platform: Fix race condition during DMA configure at IOMMU probe time

To avoid a race between the IOMMU probing thread and the device driver
async probing thread during configuration of the platform DMA, update
`platform_dma_configure()` to read `dev->driver` once and test if it's
NULL before using it. This ensures that we don't de-reference an invalid
platform driver pointer if the device driver is asynchronously bound
while configuring the DMA.

Fixes: bcb81ac ("iommu: Get DT/ACPI parsing into the proper probe path")
Signed-off-by: Will McVicker <willmcvicker@google.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20250424180420.3928523-1-willmcvicker@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: willmcvicker

drivers/base/platform.c

@@ -1440,7 +1440,7 @@ static void platform_shutdown(struct device *_dev)
 
 static int platform_dma_configure(struct device *dev)
 {
-	struct platform_driver *drv = to_platform_driver(dev->driver);
+	struct device_driver *drv = READ_ONCE(dev->driver);
 	struct fwnode_handle *fwnode = dev_fwnode(dev);
 	enum dev_dma_attr attr;
 	int ret = 0;
@@ -1451,8 +1451,8 @@ static int platform_dma_configure(struct device *dev)
 		attr = acpi_get_dma_attr(to_acpi_device_node(fwnode));
 		ret = acpi_dma_configure(dev, attr);
 	}
-	/* @drv may not be valid when we're called from the IOMMU layer */
-	if (ret || !dev->driver || drv->driver_managed_dma)
+	/* @dev->driver may not be valid when we're called from the IOMMU layer */
+	if (ret || !drv || to_platform_driver(drv)->driver_managed_dma)
 		return ret;
 
 	ret = iommu_device_use_default_domain(dev);