linux-kernel - Re: [PATCH] fs: fat: add check for dir size in fat_calc_dir

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Date:   Tue, 30 Jun 2020 20:08:12 +0900
From:   OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:     Anupam Aggarwal <anupam.al@...sung.com>
Cc:     linux-kernel@...r.kernel.org, a.sahrawat@...sung.com
Subject: Re: [PATCH] fs: fat: add check for dir size in fat_calc_dir_size

Anupam Aggarwal <anupam.al@...sung.com> writes:

> Max directory size of FAT filesystem is FAT_MAX_DIR_SIZE(2097152 bytes)
> It is possible that, due to corruption, directory size calculated in
> fat_calc_dir_size() can be greater than FAT_MAX_DIR_SIZE, i.e.
> can be in GBs, hence directory traversal can take long time.
> for example when command "ls -lR" is executed on corrupted FAT
> formatted USB, fat_search_long() function will lookup for a filename from
> position 0 till end of corrupted directory size, multiple such lookups
> will lead to long directory traversal
>
> Added sanity check for directory size fat_calc_dir_size(),
> and return EIO error, which will prevent lookup in corrupted directory
>
> Signed-off-by: Anupam Aggarwal <anupam.al@...sung.com>
> Signed-off-by: Amit Sahrawat <a.sahrawat@...sung.com>

There are many implementation that doesn't follow the spec strictly. And
when I tested in past, Windows also allowed to read the directory beyond
that limit. I can't recall though if there is in real case or just test
case though.

So if there is no strong reason to apply the limit, I don't think it is
good to limit it. (btw, the current code should detect the corruption of
infinite loop already)

Thanks.

> ---
>  fs/fat/inode.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/fs/fat/inode.c b/fs/fat/inode.c
> index a0cf99d..9b2e81e 100644
> --- a/fs/fat/inode.c
> +++ b/fs/fat/inode.c
> @@ -490,6 +490,13 @@ static int fat_calc_dir_size(struct inode *inode)
>  		return ret;
>  	inode->i_size = (fclus + 1) << sbi->cluster_bits;
>  
> +	if (i_size_read(inode) > FAT_MAX_DIR_SIZE) {
> +		fat_fs_error(inode->i_sb,
> +			     "%s corrupted directory (invalid size %lld)\n",
> +			     __func__, i_size_read(inode));
> +		return -EIO;
> +	}
> +
>  	return 0;
>  }

-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>