[PATCH -next] watchdog: Fix potential dereferencing of null pointer

[Wang Wensheng]

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 <wangwenshe...@huawei.com>
---
 drivers/watchdog/watchdog_core.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
index 423844757812..5269761ba072 100644
--- a/drivers/watchdog/watchdog_core.c
+++ b/drivers/watchdog/watchdog_core.c
@@ -252,10 +252,8 @@ static int __watchdog_register_device(struct 
watchdog_device *wdd)
                wdd->id = id;
 
                ret = watchdog_dev_register(wdd);
-               if (ret) {
-                       ida_simple_remove(&watchdog_ida, id);
-                       return ret;
-               }
+               if (ret)
+                       goto id_remove;
        }
 
        /* Module parameter to force watchdog policy on reboot. */
@@ -267,15 +265,17 @@ 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;
+               if (!wdd->ops->stop) {
+                       ret = -EINVAL;
+                       goto dev_unregister;
+               }
 
+               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;
+                       goto dev_unregister;
                }
        }
 
@@ -289,6 +289,13 @@ static int __watchdog_register_device(struct 
watchdog_device *wdd)
        }
 
        return 0;
+
+dev_unregister:
+       watchdog_dev_unregister(wdd);
+id_remove:
+       ida_simple_remove(&watchdog_ida, id);
+
+       return ret;
 }
 
 /**
-- 
2.25.0