Re: [PATCH] iommu/io-pgtable-arm: fix size_t signedness bug in unmap path

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Dec 19, 2025 at 03:28:58PM -0800, Chaitanya Kulkarni wrote:
> __arm_lpae_unmap() returns size_t but was returning -ENOENT (negative
> error code) when encountering an unmapped PTE. Since size_t is unsigned,
> -ENOENT (typically -2) becomes a huge positive value (0xFFFFFFFFFFFFFFFE
> on 64-bit systems).
> 
> This corrupted value propagates through the call chain:
>   __arm_lpae_unmap() returns -ENOENT as size_t
>   -> arm_lpae_unmap_pages() returns it
>   -> __iommu_unmap() adds it to iova address
>   -> iommu_pgsize() triggers BUG_ON due to corrupted iova
> 
> This can cause IOVA address overflow in __iommu_unmap() loop and
> trigger BUG_ON in iommu_pgsize() from invalid address alignment.
> 
> Fix by returning 0 instead of -ENOENT. The WARN_ON already signals
> the error condition, and returning 0 (meaning "nothing unmapped")
> is the correct semantic for size_t return type. This matches the
> behavior of other io-pgtable implementations (io-pgtable-arm-v7s,
> io-pgtable-dart) which return 0 on error conditions.
> 
> Fixes: 3318f7b5cefb ("iommu/io-pgtable-arm: Add quirk to quiet WARN_ON()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
> ---
>  drivers/iommu/io-pgtable-arm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

At least the iommu_pgsize() path runs with !IO_PGTABLE_QUIRK_NO_WARN
and we don't hit that WARN_ON() before the return, but the GPU folks
using this NO_WARN path might have an issue so it should be fixed..

Jason