>> +++ b/drivers/iommu/intel/iommu.c
>>> @@ -2342,9 +2342,20 @@ static inline int hardware_largepage_caps(struct
>>> dmar_domain *domain,
>>>                     * removed to make room for superpage(s).
>>>                     * We're adding new large pages, so make sure
>>>                     * we don't remove their parent tables.
>>> +                 *
>>> +                 * We also need to flush the iotlb before creating
>>> +                 * superpage to ensure it does not perserves any
>>> +                 * obsolete info.
>>>                     */
>>> -                dma_pte_free_pagetable(domain, iov_pfn, end_pfn,
>>> -                               largepage_lvl + 1);
>>> +                if (dma_pte_present(pte)) {
>>> +                    int i;
>>> +
>>> +                    dma_pte_free_pagetable(domain, iov_pfn, end_pfn,
>>> +                                   largepage_lvl + 1);
>>> +                    for_each_domain_iommu(i, domain)
>>> +                        iommu_flush_iotlb_psi(g_iommus[i], domain,
>>> +                                      iov_pfn, nr_pages, 0, 0);
>>
>> Thanks for patch!
>>
>> How about making the flushed page size accurate? For example,
>>
>> @@ -2365,8 +2365,8 @@ __domain_mapping(struct dmar_domain *domain, unsigned long
>> iov_pfn,
>>                                          dma_pte_free_pagetable(domain, iov_pfn,
>> end_pfn,
>>
>> largepage_lvl + 1);
>>                                          for_each_domain_iommu(i, domain)
>> - iommu_flush_iotlb_psi(g_iommus[i], domain,
>> - iov_pfn, nr_pages, 0, 0);
>> + iommu_flush_iotlb_psi(g_iommus[i], domain, iov_pfn,
>> + ALIGN_DOWN(nr_pages, lvl_pages), 0, 0);
>>
> Yes, make sense.
> 
> Maybe another alternative is 'end_pfn - iova_pfn + 1', it's readable because we
> free pagetable with (iova_pfn, end_pfn) above. Which one do you prefer?

Yours looks better.

By the way, if you are willing to prepare a v2, please make sure to add
Joerg (IOMMU subsystem maintainer) to the list.

Best regards,
baolu
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