linux-ext4 - Re: [PATCH] ext4: avoid arithemetic overflow that can trigger a BUG

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Date:   Fri, 31 Aug 2018 21:31:36 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Theodore Ts'o <tytso@....edu>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Cc:     stable@...r.kernel.org
Subject: Re: [PATCH] ext4: avoid arithemetic overflow that can trigger a BUG

On 08/31/2018, 07:41 PM, Theodore Ts'o wrote:
> A maliciously crafted file system can cause an overflow when the
> results of a 64-bit calculation is stored into a 32-bit length
> parameter.
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=200623
> 
> Signed-off-by: Theodore Ts'o <tytso@....edu>
> Reported-by: Wen Xu <wen.xu@...ech.edu>
> Cc: stable@...r.kernel.org
> ---
>  fs/ext4/inode.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 8f6ad7667974..1134c3473673 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3414,6 +3414,7 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  	unsigned int blkbits = inode->i_blkbits;
>  	unsigned long first_block = offset >> blkbits;
>  	unsigned long last_block = (offset + length - 1) >> blkbits;
> +	unsigned long len;
>  	struct ext4_map_blocks map;
>  	bool delalloc = false;
>  	int ret;
> @@ -3434,7 +3435,8 @@ static int ext4_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  	}
>  
>  	map.m_lblk = first_block;
> -	map.m_len = last_block - first_block + 1;
> +	len = last_block - first_block + 1;
> +	map.m_len = (len < UINT_MAX) ? len : UINT_MAX;

Can't this be just

map.m_len = min_t(unsigned long, last_block - first_block + 1, UINT_MAX)?


>  	if (flags & IOMAP_REPORT) {
>  		ret = ext4_map_blocks(NULL, inode, &map, 0);
> 

thanks,
-- 
js
suse labs