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

[Abd-Alrhman Masalkhi <abd.masalkhi@gmail.com>]

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()

> +		/*
> +		 * 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
>

-- 
Best Regards,
Abd-Alrhman