PCI: Fix pci_slot_release() NULL pointer dereference

BugLink: https://bugs.launchpad.net/bugs/1910822

commit 4684709 upstream.

If kobject_init_and_add() fails, pci_slot_release() is called to delete
slot->list from parent->slots.  But slot->list hasn't been initialized
yet, so we dereference a NULL pointer:

  Unable to handle kernel NULL pointer dereference at virtual address
00000000
  ...
  CPU: 10 PID: 1 Comm: swapper/0 Not tainted 4.4.240 #197
  task: ffffeb398a45ef10 task.stack: ffffeb398a470000
  PC is at __list_del_entry_valid+0x5c/0xb0
  LR is at pci_slot_release+0x84/0xe4
  ...
  __list_del_entry_valid+0x5c/0xb0
  pci_slot_release+0x84/0xe4
  kobject_put+0x184/0x1c4
  pci_create_slot+0x17c/0x1b4
  __pci_hp_initialize+0x68/0xa4
  pciehp_probe+0x1a4/0x2fc
  pcie_port_probe_service+0x58/0x84
  driver_probe_device+0x320/0x470

Initialize slot->list before calling kobject_init_and_add() to avoid this.

Fixes: 8a94644 ("PCI: Fix pci_create_slot() reference count leak")
Link: https://lore.kernel.org/r/1606876422-117457-1-git-send-email-zhongjubin@huawei.com
Signed-off-by: Jubin Zhong <zhongjubin@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org	# v5.9+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

Author: Jubin Zhong

drivers/pci/slot.c

@@ -308,16 +308,16 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
 		goto err;
 	}
 
+	INIT_LIST_HEAD(&slot->list);
+	list_add(&slot->list, &parent->slots);
+
 	err = kobject_init_and_add(&slot->kobj, &pci_slot_ktype, NULL,
 				   "%s", slot_name);
 	if (err) {
 		kobject_put(&slot->kobj);
 		goto err;
 	}
 
-	INIT_LIST_HEAD(&slot->list);
-	list_add(&slot->list, &parent->slots);
-
 	down_read(&pci_bus_sem);
 	list_for_each_entry(dev, &parent->devices, bus_list)
 		if (PCI_SLOT(dev->devfn) == slot_nr)