linux-ext4 - Re: [BUG && Question] question of SB_ACTIVE flag in ext4_orphan

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Message-ID: <20210322172551.GJ31783@quack2.suse.cz>
Date:   Mon, 22 Mar 2021 18:25:51 +0100
From:   Jan Kara <jack@...e.cz>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     Jan Kara <jack@...e.cz>, "Theodore Y. Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        yangerkun <yangerkun@...wei.com>
Subject: Re: [BUG && Question] question of SB_ACTIVE flag in
 ext4_orphan_cleanup()

Hi!

On Mon 22-03-21 23:24:23, Zhang Yi wrote:
> We find a use after free problem when CONFIG_QUOTA is enabled, the detail of
> this problem is below.
> 
> mount_bdev()
> 	ext4_fill_super()
> 		sb->s_root = d_make_root(root);
> 		ext4_orphan_cleanup()
> 			sb->s_flags |= SB_ACTIVE; <--- 1. mark sb active
> 			ext4_orphan_get()
> 			ext4_truncate()
> 				ext4_block_truncate_page()
> 					mark_buffer_dirty <--- 2. dirty inode
> 			iput()
> 				iput_final  <--- 3. put into lru list
> 		ext4_mark_recovery_complete  <--- 4. failed and return error
> 		sb->s_root = NULL;
> 	deactivate_locked_super()
> 		kill_block_super()
> 			generic_shutdown_super()
> 				<--- 5. did not evict_inodes
> 		put_super()
> 			__put_super()
> 				<--- 6. put super block
> 
> Because of the truncated inodes was dirty and will write them back later, it
> will trigger use after free problem. Now the question is why we need to set
> SB_ACTIVE bit when enable CONFIG_QUOTA below?
> 
>   #ifdef CONFIG_QUOTA
>           /* Needed for iput() to work correctly and not trash data */
>           sb->s_flags |= SB_ACTIVE;
> 
> This code was merged long long ago in v2.6.6, IIUC, it may not affect
> the quota statistics it we evict inode directly in the last iput.
> In order to slove this UAF problem, I'm not sure is there any side effect
> if we just remove this code, or remove SB_ACTIVE and call evict_inodes()
> in the error path of ext4_fill_super().
> 
> Could you give some suggestions?

That's a very good question. I do remember that I've added this code back
then because otherwise orphan cleanup was loosing updates to quota files.
But you're right that now I don't see how that could be happening and it
would be nice if we could get rid of this hack (and even better if it also
fixes the problem you've found). I guess I'll just try and test this change
with various quota configurations to see whether something still breaks or
not. Thanks report!

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