PCI: acpiphp_ibm: Fix null dereferences on null ibm_slot

ibm_slot_from_id() can return null if the des header signature is not
"aPCI" or if the kmalloc() for the return ACPI descriptor fails, causing
potential null pointer dereferences on the return null descriptor.

Handle the null case with appropriate check and error return.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/hotplug/acpiphp_ibm.c

@@ -154,7 +154,8 @@ static union apci_descriptor *ibm_slot_from_id(int id)
 ibm_slot_done:
 	if (ret) {
 		ret = kmalloc(sizeof(union apci_descriptor), GFP_KERNEL);
-		memcpy(ret, des, sizeof(union apci_descriptor));
+		if (ret)
+			memcpy(ret, des, sizeof(union apci_descriptor));
 	}
 	kfree(table);
 	return ret;
@@ -175,8 +176,13 @@ static int ibm_set_attention_status(struct hotplug_slot *slot, u8 status)
 	acpi_status stat;
 	unsigned long long rc;
 	union apci_descriptor *ibm_slot;
+	int id = hpslot_to_sun(slot);
 
-	ibm_slot = ibm_slot_from_id(hpslot_to_sun(slot));
+	ibm_slot = ibm_slot_from_id(id);
+	if (!ibm_slot) {
+		pr_err("APLS null ACPI descriptor for slot %d\n", id);
+		return -ENODEV;
+	}
 
 	pr_debug("%s: set slot %d (%d) attention status to %d\n", __func__,
 			ibm_slot->slot.slot_num, ibm_slot->slot.slot_id,
@@ -215,8 +221,13 @@ static int ibm_set_attention_status(struct hotplug_slot *slot, u8 status)
 static int ibm_get_attention_status(struct hotplug_slot *slot, u8 *status)
 {
 	union apci_descriptor *ibm_slot;
+	int id = hpslot_to_sun(slot);
 
-	ibm_slot = ibm_slot_from_id(hpslot_to_sun(slot));
+	ibm_slot = ibm_slot_from_id(id);
+	if (!ibm_slot) {
+		pr_err("APLS null ACPI descriptor for slot %d\n", id);
+		return -ENODEV;
+	}
 
 	if (ibm_slot->slot.attn & 0xa0 || ibm_slot->slot.status[1] & 0x08)
 		*status = 1;