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SubjectRe: [PATCH v2] mtd: break circular locks in register_mtd_blktrans
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On 18/6/21 12:09 am, 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>
> ---
>
> Changes in v1 -> v2:
>
> Break the circular dependency in register_mtd_blktrans instead of blk_request_module, as suggested by Christoph Hellwig.
>
> drivers/mtd/mtd_blkdevs.c | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
> index fb8e12d590a1..7d26cfe24d05 100644
> --- a/drivers/mtd/mtd_blkdevs.c
> +++ b/drivers/mtd/mtd_blkdevs.c
> @@ -528,14 +528,10 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
> if (!blktrans_notifier.list.next)
> register_mtd_user(&blktrans_notifier);
>
> -
> - mutex_lock(&mtd_table_mutex);
> -
> ret = register_blkdev(tr->major, tr->name);
> if (ret < 0) {
> printk(KERN_WARNING "Unable to register %s block device on major %d: %d\n",
> tr->name, tr->major, ret);
> - mutex_unlock(&mtd_table_mutex);
> return ret;
> }
>
> @@ -545,12 +541,12 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
> tr->blkshift = ffs(tr->blksize) - 1;
>
> INIT_LIST_HEAD(&tr->devs);
> - list_add(&tr->list, &blktrans_majors);
>
> + mutex_lock(&mtd_table_mutex);
> + list_add(&tr->list, &blktrans_majors);
> mtd_for_each_device(mtd)
> if (mtd->type != MTD_ABSENT)
> tr->add_mtd(tr, mtd);
> -
> mutex_unlock(&mtd_table_mutex);
> return 0;
> }
>

Hi maintainers,

Any chance to review this patch?

For additional reference, the mtd_table_mutex --> major_names_lock
hierarchy that can be removed by this patch also appears in a different
lock chain:
https://syzkaller.appspot.com/bug?id=cbf5fe846f14a90f05e10df200b08c57941dc750

Best wishes,
Desmond

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