Re: [PATCH] block: bio: Use struct_size() in kmalloc()

[Kees Cook <keescook@chromium.org>]

On Mon, Jun 10, 2019 at 10:04:12AM -0500, Gustavo A. R. Silva wrote:
> One of the more common cases of allocation size calculations is finding
> the size of a structure that has a zero-sized array at the end, along
> with memory for some number of elements for that array. For example:
> 
> struct bio_map_data {
> 	...
>         struct iovec iov[];
> };
> 
> instance = kmalloc(sizeof(sizeof(struct bio_map_data) + sizeof(struct iovec) *
>                           count, GFP_KERNEL);
> 
> Instead of leaving these open-coded and prone to type mistakes, we can
> now use the new struct_size() helper:
> 
> instance = kmalloc(struct_size(instance, iov, count), GFP_KERNEL);
> 
> This code was detected with the help of Coccinelle.
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  block/bio.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 683cbb40f051..4bcdcd3f63f4 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1120,8 +1120,7 @@ static struct bio_map_data *bio_alloc_map_data(struct iov_iter *data,
>  	if (data->nr_segs > UIO_MAXIOV)
>  		return NULL;
>  
> -	bmd = kmalloc(sizeof(struct bio_map_data) +
> -		       sizeof(struct iovec) * data->nr_segs, gfp_mask);
> +	bmd = kmalloc(struct_size(bmd, iov, data->nr_segs), gfp_mask);
>  	if (!bmd)
>  		return NULL;
>  	memcpy(bmd->iov, data->iov, sizeof(struct iovec) * data->nr_segs);
> -- 
> 2.21.0
> 

-- 
Kees Cook