[PATCH] watchdog: pretimeout: Fix UAF in watchdog_unregister_governor()

[Tzung-Bi Shih <tzungbi@kernel.org>]

When a watchdog governor is unregistered, it updates existing watchdog
devices that were using this governor by falling back to `default_gov`.

If the governor being unregistered is currently set as `default_gov`,
the `default_gov` is never cleared.  This leads to 2 use-after-free
issues:
1. New watchdog devices registered after this point will inherit the
   dangling `default_gov`.
2. Existing watchdog devices using the unregistered governor will have
   their `wdd->gov` reassigned to the dangling `default_gov`.

Fix the UAF by clearing `default_gov` if it matches the governor being
unregistered.

Fixes: da0d12ff2b82 ("watchdog: pretimeout: add panic pretimeout governor")
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
---
 drivers/watchdog/watchdog_pretimeout.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/watchdog/watchdog_pretimeout.c b/drivers/watchdog/watchdog_pretimeout.c
index 19eb2ed2c7cb..02e09b9e396d 100644
--- a/drivers/watchdog/watchdog_pretimeout.c
+++ b/drivers/watchdog/watchdog_pretimeout.c
@@ -167,6 +167,8 @@ void watchdog_unregister_governor(struct watchdog_governor *gov)
 	}
 
 	spin_lock_irq(&pretimeout_lock);
+	if (default_gov == gov)
+		default_gov = NULL;
 	list_for_each_entry(p, &pretimeout_list, entry)
 		if (p->wdd->gov == gov)
 			p->wdd->gov = default_gov;
-- 
2.55.0.rc2.803.g1fd1e6609c-goog