Commit 5f478e4e authored Feb 08, 2017 by Tejun Heo Committed by Jens Axboe Feb 08, 2017

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



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().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: a13f35e8 ("writeback: don't embed root bdi_writeback_congested in bdi_writeback")
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Jens Axboe <axboe@fb.com>

parent b35ba01e

mm/backing-dev.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -759,15 +759,20 @@ 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);
		}

		#endif /* CONFIG_CGROUP_WRITEBACK */

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