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

[Yogesh Gaur]

On Mon, Oct 5, 2026 at 10:35 PM yu kuai <yukuai@xxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=95eeb4ada2349a2170ea
> > Cc: stable@xxxxxxxxxxxxxxx
> > 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@xxxxxxx/#t>
>

Thanks Kuai. Yes just refusing the open would be going to move the
race condition. I had missed earlier patch from xiaobai050, sorry for
the duplicate.

I would send a v2 that holds reconfig_mutex from add_disk() through
kobject_add() in md_alloc(), so an ioctl issued in that window waits
instead of failing the open. RUN_ARRAY is covered as well, since
md_run() also uses mddev->kobj and runs under same lock.

Thanks
Yogesh
> >
>
> --
> Thanks,
> Kuai