[PATCH] PCIe: AER: during disable, check subordinate before walking

[Alex Chiang <achiang@hp.com>]

Commit 47a8b0cc (Enable PCIe AER only after checking firmware
support) wants to walk the PCI bus in the remove path to disable
AER, and calls pci_walk_bus for downstream bridges.

Unfortunately, in the remove path, we remove devices and bridges
in a depth-first manner, starting with the furthest downstream
bridge and working our way backwards.

The furthest downstream bridges will not have a dev->subordinate,
and we hit a NULL deref in pci_walk_bus.

Check for dev->subordinate first before attempting to walk the
PCI hierarchy below us.

Cc: Andrew Patterson <andrew.patterson@hp.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
---
Willy, this is .29 material, please push to Linus, thanks.

---
diff --git a/drivers/pci/pcie/aer/aerdrv_core.c b/drivers/pci/pcie/aer/aerdrv_core.c
index aebb5f6..677d680 100644
--- a/drivers/pci/pcie/aer/aerdrv_core.c
+++ b/drivers/pci/pcie/aer/aerdrv_core.c
@@ -133,6 +133,9 @@ static void set_downstream_devices_error_reporting(struct pci_dev *dev,
 						   bool enable)
 {
 	set_device_error_reporting(dev, &enable);
+
+	if (!dev->subordinate)
+		return;
 	pci_walk_bus(dev->subordinate, set_device_error_reporting, &enable);
 }