Re: [PATCH] md: don't hand out the array before md_alloc() has added mddev->kobj

["yu kuai" <yukuai@fygo.io>]

Hi,

在 2026/10/4 13:57, Yogesh Gaur 写道:
> md_alloc() publishes the gendisk before it is done setting the mddev up:
>
> 	disk->private_data = mddev;
> 	...
> 	error = add_disk(disk);
> 	if (error)
> 		goto out_put_disk;
>
> 	kobject_init(&mddev->kobj, &md_ktype);
> 	error = kobject_add(&mddev->kobj, &disk_to_dev(disk)->kobj, "%s", "md");
>
> add_disk() makes /dev/mdN openable, and md_open() only refuses the open
> when MD_CLOSING is set.  mddev comes from mddev_alloc() and is zeroed,
> so anything that gets in between add_disk() and kobject_add() sees an
> mddev->kobj that has never been through kobject_init() - no ktype, no
> kref, state_initialized clear.
>
> syzbot opens the array in that window and issues ADD_NEW_DISK:
>
>    kobject: '(null)' (ffff8880120640f0): is not initialized, yet kobject_get() is being called.
>    WARNING: lib/kobject.c:642 at kobject_add_internal+0xea/0xcd0 lib/kobject.c:225
>     kobject_add_varg lib/kobject.c:374 [inline]
>     kobject_add+0x163/0x240 lib/kobject.c:426
>     bind_rdev_to_array+0x80c/0xdd0 drivers/md/md.c:2621
>     md_add_new_disk+0xe3b/0x1850 drivers/md/md.c:7684
>     md_ioctl+0x200a/0x2610 drivers/md/md.c:8499
>
> bind_rdev_to_array() is not the only way in.  md_run() calls
> sysfs_create_group(&mddev->kobj, &md_redundancy_group), and
> internal_create_group() has its own WARN_ON(!kobj->sd), so RUN_ARRAY in
> the same window warns too.  Guarding the individual callers would mean
> finding all of them; the window itself is what should not be reachable.
>
> Refuse the open until md_alloc() has added the kobject.  mddev->kobj.sd
> is NULL until kobject_add() creates the directory, and stays set for the
> rest of the mddev's life: commit ca39f7502425 ("md: fix mddev->kobj
> lifetime") dropped the explicit kobject_del() and lets the final put do
> the removal.  md.c already tests mddev->kobj.sd for "is this array
> published yet" in mddev_unlock() and md_run().
>
> This cannot deadlock against add_disk() itself: device_add_disk() only
> opens the disk to scan partitions when get_capacity(disk) is non-zero,
> and md_alloc() does not set the capacity - md_run() does, long after.
>
> Fixes: ca39f7502425 ("md: fix mddev->kobj lifetime")
> Reported-by: syzbot+95eeb4ada2349a2170ea@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=95eeb4ada2349a2170ea
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> ---
>   drivers/md/md.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 680b34a63cb3..e3326b78aef7 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -8610,6 +8610,15 @@ static int md_open(struct gendisk *disk, blk_mode_t mode)
>   	if (test_bit(MD_CLOSING, &mddev->flags))
>   		goto out_unlock;
>   
> +	/*
> +	 * md_alloc() publishes the gendisk with add_disk() before it has
> +	 * initialised and added mddev->kobj, so an array opened in that window
> +	 * would let ioctls reach a kobject that is still all zeroes.  Wait for
> +	 * md_alloc() to finish rather than handing out the device early.
> +	 */
> +	if (!mddev->kobj.sd)
> +		goto out_unlock;
> +
>   	atomic_inc(&mddev->openers);
>   	mutex_unlock(&mddev->open_mutex);

This is the same problem that I already replied in another thread, the same solution
with different code.

Re: [PATCH] md: prevent opening an array before its sysfs kobject is 
ready - yu kuai <https://lore.kernel.org/all/a19e3020-272b-4f93-8535-00fa07002d91@fygo.io/#t>

>   

-- 
Thanks,
Kuai