rfkill: fix rfkill_fop_read wait_event usage

commit 6736fde9672ff6717ac576e9bba2fd5f3dfec822 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: c64fb01627e24 ("rfkill: create useful userspace interface")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index e2e2e6154fd288f06b3c04880aa551c5db5ddec5..6563cc04c5782d236644e50bf8bf5f7820f7fd09 100644 (file)
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1088,17 +1088,6 @@ 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)
 {
@@ -1115,8 +1104,11 @@ 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)