appletalk: Fix potential NULL pointer dereference in unregister_snap_client

commit 9804501f upstream.

register_snap_client may return NULL, all the callers
check it, but only print a warning. This will result in
NULL pointer dereference in unregister_snap_client and other
places.

It has always been used like this since v2.6
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to <4.15: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/atalk.h

@@ -107,7 +107,7 @@ static __inline__ struct elapaarp *aarp_hdr(struct sk_buff *skb)
 #define AARP_RESOLVE_TIME	(10 * HZ)
 
 extern struct datalink_proto *ddp_dl, *aarp_dl;
-extern void aarp_proto_init(void);
+extern int aarp_proto_init(void);
 
 /* Inter module exports */
 

net/appletalk/aarp.c

@@ -879,15 +879,24 @@ static struct notifier_block aarp_notifier = {
 
 static unsigned char aarp_snap_id[] = { 0x00, 0x00, 0x00, 0x80, 0xF3 };
 
-void __init aarp_proto_init(void)
+int __init aarp_proto_init(void)
 {
+	int rc;
+
 	aarp_dl = register_snap_client(aarp_snap_id, aarp_rcv);
-	if (!aarp_dl)
+	if (!aarp_dl) {
 		printk(KERN_CRIT "Unable to register AARP with SNAP.\n");
+		return -ENOMEM;
+	}
 	setup_timer(&aarp_timer, aarp_expire_timeout, 0);
 	aarp_timer.expires  = jiffies + sysctl_aarp_expiry_time;
 	add_timer(&aarp_timer);
-	register_netdevice_notifier(&aarp_notifier);
+	rc = register_netdevice_notifier(&aarp_notifier);
+	if (rc) {
+		del_timer_sync(&aarp_timer);
+		unregister_snap_client(aarp_dl);
+	}
+	return rc;
 }
 
 /* Remove the AARP entries associated with a device. */

net/appletalk/ddp.c

@@ -1911,9 +1911,6 @@ static unsigned char ddp_snap_id[] = { 0x08, 0x00, 0x07, 0x80, 0x9B };
 EXPORT_SYMBOL(atrtr_get_dev);
 EXPORT_SYMBOL(atalk_find_dev_addr);
 
-static const char atalk_err_snap[] __initconst =
-	KERN_CRIT "Unable to register DDP with SNAP.\n";
-
 /* Called by proto.c on kernel start up */
 static int __init atalk_init(void)
 {
@@ -1928,17 +1925,22 @@ static int __init atalk_init(void)
 		goto out_proto;
 
 	ddp_dl = register_snap_client(ddp_snap_id, atalk_rcv);
-	if (!ddp_dl)
-		printk(atalk_err_snap);
+	if (!ddp_dl) {
+		pr_crit("Unable to register DDP with SNAP.\n");
+		goto out_sock;
+	}
 
 	dev_add_pack(&ltalk_packet_type);
 	dev_add_pack(&ppptalk_packet_type);
 
 	rc = register_netdevice_notifier(&ddp_notifier);
 	if (rc)
-		goto out_sock;
+		goto out_snap;
+
+	rc = aarp_proto_init();
+	if (rc)
+		goto out_dev;
 
-	aarp_proto_init();
 	rc = atalk_proc_init();
 	if (rc)
 		goto out_aarp;
@@ -1952,11 +1954,13 @@ static int __init atalk_init(void)
 	atalk_proc_exit();
 out_aarp:
 	aarp_cleanup_module();
+out_dev:
 	unregister_netdevice_notifier(&ddp_notifier);
-out_sock:
+out_snap:
 	dev_remove_pack(&ppptalk_packet_type);
 	dev_remove_pack(&ltalk_packet_type);
 	unregister_snap_client(ddp_dl);
+out_sock:
 	sock_unregister(PF_APPLETALK);
 out_proto:
 	proto_unregister(&ddp_proto);