4.16-stable review patch. If anyone has any objections, please let me know.
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From: Jan Kara jack@suse.cz
commit b8b784958eccbf8f51ebeee65282ca3fd59ea391 upstream.
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 Reviewed-by: Dave Chinner dchinner@redhat.com Signed-off-by: Jan Kara jack@suse.cz Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/fs-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- 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);