Intel-IOMMU Alignment Issue in dma_pte_clear_range()

This issue was pointed out by Linus.

In dma_pte_clear_range() in intel-iommu.c

start = PAGE_ALIGN(start);
end &= PAGE_MASK;
npages = (end - start) / VTD_PAGE_SIZE;

In partial page case, start could be bigger than end and npages will be
negative.

Currently the issue doesn't show up as a real bug in because start and 
end have been aligned to page boundary already by all callers. So the 
issue has been hidden. But it is dangerous programming practice.
Signed-off-by: NFenghua Yu <fenghua.yu@intel.com>
Signed-off-by: NDavid Woodhouse <David.Woodhouse@intel.com>

drivers/pci/intel-iommu.c

@@ -733,8 +733,8 @@ static void dma_pte_clear_range(struct dmar_domain *domain, u64 start, u64 end)
 	start &= (((u64)1) << addr_width) - 1;
 	end &= (((u64)1) << addr_width) - 1;
 	/* in case it's partial page */
-	start = PAGE_ALIGN(start);
-	end &= PAGE_MASK;
+	start &= PAGE_MASK;
+	end = PAGE_ALIGN(end);
 	npages = (end - start) / VTD_PAGE_SIZE;
 
 	/* we don't need lock here, nobody else touches the iova range */