linux-ext4 - Re: [PATCH] ext4: Fix loff_t overflow in ext4_max_bitmap

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Message-ID: <20210607100028.GE30275@quack2.suse.cz>
Date:   Mon, 7 Jun 2021 12:00:28 +0200
From:   Jan Kara <jack@...e.cz>
To:     Ritesh Harjani <riteshh@...ux.ibm.com>
Cc:     linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] ext4: Fix loff_t overflow in ext4_max_bitmap_size()

On Sat 05-06-21 10:39:32, Ritesh Harjani wrote:
> We should use unsigned long long rather than loff_t to avoid
> overflow in ext4_max_bitmap_size() for comparison before returning.
> w/o this patch sbi->s_bitmap_maxbytes was becoming a negative
> value due to overflow of upper_limit (with has_huge_files as true)
> 
> Below is a quick test to trigger it on a 64KB pagesize system.
> 
> sudo mkfs.ext4 -b 65536 -O ^has_extents,^64bit /dev/loop2
> sudo mount /dev/loop2 /mnt
> sudo echo "hello" > /mnt/hello 	-> This will error out with
> 				"echo: write error: File too large"
> 
> Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>

OK, this works (although it's really tight ;). Won't it be somewhat safer
if we compared upper_limit and res before shifting both by blocksize_bits
to the left? Basically we need to shift only for comparison with
MAX_LFS_FILESIZE which is in bytes... But either way feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/ext4/super.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 7dc94f3e18e6..bedb66386966 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3189,17 +3189,17 @@ static loff_t ext4_max_size(int blkbits, int has_huge_files)
>   */
>  static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
>  {
> -	loff_t res = EXT4_NDIR_BLOCKS;
> +	unsigned long long upper_limit, res = EXT4_NDIR_BLOCKS;
>  	int meta_blocks;
> -	loff_t upper_limit;
> -	/* This is calculated to be the largest file size for a dense, block
> +
> +	/*
> +	 * This is calculated to be the largest file size for a dense, block
>  	 * mapped file such that the file's total number of 512-byte sectors,
>  	 * including data and all indirect blocks, does not exceed (2^48 - 1).
>  	 *
>  	 * __u32 i_blocks_lo and _u16 i_blocks_high represent the total
>  	 * number of 512-byte sectors of the file.
>  	 */
> -
>  	if (!has_huge_files) {
>  		/*
>  		 * !has_huge_files or implies that the inode i_block field
> @@ -3242,7 +3242,7 @@ static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
>  	if (res > MAX_LFS_FILESIZE)
>  		res = MAX_LFS_FILESIZE;
>  
> -	return res;
> +	return (loff_t)res;
>  }
>  
>  static ext4_fsblk_t descriptor_loc(struct super_block *sb,
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR