Re: [dm-devel] [PATCH 04/11] dm verity fec: Remove VLA usage

[Eric Biggers]

On Wed, Jun 20, 2018 at 12:04:01PM -0700, Kees Cook wrote:
> In the quest to remove all stack VLA usage from the kernel[1], this
> uses the newly defined max digest size macro. Also adds a sanity-check
> at use-time.
> 
> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@xxxxxxxxxxxxxx
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  drivers/md/dm-verity-fec.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
> index 684af08d0747..0dfcc52835bc 100644
> --- a/drivers/md/dm-verity-fec.c
> +++ b/drivers/md/dm-verity-fec.c
> @@ -212,12 +212,15 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
>  	struct dm_verity_fec_io *fio = fec_io(io);
>  	u64 block, ileaved;
>  	u8 *bbuf, *rs_block;
> -	u8 want_digest[v->digest_size];
> +	u8 want_digest[AHASH_MAX_DIGESTSIZE];
>  	unsigned n, k;
>  
>  	if (neras)
>  		*neras = 0;
>  
> +	if (WARN_ON(v->digest_size < sizeof(want_digest)))
> +		return -EINVAL;
> +

This is backwards; it should be 'v->digest_size > sizeof(want_digest)'.

- Eric