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

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Date:   Wed, 31 Mar 2021 11:11:10 +0800
From:   Zhang Yi <yi.zhang@...wei.com>
To:     Jan Kara <jack@...e.cz>
CC:     "Theodore Y. Ts'o" <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        yangerkun <yangerkun@...wei.com>, <linfeilong@...wei.com>
Subject: Re: [BUG && Question] question of SB_ACTIVE flag in
 ext4_orphan_cleanup()

On 2021/3/30 23:02, Jan Kara wrote:
> On Mon 29-03-21 17:20:35, Zhang Yi wrote:
>> On 2021/3/23 1:25, Jan Kara wrote:
>>> 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!
>>>
>>
>> Thanks for taking time to look at this, is this change OK under your various
>> quota test cases?
> 
> Yes, I did tests both with journalled quotas and with ext4 quota feature
> and the quota accounting was correct after orphan recovery. So just
> removing the SB_ACTIVE setting is fine AFAICT. Will you send a patch or
> should I do it?
> 

Thanks for testing this change, I will send a patch.

Yi.