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SubjectRe: [PATCH v2] mtd: break circular locks in register_mtd_blktrans
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On 30/6/21 4:43 pm, Miquel Raynal wrote:
> Hello,
>
> Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> wrote on Wed, 30 Jun
> 2021 16:04:17 +0800:
>
>> 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
>
> I'm fine with the patch, but it came too late in the release cycle so
> now I'm waiting -rc1 to apply it.
>
> Thanks,
> Miquèl
>

Got it. Thanks for the review, Miquèl.

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