Re: [PATCH] iommu/intel: Fix __domain_mapping()'s usage of switch_to_super_page()

[Nicolas Saenz Julienne <nsaenz@amazon.com>]

On Tue Aug 26, 2025 at 2:38 PM UTC, Eugene Koira wrote:
> switch_to_super_page() assumes the memory range it's working on is aligned
> to the target large page level. Unfortunately, __domain_mapping() doesn't
> take this into account when using it, and will pass unaligned ranges
> ultimately freeing a PTE range larger than expected.
>
> Take for example a mapping with the following iov_pfn range [0x3fe400,
> 0x4c0600], which should be backed by the following mappings:
>
>    iov_pfn [0x3fe400, 0x3fffff] covered by 2MiB pages
>    iov_pfn [0x400000, 0x4bffff] covered by 1GiB pages
>    iov_pfn [0x4c0000, 0x4c05ff] covered by 2MiB pages
>
> Under this circumstance, __domain_mapping() will pass [0x400000, 0x4c05ff]
> to switch_to_super_page() at a 1 GiB granularity, which will in turn
> free PTEs all the way to iov_pfn 0x4fffff.
>
> Mitigate this by rounding down the iov_pfn range passed to
> switch_to_super_page() in __domain_mapping()
> to the target large page level.
>
> Additionally add range alignment checks to switch_to_super_page.
>
> Fixes: 9906b9352a35 ("iommu/vt-d: Avoid duplicate removing in __domain_mapping()")
> Signed-off-by: Eugene Koira <eugkoira@amazon.com>
> Cc: stable@vger.kernel.org
> ---

>  drivers/iommu/intel/iommu.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 9c3ab9d9f69a..dff2d895b8ab 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -1575,6 +1575,10 @@ static void switch_to_super_page(struct dmar_domain *domain,
>  	unsigned long lvl_pages = lvl_to_nr_pages(level);
>  	struct dma_pte *pte = NULL;
>  
> +	if (WARN_ON(!IS_ALIGNED(start_pfn, lvl_pages) ||
> +		    !IS_ALIGNED(end_pfn + 1, lvl_pages)))

It might make sense to downgrade the warning to WARN_ON_ONCE().

Other than that:

	Reviewed-by: Nicolas Saenz Julienne <nsaenz@amazon.com>

Regards,
Nicolas