Patch "block: fix double-free in the failure path of cgwb_bdi_init()" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

block: fix double-free in the failure path of cgwb_bdi_init()

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
block-fix-double-free-in-the-failure-path-of-cgwb_bdi_init.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Tejun Heo <[email protected]>
Date: Wed, 8 Feb 2017 15:19:07 -0500
Subject: block: fix double-free in the failure path of cgwb_bdi_init()

From: Tejun Heo <[email protected]>

commit 5f478e4ea5c5560b4e40eb136991a09f9389f331 upstream.

When !CONFIG_CGROUP_WRITEBACK, bdi has single bdi_writeback_congested at bdi->wb_congested. cgwb_bdi_init() allocates it with kzalloc() and doesn't do further initialization. This usually works fine as the reference count gets bumped to 1 by wb_init() and the put from wb_exit() releases it.

However, when wb_init() fails, it puts the wb base ref automatically freeing the wb and the explicit kfree() in cgwb_bdi_init() error path ends up trying to free the same pointer the second time causing a double-free.

Fix it by explicitly initilizing the refcnt to 1 and putting the base ref from cgwb_bdi_destroy().

---
 mm/backing-dev.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

@@ -757,15 +757,20 @@ static int cgwb_bdi_init(struct backing_
 	if (!bdi->wb_congested)
 		return -ENOMEM;
 
+	atomic_set(&bdi->wb_congested->refcnt, 1);
+
 	err = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);
 	if (err) {
-		kfree(bdi->wb_congested);
+		wb_congested_put(bdi->wb_congested);
 		return err;
 	}
 	return 0;
 }
 
-static void cgwb_bdi_destroy(struct backing_dev_info *bdi) { }
+static void cgwb_bdi_destroy(struct backing_dev_info *bdi)
+{
+	wb_congested_put(bdi->wb_congested);
+}
 
 #endif	/* CONFIG_CGROUP_WRITEBACK */
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/block-fix-double-free-in-the-failure-path-of-cgwb_bdi_init.patch