Re: [PATCH v3] md: Fix the null-ptr-deref of 'mddev->private' while submitting IO

[Zhihao Cheng <chengzhihao1@huawei.com>]

在 2026/9/18 16:20, Abd-Alrhman Masalkhi 写道:
> On Fri, Sep 18, 2026 at 15:38 +0800, Zhihao Cheng wrote:
>> 在 2026/9/18 15:29, Abd-Alrhman Masalkhi 写道:
>>>
>>> Hi Zhihao,
>>>
>>> On Fri, Sep 18, 2026 at 15:09 +0800, Zhihao Cheng wrote:
>>>> Concurrent processes md_stop and IO submitting could trigger a
>>>> null-ptr-deref of 'mddev->private':
>>>>
>>>>    BUG: kernel NULL pointer dereference, address: 0000000000000070
>>>>    RIP: 0010:_wait_barrier+0x2f/0x250
>>>>    Call Trace:
>>>>     raid1_make_request+0x150/0xf50
>>>>     md_handle_request+0x104/0x530
>>>>     md_submit_bio+0x76/0x130
>>>>     submit_bio+0xdd/0x250
>>>>     submit_bio_wait+0x1f/0x40
>>>>     __blkdev_direct_IO_simple+0x1f6/0x370
>>>>     blkdev_write_iter+0x3b2/0x520
>>>>     ksys_write+0x7d/0x190
>>>>
>>>>                 P1
>>>>     fd = open(/dev/md0, O_RDWR)
>>>> 	                      P2 (forked from P1, fd' <= fd)
>>>>     write(fd)
>>>>      submit_bio
>>>>       md_handle_request
>>>>        raid1_make_request
>>>>         raid1_write_request
>>>>                                     ioctl(fd, STOP_ARRAY)
>>>> 		                   mddev_set_closing_and_sync_blockdev
>>>> 				   // check passed, mddev->openers = 1,
>>>> 				   // because md_open() is only called
>>>> 				   // once in P1->open
>>>> 				   do_md_stop
>>>> 				    __md_stop
>>>> 				     mddev->private = NULL
>>>>
>>>>          conf = mddev->private // NULL
>>>>           wait_barrier(conf, sector) // null-ptr-deref !
>>>>
>>>> It is a common problem for raid0/1/10/5, and __md_stop could be triggered
>>>> by several paths(eg. ioctl, sysfs, ->dtr). Fix it by replacing
>>>> mddev_lock() with mddev_suspend_and_lock() for all __md_stop() callers.
>>>> The caller dm_table_destroy() is guaranteed being invoked with device
>>>> suspended, so raid_dtr() could keep using mddev_lock_nointr().
>>>> The caller array_state_store() is guaranteed by the check
>>>> mddev_set_closing_and_sync_blockdev(mddev, 0). For example, someone open
>>>> /dev/mdx, write something and close /dev/mdx, it won't trigger the
>>>> problem, all dirty pages can be flushed before mddev->openers decrement.
>>>> Besides, fail the submitting IO in md_handle_request() if the
>>>> 'mddev->pers' becomes NULL.
>>>>
>>>> Fetch a reproducer in https://bugzilla.kernel.org/show_bug.cgi?id=222020
>>>>
>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>> Reported-by: syzbot+3fe892ea5fc292e1353f@syzkaller.appspotmail.com
>>>> Closes: https://syzkaller.appspot.com/bug?extid=3fe892ea5fc292e1353f
>>>> Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
>>>> ---
>>>>    v1->v2:
>>>>     1. Add 'mddev->pers != NULL' check before make_request
>>>>     2. Delete dm-raid caller(->dtr) modifications
>>>>     3. Move memalloc_noio_restore after mddev_unlock_and_resume
>>>>    v2->v3:
>>>>     1. Remove modifications in array_state_store()
>>>>     2. update commit msg
>>>>    drivers/md/md.c | 15 +++++++++++++++
>>>>    1 file changed, 15 insertions(+)
>>>>
>>>> diff --git a/drivers/md/md.c b/drivers/md/md.c
>>>> index 680b34a63cb3..e73338c26d53 100644
>>>> --- a/drivers/md/md.c
>>>> +++ b/drivers/md/md.c
>>>> @@ -414,6 +414,20 @@ bool md_handle_request(struct mddev *mddev, struct bio *bio)
>>>>    		if (!percpu_ref_tryget_live(&mddev->active_io))
>>>>    			goto check_suspended;
>>>>    	}
>>>> +	if (!mddev->pers) {
>>> Isn't this case already handled by md_submit_bio(). take a look in
>>> md_submit_bio()
>> Hi,Abd-Alrhman
>> I understand that 'mddev == NULL || mddev->pers == NULL' in
>> md_submit_bio() is a qiuck check, there still exists a small race window:
>>        P1                     P2
>> md_submit_bio
>>    if (mddev == NULL || mddev->pers == NULL)
>>                           md_ioctl
>>                            mddev_suspend_and_lock
>>                            do_md_stop->__md_stop
>>                            // set mddev->pers/private as NULL
>>                            mddev_unlock_and_resume
>>    md_handle_request
>>     if (is_suspended(mddev, bio))
>>     percpu_ref_tryget_live(&mddev->active_io)
>>     mddev->pers // null-ptr-deref
> 
> It makes sense. It would solve only the null pointer reference, but I
> see another issue. Look at the comment before calling
> mddev_set_closing_and_sync_blockdev() in md_ioctl(). it says "Need to
> flush page cache, and ensure no-one else opens and writes". The
> suspending happens after flushing the page cache, in this case,
> there might be other writes in-flight.

At present, the current implementation does not align with the expected 
annotations. Yu Kai privately gave me a feasible suggestion, which is to 
have md_ioctl operate on a character device(like /dev/mapper/control) 
instead of an md block device. This fix patch will serve as a temporary 
solution before the official implementation is merged. By the way, I 
don't have much time to implment it, do you have time to deal with the 
official solution?
> 
>>>
>>>> +		/*
>>>> +		 * The __md_stop() sets 'mddev->private' to NULL during
>>>> +		 * the IO submitting, check 'mddev->pers' before the IO
>>>> +		 * being processed by specific driver to avoid the
>>>> +		 * null-ptr-deref of 'mddev-><member>'. The check is
>>>> +		 * safe because the IO has got the 'mddev->active_io'
>>>> +		 * reference, and all __md_stop() callers will wait for
>>>> +		 * the reference to be zero.
>>>> +		 */
>>>> +		bio_io_error(bio);
>>>> +		percpu_ref_put(&mddev->active_io);
>>>> +		return true;
>>>> +	}
>>>>    	if (!mddev->pers->make_request(mddev, bio)) {
>>>>    		percpu_ref_put(&mddev->active_io);
>>>>    		if (mddev_is_dm(mddev) && mddev->pers->prepare_suspend)
>>>> @@ -8299,6 +8313,7 @@ static bool md_ioctl_need_suspend(unsigned int cmd)
>>>>    	case HOT_REMOVE_DISK:
>>>>    	case SET_BITMAP_FILE:
>>>>    	case SET_ARRAY_INFO:
>>>> +	case STOP_ARRAY:
>>>>    		return true;
>>>>    	default:
>>>>    		return false;
>>>> -- 
>>>> 2.52.0
>>>>
>>>
>>
>