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

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 <redacted>
---
 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