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

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.  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.

Every ioctl that can reach mddev->kobj runs under reconfig_mutex, so
hold it across add_disk() and kobject_add().  An ioctl issued in the
window now waits for md_alloc() to finish instead of seeing a
half-built mddev, and opening the array still succeeds.  The ioctls
md_ioctl() serves without the mutex only read array state and return
-ENODEV on an array with no disks.

Hoisting kobject_init() above add_disk() is not an option: commit
ca39f7502425 ("md: fix mddev->kobj lifetime") moved it below add_disk()
so that md_alloc()'s error paths, which free the mddev directly, never
see an initialised kobject.

Fixes: ca39f7502425 ("md: fix mddev->kobj lifetime")
Reported-by: syzbot+95eeb4ada2349a2170ea@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6aaf3284.514bccd6.255447.0003.GAE@google.com/
Suggested-by: Yu Kuai <yukuai@fygo.io>
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Yogesh Gaur <yogeshgaur.83@gmail.com>
Link: https://patch.msgid.link/20261006122904.1538-1-yogeshgaur.83@gmail.com
Signed-off-by: Yu Kuai <yukuai@fygo.io>

0 files changed, 0 insertions, 0 deletions