linux-kernel - Re: f2fs: Possible use-after-free when umount filesystem

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Date:	Tue, 22 Jul 2014 14:04:18 +0400
From:	Andrey Tsyvarev <tsyvarev@...ras.ru>
To:	Gu Zheng <guz.fnst@...fujitsu.com>
CC:	Jaegeuk Kim <jaegeuk@...nel.org>,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Re: 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.

-- 
Best regards,

Andrey Tsyvarev
Linux Verification Center, ISPRAS
web:http://linuxtesting.org

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