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

["Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" <longpeng2@huawei.com>]

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?

> 
>> +                }
>>               } else {
>>                   pteval &= ~(uint64_t)DMA_PTE_LARGE_PAGE;
>>               }
>>
> 
> Best regards,
> baolu
> .
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