[PATCH 2/3] ARM: dma-mapping: reset DMA ops before detaching from the IOMMU

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

arm_iommu_attach_device() installs the IOMMU-aware dma_map_ops only
after iommu_attach_device() has run, so a driver that builds its page
tables during attach (msm_iommu via io-pgtable) maps them while the
device still has the direct DMA ops: the resulting dma_handle equals the
physical address.

arm_iommu_detach_device() does the reverse in the wrong order. It calls
iommu_detach_device() first and clears the DMA ops afterwards. When
detaching frees those page tables (msm_iommu_identity_attach() ->
free_io_pgtable_ops()), the freeing dma_unmap now goes through the still
installed IOMMU ops, which treat the stored physical address as an IOVA.
iommu_unmap() warns "region not mapped" and __free_iova() indexes an
unallocated bitmap extension, dereferencing NULL:

  __bitmap_clear from arm_iommu_unmap_phys
  arm_iommu_unmap_phys from dma_unmap_phys
  dma_unmap_phys from __arm_v7s_free_table
  __arm_v7s_free_table from arm_v7s_free_pgtable
  arm_v7s_free_pgtable from msm_iommu_identity_attach
  msm_iommu_identity_attach from __iommu_attach_device
  __iommu_attach_device from iommu_detach_device
  iommu_detach_device from arm_iommu_detach_device
  arm_iommu_detach_device from arch_teardown_dma_ops
  arch_teardown_dma_ops from device_unbind_cleanup

This is reached on apq8064 (ifc6410) when the adreno GPU probe fails and
its DMA ops are torn down.

Reset the DMA ops before iommu_detach_device() so page-table teardown
runs with the same direct ops that mapped them at attach time.

Fixes: 78fc30b4bb35 ("iommu/msm: Implement an IDENTITY domain")
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Assisted-by: Claude:claude-opus-5
---
 arch/arm/mm/dma-mapping.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index f9bc53b60f99..a467e0d01a5b 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -1642,10 +1642,13 @@ void arm_iommu_detach_device(struct device *dev)
 		return;
 	}
 
+	/* Reset DMA ops before detach so page-table teardown uses the same
+	 * direct ops that mapped them at attach time.
+	 */
+	set_dma_ops(dev, NULL);
 	iommu_detach_device(mapping->domain, dev);
 	kref_put(&mapping->kref, release_iommu_mapping);
 	to_dma_iommu_mapping(dev) = NULL;
-	set_dma_ops(dev, NULL);
 
 	pr_debug("Detached IOMMU controller from %s device.\n", dev_name(dev));
 }

-- 
2.47.3