Re: [PATCH 1/2] fs/super.c: fix race between freeze_super() and thaw_super()

Jan Kara
2016-09-26T16:11:29Z

On Mon 26-09-16 18:07:48, Oleg Nesterov wrote:
> Change thaw_super() to check frozen != SB_FREEZE_COMPLETE rather than
> frozen == SB_UNFROZEN, otherwise it can race with freeze_super() which
> drops sb->s_umount after SB_FREEZE_WRITE to preserve the lock ordering.
>
> In this case thaw_super() will wrongly call s_op->unfreeze_fs() before
> it was actually frozen, and call sb_freeze_unlock() which leads to the
> unbalanced percpu_up_write(). Unfortunately lockdep can't detect this,
> so this triggers misc BUG_ON()'s in kernel/rcu/sync.c.
>
> Reported-and-tested-by: Nikolay Borisov <kernel@kyup.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Cc: stable@vger.kernel.org

The patch looks good. Thanks!

Reviewed-by: Jan Kara <jack@suse.cz>

Honza
> ---
> fs/super.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index d78b984..2549896c 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1324,8 +1324,8 @@ int freeze_super(struct super_block *sb)
> }
> }
> /*
> - * This is just for debugging purposes so that fs can warn if it
> - * sees write activity when frozen is set to SB_FREEZE_COMPLETE.
> + * For debugging purposes so that fs can warn if it sees write activity
> + * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super().
> */
> sb->s_writers.frozen = SB_FREEZE_COMPLETE;
> up_write(&sb->s_umount);
> @@ -1344,7 +1344,7 @@ int thaw_super(struct super_block *sb)
> int error;
>
> down_write(&sb->s_umount);
> - if (sb->s_writers.frozen == SB_UNFROZEN) {
> + if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) {
> up_write(&sb->s_umount);
> return -EINVAL;
> }
> --
> 2.5.0
>
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR