Re: [BUG && Question] question of SB_ACTIVE flag in ext4_orphan_cleanup()

[Zhang Yi <yi.zhang@huawei.com>]

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?

Thanks,
Yi.