[added to the 4.1 stable tree] rfkill: fix rfkill_fop_read wait_event usage

From: Johannes Berg <redacted>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit 6736fde9672ff6717ac576e9bba2fd5f3dfec822 ]

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.

Cc: stable@vger.kernel.org
Fixes: c64fb01627e24 ("rfkill: create useful userspace interface")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Johannes Berg <redacted>
Signed-off-by: Sasha Levin <redacted>
---
 net/rfkill/core.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index fa7cd79..a97bb73 100644
--- a/net/rfkill/core.c
+++ b/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)

-- 
2.5.0