linux-kernel - Re: [PATCH v2] mtd: break circular locks in register_mtd

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Message-Id: <20210715225040.66117-1-miquel.raynal@bootlin.com>
Date:   Fri, 16 Jul 2021 00:50:40 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        miquel.raynal@...tlin.com, richard@....at, vigneshr@...com
Cc:     linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+6a8a0d93c91e8fbf2e80@...kaller.appspotmail.com,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] mtd: break circular locks in register_mtd_blktrans

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@...kaller.appspotmail.com
> Co-developed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>

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

Miquel