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

[Andreas Dilger <adilger@dilger.ca>]

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On Aug 31, 2018, at 11:41 AM, Theodore Ts'o <tytso@mit.edu> 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@mit.edu>
> Reported-by: Wen Xu <wen.xu@gatech.edu>
> Cc: stable@vger.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,
> 	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,
> 	}
> 
> 	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;

Wouldn't "(len < UINT_MAX)" always be true on a 32-bit system, or is there some
other limitation in that case (e.g. filesystem < 16TB) that prevents it from
being an issue?  Otherwise, this should use "unsigned long long len".

Cheers, Andreas






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