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 <willmcvicker@google.com>
Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Will McVicker <willmcvicker@google.com>
Link: https://lore.kernel.org/r/20250425133929.646493-3-robin.murphy@arm.com
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

diff --git a/drivers/bus/fsl-mc/fsl-mc-bus.c b/drivers/bus/fsl-mc/fsl-mc-bus.c
index 0c3a38d7f3358c25d5a475fb123021b65e70afad..7671bd1585455183e2ae402c703e6b812a8071c9 100644 (file)
--- a/drivers/bus/fsl-mc/fsl-mc-bus.c
+++ b/drivers/bus/fsl-mc/fsl-mc-bus.c
@@ -139,9 +139,9 @@ static int fsl_mc_bus_uevent(const struct device *dev, struct kobj_uevent_env *e
 
 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;
 
@@ -153,8 +153,8 @@ static int fsl_mc_dma_configure(struct device *dev)
        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);