locking/rtmutex: Avoid a NULL pointer dereference on deadlock

Sebastian Andrzej Siewior · Ingo Molnar · commit 8d1e5a1a1ccf · 2015-02-18T10:20:32.000+01:00
With task_blocks_on_rt_mutex() returning early -EDEADLK we never add the waiter to the waitqueue. Later, we try to remove it via remove_waiter() and go boom in rt_mutex_top_waiter() because rb_entry() gives a NULL pointer. ( Tested on v3.18-RT where rtmutex is used for regular mutex and I tried to get one twice in a row. ) Not sure when this started but I guess 397335f ("rtmutex: Fix deadlock detector for real") or commit 3d5c934 ("rtmutex: Handle deadlock detection smarter"). Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Acked-by: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: <stable@vger.kernel.org> # for v3.16 and later kernels Link: http://lkml.kernel.org/r/1424187823-19600-1-git-send-email-bigeasy@linutronix.de Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
@@ -1193,7 +1193,8 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 		ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);
 
 	if (unlikely(ret)) {
-		remove_waiter(lock, &waiter);
+		if (rt_mutex_has_waiters(lock))
+			remove_waiter(lock, &waiter);
 		rt_mutex_handle_deadlock(ret, chwalk, &waiter);
 	}
 

Original file line number	Diff line number	Diff line change
`@@ -1193,7 +1193,8 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,`
`1193`	`1193`	`ret = __rt_mutex_slowlock(lock, state, timeout, &waiter);`
`1194`	`1194`
`1195`	`1195`	`if (unlikely(ret)) {`
`1196`		`- remove_waiter(lock, &waiter);`
	`1196`	`+ if (rt_mutex_has_waiters(lock))`
	`1197`	`+ remove_waiter(lock, &waiter);`
`1197`	`1198`	`rt_mutex_handle_deadlock(ret, chwalk, &waiter);`
`1198`	`1199`	`}`
`1199`	`1200`