PCI/DPC: Fix use-after-free on concurrent DPC and hot-removal

Keith reports a use-after-free when a DPC event occurs concurrently to
hot-removal of the same portion of the hierarchy:

The dpc_handler() awaits readiness of the secondary bus below the
Downstream Port where the DPC event occurred.  To do so, it polls the
config space of the first child device on the secondary bus.  If that
child device is concurrently removed, accesses to its struct pci_dev
cause the kernel to oops.

That's because pci_bridge_wait_for_secondary_bus() neglects to hold a
reference on the child device.  Before v6.3, the function was only
called on resume from system sleep or on runtime resume.  Holding a
reference wasn't necessary back then because the pciehp IRQ thread
could never run concurrently.  (On resume from system sleep, IRQs are
not enabled until after the resume_noirq phase.  And runtime resume is
always awaited before a PCI device is removed.)

However starting with v6.3, pci_bridge_wait_for_secondary_bus() is also
called on a DPC event.  Commit 53b54ad0 ("PCI/DPC: Await readiness
of secondary bus after reset"), which introduced that, failed to
appreciate that pci_bridge_wait_for_secondary_bus() now needs to hold a
reference on the child device because dpc_handler() and pciehp may
indeed run concurrently.  The commit was backported to v5.10+ stable
kernels, so that's the oldest one affected.

Add the missing reference acquisition.

Abridged stack trace:

  BUG: unable to handle page fault for address: 00000000091400c0
  CPU: 15 PID: 2464 Comm: irq/53-pcie-dpc 6.9.0
  RIP: pci_bus_read_config_dword+0x17/0x50
  pci_dev_wait()
  pci_bridge_wait_for_secondary_bus()
  dpc_reset_link()
  pcie_do_recovery()
  dpc_handler()

Fixes: 53b54ad0 ("PCI/DPC: Await readiness of secondary bus after reset")
Closes: https://lore.kernel.org/r/20240612181625.3604512-3-kbusch@meta.com/
Link: https://lore.kernel.org/linux-pci/8e4bcd4116fd94f592f2bf2749f168099c480ddf.1718707743.git.lukas@wunner.deReported-by: Keith Busch <kbusch@kernel.org>
Tested-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: stable@vger.kernel.org # v5.10+

drivers/pci/pci.c

@@ -4753,7 +4753,7 @@ static int pci_bus_max_d3cold_delay(const struct pci_bus *bus)
  */
 int pci_bridge_wait_for_secondary_bus(struct pci_dev *dev, char *reset_type)
 {
-	struct pci_dev *child;
+	struct pci_dev *child __free(pci_dev_put) = NULL;
 	int delay;
 
 	if (pci_dev_is_disconnected(dev))
@@ -4782,8 +4782,8 @@ int pci_bridge_wait_for_secondary_bus(struct pci_dev *dev, char *reset_type)
 		return 0;
 	}
 
-	child = list_first_entry(&dev->subordinate->devices, struct pci_dev,
-				 bus_list);
+	child = pci_dev_get(list_first_entry(&dev->subordinate->devices,
+					     struct pci_dev, bus_list));
 	up_read(&pci_bus_sem);
 
 	/*