rfkill: fix rfkill_fop_read wait_event usage

commit 6736fde9 upstream.

The code within wait_event_interruptible() is called with
!TASK_RUNNING, so mustn't call any functions that can sleep,
like mutex_lock().

Since we re-check the list_empty() in a loop after the wait,
it's safe to simply use list_empty() without locking.

This bug has existed forever, but was only discovered now
because all userspace implementations, including the default
'rfkill' tool, use poll() or select() to get a readable fd
before attempting to read.

Fixes: c64fb016 ("rfkill: create useful userspace interface")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/rfkill/core.c

@@ -1081,17 +1081,6 @@ static unsigned int rfkill_fop_poll(struct file *file, poll_table *wait)
 	return res;
 }
 
-static bool rfkill_readable(struct rfkill_data *data)
-{
-	bool r;
-
-	mutex_lock(&data->mtx);
-	r = !list_empty(&data->events);
-	mutex_unlock(&data->mtx);
-
-	return r;
-}
-
 static ssize_t rfkill_fop_read(struct file *file, char __user *buf,
 			       size_t count, loff_t *pos)
 {
@@ -1108,8 +1097,11 @@ static ssize_t rfkill_fop_read(struct file *file, char __user *buf,
 			goto out;
 		}
 		mutex_unlock(&data->mtx);
+		/* since we re-check and it just compares pointers,
+		 * using !list_empty() without locking isn't a problem
+		 */
 		ret = wait_event_interruptible(data->read_wait,
-					       rfkill_readable(data));
+					       !list_empty(&data->events));
 		mutex_lock(&data->mtx);
 
 		if (ret)