Re: [PATCH] amba: Fix driver_managed_dma check

[Ketan Kishore <ketan.kishore@oss.qualcomm.com>]

On 4/25/2025 7:09 PM, Robin Murphy wrote:
> Since it's not currently safe to take device_lock() in the IOMMU probe
> path, that can race against really_probe() setting dev->driver before
> attempting to bind. The race itself isn't so bad, since we're only
> concerned with dereferencing dev->driver itself anyway, but sadly my
> attempt to implement the check with minimal churn leads to a kind of
> TOCTOU issue, where dev->driver becomes valid after to_amba_driver(NULL)
> is already computed, and thus the check fails to work as intended.
> 
> Will and I both hit this with the platform bus, but the pattern here is
> the same, so fix it for correctness too.
> 
> Reported-by: Will McVicker <willmcvicker@google.com>
> Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
			arch_teardown_dma_ops(dev);

Hello Robin

We faced this issue with amba_dma_configure race internally as well:
Call trace:
   amba_dma_configure
   __iommu_probe_device
   probe_iommu_group
   bus_for_each_dev
   iommu_device_register
   arm_smmu_device_probe
   platform_probe
   really_probe
   __driver_probe_device
   driver_probe_device
   __device_attach_driver
   bus_for_each_drv
   __device_attach
   device_initial_probe
   bus_probe_device
   deferred_probe_work_func
   process_scheduled_works
   worker_thread
   kthread
   ret_from_fork

We raised change for this based on already merged changes that fixed the 
same TOCTOU race for the platform bus in
commit 95deee37a123 ("platform: Fix race condition during DMA
configure at IOMMU probe time") and for fsl-mc in commit 152f33ee30ee
("bus: fsl_mc: Fix driver_managed_dma check").
Link: 
https://lore.kernel.org/all/20260825-iommu_races-v2-1-49de6de71357@oss.qualcomm.com/

We later found this change you raised which was never merged.
Can you please help to revive and take this change forward for merge.

Reviewed-by: Ketan Kishore <ketan.kishore@oss.qualcomm.com>

Regards
Ketan