[4.2.y-ckt stable] Patch "rfkill: fix rfkill_fop_read wait_event usage" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    rfkill: fix rfkill_fop_read wait_event usage

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt5.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 2b527d33bbe250d92ced63b93b760f2cf35c20a9 Mon Sep 17 00:00:00 2001
From: Johannes Berg <johannes.berg at intel.com>
Date: Tue, 26 Jan 2016 11:29:03 +0100
Subject: 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 at google.com>
Signed-off-by: Johannes Berg <johannes.berg at intel.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 31b5c63..c20ea1b 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1087,17 +1087,6 @@ 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)
 {
@@ -1114,8 +1103,11 @@ 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)
--
2.7.0