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Subject: Re: [PATCH] block: fix double-free in the failure path of
 cgwb_bdi_init()
References: <CACT4Y+ZsX1gQHdr7+tqhhB6CeKHBU=4VTMDj-meNbZ=uEPLKWA@mail.gmail.com>
 <20170208201907.GC25826@htj.duckdns.org>
From: Jens Axboe <axboe@fb.com>
Message-ID: <66bc2094-3f13-91da-41c2-78fa1e8a81e8@fb.com>
Date: Wed, 8 Feb 2017 13:52:52 -0700
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Sender: owner-linux-mm@kvack.org
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To: Tejun Heo <tj@kernel.org>, Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, xiakaixu@huawei.com, Vlastimil Babka <vbabka@suse.cz>, Joe Perches <joe@perches.com>, Mel Gorman <mgorman@techsingularity.net>, Michal Hocko <mhocko@suse.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, syzkaller <syzkaller@googlegroups.com>

On 02/08/2017 01:19 PM, Tejun Heo wrote:
> 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().

Queued up for 4.11.

-- 
Jens Axboe

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