Re: [PATCH 1/3] fs: Avoid leaving freed inode on dirty list

Xiaoguang Wang
2020-04-25T06:42:09Z

hi,

> evict() can race with writeback_sb_inodes() and so
> list_empty(&inode->i_io_list) check can race with list_move() from
> redirty_tail() possibly resulting in list_empty() returning false and
^^^^^^^^^^^^^^^
returning true?
if (!list_empty(&inode->i_io_list))
inode_io_list_del(inode);
so "!list_empty(&inode->i_io_list)" returns false, and will not remove
inode for wb->b_dirty list.
> thus we end up leaving freed inode in wb->b_dirty list leading to
> use-after-free issues.
>
> Fix the problem by using list_empty_careful() check and add assert that
> inode's i_io_list is empty in clear_inode() to catch the problem earlier
> in the future.
From list_empty_careful()'s comments, using list_empty_careful() without
synchronization can only be safe if the only activity that can happen to the
list entry is list_del_init(), but list_move() does not use list_del_init().

static inline void list_move(struct list_head *list, struct list_head *head)
{
__list_del_entry(list);
list_add(list, head);
}

So I wonder whether list_empty(&inode->i_io_list) check in evict() can race with
list_move() from redirty_tail()?

Regards,
Xiaoguang Wang
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
> fs/inode.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/fs/inode.c b/fs/inode.c
> index 93d9252a00ab..a73c8a7aa71a 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -534,6 +534,7 @@ void clear_inode(struct inode *inode)
> BUG_ON(!(inode->i_state & I_FREEING));
> BUG_ON(inode->i_state & I_CLEAR);
> BUG_ON(!list_empty(&inode->i_wb_list));
> + BUG_ON(!list_empty(&inode->i_io_list));
> /* don't need i_lock here, no concurrent mods to i_state */
> inode->i_state = I_FREEING | I_CLEAR;
> }
> @@ -559,7 +560,13 @@ static void evict(struct inode *inode)
> BUG_ON(!(inode->i_state & I_FREEING));
> BUG_ON(!list_empty(&inode->i_lru));
>
> - if (!list_empty(&inode->i_io_list))
> + /*
> + * We are the only holder of the inode so it cannot be marked dirty.
> + * Flusher thread won't start new writeback but there can be still e.g.
> + * redirty_tail() running from writeback_sb_inodes(). So we have to be
> + * careful to remove inode from dirty/io list in all the cases.
> + */
> + if (!list_empty_careful(&inode->i_io_list))
> inode_io_list_del(inode);
>
> inode_sb_list_del(inode);
>