Re: [PATCH -next v7 5/5] md: protect md_thread with rcu

[Song Liu]

On Tue, Apr 25, 2023 at 4:54 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Our test reports a uaf for 'mddev->sync_thread':
>
> T1                      T2
> md_start_sync
>  md_register_thread
>  // mddev->sync_thread is set
>                         raid1d
>                          md_check_recovery
>                           md_reap_sync_thread
>                            md_unregister_thread
>                             kfree
>
>  md_wakeup_thread
>   wake_up
>   ->sync_thread was freed
>
> Root cause is that there is a small windown between register thread and
> wake up thread, where the thread can be freed concurrently.
>
> Currently, a global spinlock 'pers_lock' is borrowed to protect
> 'mddev->thread', this problem can be fixed likewise, however, there are
> similar problems elsewhere, and use a global lock for all the cases is
> not good.
>
> This patch protect all md_thread with rcu.
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  block/blk-cgroup.c        |  3 ++
>  drivers/md/md-bitmap.c    | 10 ++++--
>  drivers/md/md-cluster.c   | 17 ++++++----
>  drivers/md/md-multipath.c |  4 +--
>  drivers/md/md.c           | 69 ++++++++++++++++++---------------------
>  drivers/md/md.h           |  8 ++---
>  drivers/md/raid1.c        |  7 ++--
>  drivers/md/raid1.h        |  2 +-
>  drivers/md/raid10.c       | 20 +++++++-----
>  drivers/md/raid10.h       |  2 +-
>  drivers/md/raid5-cache.c  | 22 ++++++++-----
>  drivers/md/raid5.c        | 15 +++++----
>  drivers/md/raid5.h        |  2 +-
>  13 files changed, 100 insertions(+), 81 deletions(-)
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 1c1ebeb51003..0ecb4cce8af2 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -527,6 +527,9 @@ static void blkg_destroy_all(struct gendisk *disk)
>         list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
>                 struct blkcg *blkcg = blkg->blkcg;
>
> +               if (hlist_unhashed(&blkg->blkcg_node))
> +                       continue;
> +

This change is not related, right?

I don't think we can rush this change in the 6.4 merge window. Let's
test it more thoroughly and ship it in the next merge window.

Thanks for working on this!

Song
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