linux-kernel - Re: [PATCH] fs: do not push freeing inode to b_dirty

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Date:   Mon, 14 Nov 2022 11:46:53 +0100
From:   Jan Kara <jack@...e.cz>
To:     Svyatoslav Feldsherov <feldsherov@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Lukas Czerner <lczerner@...hat.com>,
        Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
        syzbot+6ba92bd00d5093f7e371@...kaller.appspotmail.com,
        oferz@...gle.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: do not push freeing inode to b_dirty_time list

On Sun 13-11-22 17:24:39, Svyatoslav Feldsherov wrote:
> After commit cbfecb927f42 ("fs: record I_DIRTY_TIME even if inode
> already has I_DIRTY_INODE") writeiback_single_inode can push inode with
> I_DIRTY_TIME set to b_dirty_time list. In case of freeing inode with
> I_DIRTY_TIME set this can happened after deletion of inode io_list at
> evict. Stack trace is following.
> 
> evict
> fat_evict_inode
> fat_truncate_blocks
> fat_flush_inodes
> writeback_inode
> sync_inode_metadata
> writeback_single_inode
> 
> This will lead to use after free in flusher thread.
> 
> Fixes: cbfecb927f42 ("fs: record I_DIRTY_TIME even if inode already has I_DIRTY_INODE")
> Reported-by: syzbot+6ba92bd00d5093f7e371@...kaller.appspotmail.com
> Signed-off-by: Svyatoslav Feldsherov <feldsherov@...gle.com>

Thanks for the analysis! I was scratching my head over this syzbot report
for a while and it didn't occur to me somebody could be calling
writeback_single_inode() from the .evict callback.

Also what contributes to the problem is that FAT calls
sync_inode_metadata(inode, 0) so it is not marking this final flush as data
integrity sync and so we happily leave the I_DIRTY_TIME bit set.

> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 443f83382b9b..31c93cbdb3fe 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -1718,7 +1718,7 @@ static int writeback_single_inode(struct inode *inode,
>  	 */
>  	if (!(inode->i_state & I_DIRTY_ALL))
>  		inode_cgwb_move_to_attached(inode, wb);
> -	else if (!(inode->i_state & I_SYNC_QUEUED)) {
> +	else if (!(inode->i_state & (I_SYNC_QUEUED | I_FREEING))) {
>  		if ((inode->i_state & I_DIRTY))
>  			redirty_tail_locked(inode, wb);
>  		else if (inode->i_state & I_DIRTY_TIME) {

So even calling inode_cgwb_move_to_attached() is not safe when I_FREEING is
already set. So I belive the I_FREEING bit check needs to be before this
whole if block.

I also think we should add some assertions into i_io_list handling
functions to complain if I_FREEING bit is set to catch these problems
earlier which means to be also more careful in __mark_inode_dirty(). But
this is for a separate cleanup.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR