Re: [PATCH rc] iommu/dma: Do not try to iommu_map a 0 length region in swiotlb

[Leon Romanovsky <leonro@nvidia.com>]

On Mon, Jun 08, 2026 at 03:10:04PM -0300, Jason Gunthorpe wrote:
> iommu_dma_iova_link_swiotlb() processes a mapping that is unaligned in three
> parts, the head, middle and trailer. If the middle is empty because there
> are no aligned pages it will call down to iommu_map() with a 0 size
> which the iommupt implementation will fail as illegal.
> 
> It then tries to do an error unwind and starts from the wrong spot
> corrupting the mapping so the eventual destruction triggers a WARN_ON.
> 
> Check for 0 length and avoid mapping and use offset not 0 as the starting
> point to unlink.
> 
> This is frequently triggered by using some kinds of thunderbolt NVMe
> drives that trigger forced SWIOTLB for unaligned memory. NVMe seems to
> pass in oddly aligned buffers for the passthrough commands from smartctl
> that hit this condition.
> 
> Cc: stable@vger.kernel.org
> Fixes: 433a76207dcf ("dma-mapping: Implement link/unlink ranges API")
> Reported-by: Mark Lord <mlord@pobox.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/iommu/dma-iommu.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> This was discovered because iommupt errors on mapping length=0 instead of
> making it a NOP, so it is an became an issue since commit d6c65b0fd621
> ("iommupt: Avoid rewalking during map") making it a regression this merge
> window.
> 

Thanks,
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>