Re: [PATCH v3] xfs: prevent creating negative-sized file via INSERT_RANGE

[Eric Biggers <ebiggers3@gmail.com>]

Hi Darrick,

On Tue, Apr 17, 2018 at 10:55:30AM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> During the "insert range" fallocate operation, i_size grows by the
> specified 'len' bytes.  XFS verifies that i_size + len < s_maxbytes, as
> it should.  But this comparison is done using the signed 'loff_t', and
> 'i_size + len' can wrap around to a negative value, causing the check to
> incorrectly pass, resulting in an inode with "negative" i_size.  This is
> possible on 64-bit platforms, where XFS sets s_maxbytes = LLONG_MAX.
> ext4 and f2fs don't run into this because they set a smaller s_maxbytes.
> 
> Fix it by doing an unsigned comparison instead.
> 

Can you fix this sentence of the commit message?  It's not doing an unsigned
comparison anymore.  Otherwise this looks fine -- thanks!

> Reproducer:
>     xfs_io -f file -c "truncate $(((1<<63)-1))" -c "finsert 0 4096"
> 
> Fixes: a904b1ca5751 ("xfs: Add support FALLOC_FL_INSERT_RANGE for fallocate")
> Cc: <stable@vger.kernel.org> # v4.1+
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> Originally-From: Eric Biggers <ebiggers@google.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> [darrick: fix signed integer addition overflow too]
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> v3: rearrange the changes to churn less
> v2: fix signed integer overflow when adding isize and len
> ---
>  fs/xfs/xfs_file.c |   14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 9fd9dd7..1ac05ab 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -778,22 +778,26 @@ xfs_file_fallocate(
>  		if (error)
>  			goto out_unlock;
>  	} else if (mode & FALLOC_FL_INSERT_RANGE) {
> -		unsigned int blksize_mask = i_blocksize(inode) - 1;
> +		unsigned int	blksize_mask = i_blocksize(inode) - 1;
> +		loff_t		isize = i_size_read(inode);
>  
> -		new_size = i_size_read(inode) + len;
>  		if (offset & blksize_mask || len & blksize_mask) {
>  			error = -EINVAL;
>  			goto out_unlock;
>  		}
>  
> -		/* check the new inode size does not wrap through zero */
> -		if (new_size > inode->i_sb->s_maxbytes) {
> +		/*
> +		 * New inode size must not exceed ->s_maxbytes, accounting for
> +		 * possible signed overflow.
> +		 */
> +		if (inode->i_sb->s_maxbytes - isize < len) {
>  			error = -EFBIG;
>  			goto out_unlock;
>  		}
> +		new_size = isize + len;
>  
>  		/* Offset should be less than i_size */
> -		if (offset >= i_size_read(inode)) {
> +		if (offset >= isize) {
>  			error = -EINVAL;
>  			goto out_unlock;
>  		}