Re: [PATCH] iommu/vt-d: Fix unmap_pages support

[Giovanni Cabiddu <giovanni.cabiddu@intel.com>]

I was able to hit the issue that this patch is fixing using the memory
allocator in QATlib (https://github.com/intel/qatlib/) which uses vfio
to map and unmap memory.

This patch fixes the problem.

On Wed, Nov 10, 2021 at 05:32:50PM -0700, Alex Williamson wrote:
> When supporting only the .map and .unmap callbacks of iommu_ops,
> the IOMMU driver can make assumptions about the size and alignment
> used for mappings based on the driver provided pgsize_bitmap.  VT-d
> previously used essentially PAGE_MASK for this bitmap as any power
> of two mapping was acceptably filled by native page sizes.
> 
> However, with the .map_pages and .unmap_pages interface we're now
> getting page-size and count arguments.  If we simply combine these
> as (page-size * count) and make use of the previous map/unmap
> functions internally, any size and alignment assumptions are very
> different.
> 
> As an example, a given vfio device assignment VM will often create
> a 4MB mapping at IOVA pfn [0x3fe00 - 0x401ff].  On a system that
> does not support IOMMU super pages, the unmap_pages interface will
> ask to unmap 1024 4KB pages at the base IOVA.  dma_pte_clear_level()
> will recurse down to level 2 of the page table where the first half
> of the pfn range exactly matches the entire pte level.  We clear the
> pte, increment the pfn by the level size, but (oops) the next pte is
> on a new page, so we exit the loop an pop back up a level.  When we
> then update the pfn based on that higher level, we seem to assume
> that the previous pfn value was at the start of the level.  In this
> case the level size is 256K pfns, which we add to the base pfn and
> get a results of 0x7fe00, which is clearly greater than 0x401ff,
> so we're done.  Meanwhile we never cleared the ptes for the remainder
> of the range.  When the VM remaps this range, we're overwriting valid
> ptes and the VT-d driver complains loudly, as reported by the user
> report linked below.
> 
> The fix for this seems relatively simple, if each iteration of the
> loop in dma_pte_clear_level() is assumed to clear to the end of the
> level pte page, then our next pfn should be calculated from level_pfn
> rather than our working pfn.
> 
> Fixes: 3f34f1259776 ("iommu/vt-d: Implement map/unmap_pages() iommu_ops callback")
> Reported-by: Ajay Garg <ajaygargnsit@gmail.com>
> Link: https://lore.kernel.org/all/20211002124012.18186-1-ajaygargnsit@gmail.com/
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
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