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From: Jan Kara <jack@suse.cz>
To: <linux-fsdevel@vger.kernel.org>
Cc: linux-block@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
	Jan Kara <jack@suse.cz>,
	Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
	Tejun Heo <tj@kernel.org>
Subject: [PATCH] bdi: Fix oops in wb_workfn()
Date: Thu,  3 May 2018 18:26:26 +0200	[thread overview]
Message-ID: <20180503162626.27753-1-jack@suse.cz> (raw)

Syzbot has reported that it can hit a NULL pointer dereference in
wb_workfn() due to wb->bdi->dev being NULL. This indicates that
wb_workfn() was called for an already unregistered bdi which should not
happen as wb_shutdown() called from bdi_unregister() should make sure
all pending writeback works are completed before bdi is unregistered.
Except that wb_workfn() itself can requeue the work with:

	mod_delayed_work(bdi_wq, &wb->dwork, 0);

and if this happens while wb_shutdown() is waiting in:

	flush_delayed_work(&wb->dwork);

the dwork can get executed after wb_shutdown() has finished and
bdi_unregister() has cleared wb->bdi->dev.

Make wb_workfn() use wakeup_wb() for requeueing the work which takes all
the necessary precautions against racing with bdi unregistration.

CC: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
CC: Tejun Heo <tj@kernel.org>
Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977
Reported-by: syzbot <syzbot+9873874c735f2892e7e9@syzkaller.appspotmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/fs-writeback.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 47d7c151fcba..471d863958bc 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1961,7 +1961,7 @@ void wb_workfn(struct work_struct *work)
 	}
 
 	if (!list_empty(&wb->work_list))
-		mod_delayed_work(bdi_wq, &wb->dwork, 0);
+		wb_wakeup(wb);
 	else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
 		wb_wakeup_delayed(wb);
 
-- 
2.13.6

             reply	other threads:[~2018-05-03 16:26 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-03 16:26 Jan Kara [this message]
2018-05-03 21:55 ` Dave Chinner
2018-05-03 21:57   ` Jens Axboe
2018-05-09  9:48   ` Jan Kara
2018-05-03 22:35 ` Tetsuo Handa
2018-05-09  9:47   ` Jan Kara
2018-05-19 14:27   ` Tetsuo Handa
2018-05-21  9:38     ` Jan Kara
2018-05-25 10:15       ` Tetsuo Handa
2018-05-09 10:31 ` Jan Kara
2018-05-09 14:42   ` Jens Axboe

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