[PATCH v1] platform: Fix race condition during DMA configure at IOMMU probe time

[Will McVicker <willmcvicker@google.com>]

If devices are probed asynchronously, then there is a chance that during
the IOMMU probe the driver is bound to the device in parallel. If this
happens after getting the platform_driver pointer while in the function
`platform_dma_configure()`, then the invalid `drv` pointer
(drv==0xf...ffd8) will be de-referenced since `dev->driver != NULL`.

To avoid a kernel panic and eliminate the race condition, we should
guard the usage of `dev->driver` by only reading it once at the
beginning of the function.

Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Signed-off-by: Will McVicker <willmcvicker@google.com>
---
 drivers/base/platform.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/base/platform.c b/drivers/base/platform.c
index 1813cfd0c4bd..b948c6e8e939 100644
--- a/drivers/base/platform.c
+++ b/drivers/base/platform.c
@@ -1440,7 +1440,8 @@ 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 platform_driver *pdrv = to_platform_driver(drv);
 	struct fwnode_handle *fwnode = dev_fwnode(dev);
 	enum dev_dma_attr attr;
 	int ret = 0;
@@ -1451,8 +1452,8 @@ 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 || pdrv->driver_managed_dma)
 		return ret;
 
 	ret = iommu_device_use_default_domain(dev);
-- 
2.49.0.805.g082f7c87e0-goog