NULL pointer dereference in Linux kernel PCI device tree helpers

In the Linux kernel, the following vulnerability has been resolved: PCI: of_property: Omit bus properties without a subordinate bus A bridge (a device with a Type 1 header) may not have a secondary bus allocated (pdev->subordinate), e.g., if there are no available bus numbers or the bridge secondary/subordinate bus numbers are not writable. The dynamic OF helpers of_pci_prop_bus_range() and of_pci_prop_intr_map() dereference pdev->subordinate without checking it. When CONFIG_PCI_DYNAMIC_OF_NODES is enabled, this can cause a NULL pointer dereference and early boot hang. Generate 'bus-range' and 'interrupt-map' properties only when a subordinate bus exists. Keep the node and its remaining properties for bridges without one. The problem was latent since 407d1a51921e ("PCI: Create device tree node for bridge"), but wasn't reachable until 1f340724419e ("PCI: of: Create device tree PCI host bridge node"), which appeared in v6.15. Before 1f340724419e, of_pci_make_dev_node() returned early because the parent OF node was missing. [bhelgaas: move pdev->subordinate test to callees, commit log]