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Date:	Wed, 23 Jul 2014 11:39:13 +0800
From:	Gu Zheng <guz.fnst@...fujitsu.com>
To:	Chao Yu <chao2.yu@...sung.com>,
	"'Andrey Tsyvarev'" <tsyvarev@...ras.ru>
CC:	"'Jaegeuk Kim'" <jaegeuk@...nel.org>,
	"'linux-kernel'" <linux-kernel@...r.kernel.org>,
	"'Alexey Khoroshilov'" <khoroshilov@...ras.ru>,
	<linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] f2fs: Possible use-after-free when umount filesystem

Hi,
On 07/23/2014 10:12 AM, Chao Yu wrote:

> Hi Andrey Gu,
> 
>> -----Original Message-----
>> From: Andrey Tsyvarev [mailto:tsyvarev@...ras.ru]
>> Sent: Tuesday, July 22, 2014 6:04 PM
>> To: Gu Zheng
>> Cc: Jaegeuk Kim; linux-kernel; Alexey Khoroshilov; linux-f2fs-devel@...ts.sourceforge.net
>> Subject: Re: [f2fs-dev] f2fs: Possible use-after-free when umount filesystem
>>
>> Hi Gu,
>>
>>>> Investigation shows, that f2fs_evict_inode, when called for 'meta_inode', uses
>> invalidate_mapping_pages() for 'node_inode'.
>>>> But 'node_inode' is deleted before 'meta_inode' in f2fs_put_super via iput().
>>>>
>>>> It seems that in common usage scenario this use-after-free is benign, because 'node_inode'
>> remains partially valid data even after kmem_cache_free().
>>>> But things may change if, while 'meta_inode' is evicted in one f2fs filesystem, another (mounted)
>> f2fs filesystem requests inode from cache, and formely
>>>> 'node_inode' of the first filesystem is returned.
>>> The analysis seems reasonable. Have you tried to swap the reclaim order of node_inde
>>> and meta_inode?
>>>
>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>> index 870fe19..e114418 100644
>>> --- a/fs/f2fs/super.c
>>> +++ b/fs/f2fs/super.c
>>> @@ -430,8 +430,8 @@ static void f2fs_put_super(struct super_block *sb)
>>>          if (sbi->s_dirty && get_pages(sbi, F2FS_DIRTY_NODES))
>>>                  write_checkpoint(sbi, true);
>>>
>>> -       iput(sbi->node_inode);
>>>          iput(sbi->meta_inode);
>>> +       iput(sbi->node_inode);
>>>
>>>          /* destroy f2fs internal modules */
>>>          destroy_node_manager(sbi);
>>>
>>> Thanks,
>>> Gu
>>
>> With reclaim order of node_inode and meta_inode swapped, use-after-free
>> error disappears.
>>
>> But shouldn't initialization order of these inodes be swapped too?
>> As meta_inode uses node_inode, it seems logical that it should be
>> initialized after it.

The initialization order dose not affect anything, so swapping the order dose not
make more sense here.

> 
> IMO, it's not easy to exchange order of initialization between meta_inode and
> node_inode, because we should use meta_inode in get_valid_checkpoint for valid
> cp first for usual verification, then init node_inode.

Yeah, but I think just moving node_inode's initialization to the front of meta_inode
dose not break anything.

> 
> As I checked, nids for both meta_inode and node_inode are reservation, so it's not
> necessary for us to invalidate pages which will never alloced.
> 
> How about skipping it as following?

It seems the right way to fix this issue.

To Andrey:
Could you please try this one?

Thanks,
Gu

> 
> diff --git a/fs/f2fs/inode.c b/fs/f2fs/inode.c
> index 2cf6962..cafba3c 100644
> --- a/fs/f2fs/inode.c
> +++ b/fs/f2fs/inode.c
> @@ -273,7 +273,7 @@ void f2fs_evict_inode(struct inode *inode)
>  
>  	if (inode->i_ino == F2FS_NODE_INO(sbi) ||
>  			inode->i_ino == F2FS_META_INO(sbi))
> -		goto no_delete;
> +		goto out_clear;
>  
>  	f2fs_bug_on(get_dirty_dents(inode));
>  	remove_dirty_dir_inode(inode);
> @@ -295,6 +295,7 @@ void f2fs_evict_inode(struct inode *inode)
>  
>  	sb_end_intwrite(inode->i_sb);
>  no_delete:
> -	clear_inode(inode);
>  	invalidate_mapping_pages(NODE_MAPPING(sbi), inode->i_ino, inode->i_ino);
> +out_clear:
> +	clear_inode(inode);
>  }
> 
>>
>> --
>> Best regards,
>>
>> Andrey Tsyvarev
>> Linux Verification Center, ISPRAS
>> web:http://linuxtesting.org
>>
>>
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> 
> .
> 


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