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

From: Tejun Heo
Date: Wed Feb 08 2017 - 16:32:34 EST

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

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

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Fixes: a13f35e87140 ("writeback: don't embed root bdi_writeback_congested in bdi_writeback")
Cc: stable@xxxxxxxxxxxxxxx # v4.2+

ISTR seeing another fix for this bug but can't find it right now. If
I'm imagining things, please apply this one. If not, either one is


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

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 3bfed5ab..61b3407 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -758,15 +758,20 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi)
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);