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Subject: Re: [PATCH v2] mtd: break circular locks in register_mtd_blktrans
To:     Miquel Raynal <miquel.raynal@bootlin.com>
Cc:     richard@nod.at, vigneshr@ti.com, linux-mtd@lists.infradead.org,
        linux-kernel@vger.kernel.org, skhan@linuxfoundation.org,
        gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com,
        Christoph Hellwig <hch@lst.de>
References: <20210617160904.570111-1-desmondcheongzx@gmail.com>
 <03e19ec8-7479-9be2-3563-a2fcf9d0ec0c@gmail.com>
 <20210630104353.7575e920@xps13>
From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Message-ID: <99b6573c-8c6d-8bcc-af8d-ce63cdfb74e4@gmail.com>
Date:   Wed, 30 Jun 2021 17:21:23 +0800
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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.