watchdog: Fix potential dereferencing of null pointer

A reboot notifier, which stops the WDT by calling the stop hook without
any check, would be registered when we set WDOG_STOP_ON_REBOOT flag.

Howerer we allow the WDT driver to omit the stop hook since commit
"d0684c8a93549" ("watchdog: Make stop function optional") and provide
a module parameter for user that controls the WDOG_STOP_ON_REBOOT flag
in commit 9232c80659e94 ("watchdog: Add stop_on_reboot parameter to
control reboot policy"). Together that commits make user potential to
insert a watchdog driver that don't provide a stop hook but with the
stop_on_reboot parameter set, then dereferencing of null pointer occurs
on system reboot.

Check the stop hook before registering the reboot notifier to fix the
issue.

Fixes: d0684c8a9354 ("watchdog: Make stop function optional")
Signed-off-by: Wang Wensheng <wangwensheng4@huawei.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201109130512.28121-1-wangwensheng4@huawei.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
index 4238447..0e9a995 100644 (file)
--- a/drivers/watchdog/watchdog_core.c
+++ b/drivers/watchdog/watchdog_core.c
@@ -267,15 +267,19 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
        }
 
        if (test_bit(WDOG_STOP_ON_REBOOT, &wdd->status)) {
-               wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
-
-               ret = register_reboot_notifier(&wdd->reboot_nb);
-               if (ret) {
-                       pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
-                              wdd->id, ret);
-                       watchdog_dev_unregister(wdd);
-                       ida_simple_remove(&watchdog_ida, id);
-                       return ret;
+               if (!wdd->ops->stop)
+                       pr_warn("watchdog%d: stop_on_reboot not supported\n", wdd->id);
+               else {
+                       wdd->reboot_nb.notifier_call = watchdog_reboot_notifier;
+
+                       ret = register_reboot_notifier(&wdd->reboot_nb);
+                       if (ret) {
+                               pr_err("watchdog%d: Cannot register reboot notifier (%d)\n",
+                                       wdd->id, ret);
+                               watchdog_dev_unregister(wdd);
+                               ida_simple_remove(&watchdog_ida, id);
+                               return ret;
+                       }
                }
        }