PCI: 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
Time-of-Check to Time-of-Use (TOCTOU) issue, where dev->driver becomes
valid after to_pci_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.

Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Reported-by: Will McVicker <willmcvicker@google.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Will McVicker <willmcvicker@google.com>
Link: https://patch.msgid.link/20250425133929.646493-4-robin.murphy@arm.com

diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index 67db34fd10ee7101baeeaae1bb9bec3b13e2fdeb..01e6aea1b0c7a112704d3dbcc41cee84df874317 100644 (file)
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -1628,7 +1628,7 @@ static int pci_bus_num_vf(struct device *dev)
  */
 static int pci_dma_configure(struct device *dev)
 {
-       struct pci_driver *driver = to_pci_driver(dev->driver);
+       const struct device_driver *drv = READ_ONCE(dev->driver);
        struct device *bridge;
        int ret = 0;
 
@@ -1645,8 +1645,8 @@ static int pci_dma_configure(struct device *dev)
 
        pci_put_host_bridge_device(bridge);
 
-       /* @driver may not be valid when we're called from the IOMMU layer */
-       if (!ret && dev->driver && !driver->driver_managed_dma) {
+       /* @drv may not be valid when we're called from the IOMMU layer */
+       if (!ret && drv && !to_pci_driver(drv)->driver_managed_dma) {
                ret = iommu_device_use_default_domain(dev);
                if (ret)
                        arch_teardown_dma_ops(dev);