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SubjectRe: [PATCH v2] mtd: break circular locks in register_mtd_blktrans
Date
On Thu, 2021-06-17 at 16:09:04 UTC, Desmond Cheong Zhi Xi wrote:
> Syzbot reported a circular locking dependency:
> https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb
>
> This happens because of the following lock dependencies:
>
> 1. loop_ctl_mutex -> bdev->bd_mutex (when loop_control_ioctl calls
> loop_remove, which then calls del_gendisk; this also happens in
> loop_exit which eventually calls loop_remove)
>
> 2. bdev->bd_mutex -> mtd_table_mutex (when blkdev_get_by_dev calls
> __blkdev_get, which then calls blktrans_open)
>
> 3. mtd_table_mutex -> major_names_lock (when register_mtd_blktrans
> calls __register_blkdev)
>
> 4. major_names_lock -> loop_ctl_mutex (when blk_request_module calls
> loop_probe)
>
> Hence there's an overall dependency of:
>
> loop_ctl_mutex ----------> bdev->bd_mutex
> ^ |
> | |
> | v
> major_names_lock <--------- mtd_table_mutex
>
> We can break this circular dependency by holding mtd_table_mutex only
> for the required critical section in register_mtd_blktrans. This
> avoids the mtd_table_mutex -> major_names_lock dependency.
>
> Reported-and-tested-by: syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com
> Co-developed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git mtd/fixes, thanks.

Miquel

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