clk: fix invalid usage of list cursor in unregister

Fix invalid usage of a list_for_each_entry cursor in
clk_notifier_unregister(). When list is empty or if the list
is completely traversed (without breaking from the loop on one
of the entries) then the list cursor does not point to a valid
entry and therefore should not be used. The patch fixes a logical
bug that hasn't been seen in pratice however it is analogus
to the bug fixed in clk_notifier_register().

The issue was dicovered when running 5.12-rc1 kernel on x86_64
with KASAN enabled:
BUG: KASAN: global-out-of-bounds in clk_notifier_register+0xab/0x230
Read of size 8 at addr ffffffffa0d10588 by task swapper/0/1

CPU: 1 PID: 1 Comm: swapper/0 Not tainted 5.12.0-rc1 #1
Hardware name: Google Caroline/Caroline,
BIOS Google_Caroline.7820.430.0 07/20/2018
Call Trace:
 dump_stack+0xee/0x15c
 print_address_description+0x1e/0x2dc
 kasan_report+0x188/0x1ce
 ? clk_notifier_register+0xab/0x230
 ? clk_prepare_lock+0x15/0x7b
 ? clk_notifier_register+0xab/0x230
 clk_notifier_register+0xab/0x230
 dw8250_probe+0xc01/0x10d4
 ...
 Memory state around the buggy address:
  ffffffffa0d10480: 00 00 00 00 00 03 f9 f9 f9 f9 f9 f9 00 00 00 00
  ffffffffa0d10500: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f9 f9
 >ffffffffa0d10580: f9 f9 f9 f9 00 00 00 00 00 00 00 00 00 00 00 00
                          ^
  ffffffffa0d10600: 00 00 00 00 00 00 f9 f9 f9 f9 f9 f9 00 00 00 00
  ffffffffa0d10680: 00 00 00 00 00 00 00 00 f9 f9 f9 f9 00 00 00 00
  ==================================================================

Fixes: b2476490 ("clk: introduce the common clock framework")
Reported-by: Lukasz Majczak <lma@semihalf.com>
Signed-off-by: Lukasz Bartosik <lb@semihalf.com>
Link: https://lore.kernel.org/r/20210401225149.18826-2-lb@semihalf.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>

drivers/clk/clk.c

@@ -4394,32 +4394,28 @@ EXPORT_SYMBOL_GPL(clk_notifier_register);
  */
 int clk_notifier_unregister(struct clk *clk, struct notifier_block *nb)
 {
-	struct clk_notifier *cn = NULL;
-	int ret = -EINVAL;
+	struct clk_notifier *cn;
+	int ret = -ENOENT;
 
 	if (!clk || !nb)
 		return -EINVAL;
 
 	clk_prepare_lock();
 
-	list_for_each_entry(cn, &clk_notifier_list, node)
-		if (cn->clk == clk)
-			break;
-
-	if (cn->clk == clk) {
-		ret = srcu_notifier_chain_unregister(&cn->notifier_head, nb);
+	list_for_each_entry(cn, &clk_notifier_list, node) {
+		if (cn->clk == clk) {
+			ret = srcu_notifier_chain_unregister(&cn->notifier_head, nb);
 
-		clk->core->notifier_count--;
+			clk->core->notifier_count--;
 
-		/* XXX the notifier code should handle this better */
-		if (!cn->notifier_head.head) {
-			srcu_cleanup_notifier_head(&cn->notifier_head);
-			list_del(&cn->node);
-			kfree(cn);
+			/* XXX the notifier code should handle this better */
+			if (!cn->notifier_head.head) {
+				srcu_cleanup_notifier_head(&cn->notifier_head);
+				list_del(&cn->node);
+				kfree(cn);
+			}
+			break;
 		}
-
-	} else {
-		ret = -ENOENT;
 	}
 
 	clk_prepare_unlock();