Re: [PATCH v2] md: Fix the null-ptr-deref of 'mddev->private' while submitting IO

[yu kuai]

Hi,

在 2026/9/17 10:42, Zhihao Cheng 写道:
> 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(). 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=3fe892ea5fc292e1353f
> Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
> ---
>   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
>   drivers/md/md.c | 32 +++++++++++++++++++++++++++++---
>   1 file changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 680b34a63cb3..3dfc34caa2dc 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) {
> +		/*
> +		 * 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)
> @@ -4657,6 +4671,8 @@ array_state_store(struct mddev *mddev, const char *buf, size_t len)
>   {
>   	int err = 0;
>   	enum array_state st = match_word(buf, array_states);
> +	unsigned int noio_flags = 0;
> +	bool suspend = false;
>   
>   	/* No lock dependent actions */
>   	switch (st) {
> @@ -4666,9 +4682,11 @@ array_state_store(struct mddev *mddev, const char *buf, size_t len)
>   	case broken:		/* cannot be set */
>   	case bad_word:
>   		return -EINVAL;
> +	case inactive:
>   	case clear:
> +		suspend = true;
> +		fallthrough;
>   	case readonly:
> -	case inactive:
>   	case read_auto:
>   		if (!mddev->pers || !md_is_rdwr(mddev))
>   			break;
> @@ -4702,9 +4720,11 @@ array_state_store(struct mddev *mddev, const char *buf, size_t len)
>   		spin_unlock(&mddev->lock);
>   		return err ?: len;
>   	}
> -	err = mddev_lock(mddev);
> +	err = suspend ? mddev_suspend_and_lock(mddev) : mddev_lock(mddev);

Unlike ioctl path, where the checking of opener is just 1, mddev_set_closing_and_sync_blockdev() here
already check there is no opener, so there can't be any IO inflight.

>   	if (err)
>   		return err;
> +	if (suspend)
> +		noio_flags = memalloc_noio_save();
>   
>   	switch (st) {
>   	case inactive:
> @@ -4775,7 +4795,12 @@ array_state_store(struct mddev *mddev, const char *buf, size_t len)
>   			mddev->hold_active = 0;
>   		sysfs_notify_dirent_safe(mddev->sysfs_state);
>   	}
> -	mddev_unlock(mddev);
> +	if (suspend) {
> +		mddev_unlock_and_resume(mddev);
> +		memalloc_noio_restore(noio_flags);
> +	} else {
> +		mddev_unlock(mddev);
> +	}
>   
>   	if (st == readonly || st == read_auto || st == inactive ||
>   	    (err && st == clear))
> @@ -8299,6 +8324,7 @@ 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;

-- 
Thanks,
Kuai