bus: fsl_mc: Fix driver_managed_dma check

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_fsl_mc_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 <[e-mail]>
Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Signed-off-by: Robin Murphy <[e-mail]>
Reviewed-by: Will McVicker <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christophe Leroy <[e-mail]>

drivers/bus/fsl-mc/fsl-mc-bus.c

···
 
 static int fsl_mc_dma_configure(struct device *dev)
 {
+	const struct device_driver *drv = READ_ONCE(dev->driver);
 	struct device *dma_dev = dev;
 	struct fsl_mc_device *mc_dev = to_fsl_mc_device(dev);
-	struct fsl_mc_driver *mc_drv = to_fsl_mc_driver(dev->driver);
 	u32 input_id = mc_dev->icid;
 	int ret;
 
···
 	else
 		ret = acpi_dma_configure_id(dev, DEV_DMA_COHERENT, &input_id);
 
-	/* @mc_drv may not be valid when we're called from the IOMMU layer */
-	if (!ret && dev->driver && !mc_drv->driver_managed_dma) {
+	/* @drv may not be valid when we're called from the IOMMU layer */
+	if (!ret && drv && !to_fsl_mc_driver(drv)->driver_managed_dma) {
 		ret = iommu_device_use_default_domain(dev);
 		if (ret)
 			arch_teardown_dma_ops(dev);