rfkill: fix rfkill_fop_read wait_event usage

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 <johannes.berg@intel.com>

1 files changed, 4 insertions, 12 deletions

diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index f53bf3b6558b..cf5b69ab1829 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1095,17 +1095,6 @@ 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)
 {
@@ -1122,8 +1111,11 @@ 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)