In the Linux kernel, the following vulnerability has been resolved:

PCI: Add missing bridge lock to pci_bus_lock()

One of the true positives that the cfg_access_lock lockdep effort
identified is this sequence:

  WARNING: CPU: 14 PID: 1 at drivers/pci/pci.c:4886 pci_bridge_secondary_bus_reset+0x5d/0x70
  RIP: 0010:pci_bridge_secondary_bus_reset+0x5d/0x70
  Call Trace:
   <TASK>
   ? __warn+0x8c/0x190
   ? pci_bridge_secondary_bus_reset+0x5d/0x70
   ? report_bug+0x1f8/0x200
   ? handle_bug+0x3c/0x70
   ? exc_invalid_op+0x18/0x70
   ? asm_exc_invalid_op+0x1a/0x20
   ? pci_bridge_secondary_bus_reset+0x5d/0x70
   pci_reset_bus+0x1d8/0x270
   vmd_probe+0x778/0xa10
   pci_device_probe+0x95/0x120

Where pci_reset_bus() users are triggering unlocked secondary bus resets.
Ironically pci_bus_reset(), several calls down from pci_reset_bus(), uses
pci_bus_lock() before issuing the reset which locks everything *but* the
bridge itself.

For the same motivation as adding:

  bridge = pci_upstream_bridge(dev);
  if (bridge)
    pci_dev_lock(bridge);

to pci_reset_function() for the "bus" and "cxl_bus" reset cases, add
pci_dev_lock() for @bus->self to pci_bus_lock().

[bhelgaas: squash in recursive locking deadlock fix from Keith Busch:
https://lore.kernel.org/r/20240711193650.701834-1-kbusch@meta.com]