Re: [BUG] possible race between md_free_disk and md_notify_reboot

[Yu Kuai]

Hi,

在 2025/02/20 12:06, Yu Kuai 写道:
> Hi,
>
> 在 2025/02/20 11:45, Guillaume Morin 写道:
> > On 20 Feb 11:19, Yu Kuai wrote:
> > > Hi,
> > >
> > > 在 2025/02/20 11:05, Guillaume Morin 写道:
> > > > how it was guaranteed that mddev_get() would fail as mddev_free() 
> > > > does not check or synchronize with the active atomic
> > >
> > > Please check how mddev is freed, start from mddev_put(). There might be
> > > something wrong, but it's not what you said.
> >
> > I will take a look. Though if you're confident that this logic protects
> > any uaf, that makes sense to me.
> >
> > However as I mentioned this is not what the crash was about (I mentioned
> > the UAF in passing). The GPF seems to be about deleting the _next_
> > pointer while iterating over all mddevs. The mddev_get on the
> > current item is not going to help with this.
>
> You don't need to emphasize this, it is still speculate without solid
> theoretical analysis. The point about mddev_get() is that it's done
> inside the lock, it shoud gurantee continue iterating should be fine.
>
> I just take a quick look, the problem looks obviously to me, see how
> md_seq_show() handle the iteration.
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 465ca2af1e6e..7c7a58f618c1 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -9911,8 +9911,11 @@ static int md_notify_reboot(struct notifier_block 
> *this,
>                          mddev_unlock(mddev);
>                  }
>                  need_delay = 1;
> -               mddev_put(mddev);
> -               spin_lock(&all_mddevs_lock);
> +
> +               spin_lock(&all_mddevs_lock)
> +               if (atomic_dec_and_test(&mddev->active))
> +                       __mddev_put(mddev);
> +
>          }
>          spin_unlock(&all_mddevs_lock);

While cooking the patch, this is not enough, list_for_each_entry_safe()
should be replaced with list_for_each_entry() as well.

Will send the patch soon, with:

Reported-by: Guillaume Morin <guillaume@xxxxxxxxxxx>

Thanks,
Kuai
> Thanks,
> Kuai
>
> .