Re: [PATCH] block/rnbd-srv: Check for unlikely string overflow

[Guoqing Jiang <guoqing.jiang@linux.dev>]

On 12/13/23 05:47, Kees Cook wrote:
> Since "dev_search_path" can technically be as large as PATH_MAX,
> there was a risk of truncation when copying it and a second string
> into "full_path" since it was also PATH_MAX sized. The W=1 builds were
> reporting this warning:
>
> drivers/block/rnbd/rnbd-srv.c: In function 'process_msg_open.isra':
> drivers/block/rnbd/rnbd-srv.c:616:51: warning: '%s' directive output may be truncated writing up to 254 bytes into a region of size between 0 and 4095 [-Wformat-truncation=]
>    616 |                 snprintf(full_path, PATH_MAX, "%s/%s",
>        |                                                   ^~
> In function 'rnbd_srv_get_full_path',
>      inlined from 'process_msg_open.isra' at drivers/block/rnbd/rnbd-srv.c:721:14: drivers/block/rnbd/rnbd-srv.c:616:17: note: 'snprintf' output between 2 and 4351 bytes into a destination of size 4096
>    616 |                 snprintf(full_path, PATH_MAX, "%s/%s",
>        |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>    617 |                          dev_search_path, dev_name);
>        |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> To fix this, unconditionally check for truncation (as was already done
> for the case where "%SESSNAME%" was present).
>
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202312100355.lHoJPgKy-lkp@intel.com/
> Cc: "Md. Haris Iqbal" <haris.iqbal@ionos.com>
> Cc: Jack Wang <jinpu.wang@ionos.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: linux-block@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>   drivers/block/rnbd/rnbd-srv.c | 19 ++++++++++---------
>   1 file changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/block/rnbd/rnbd-srv.c b/drivers/block/rnbd/rnbd-srv.c
> index 65de51f3dfd9..ab78eab97d98 100644
> --- a/drivers/block/rnbd/rnbd-srv.c
> +++ b/drivers/block/rnbd/rnbd-srv.c
> @@ -585,6 +585,7 @@ static char *rnbd_srv_get_full_path(struct rnbd_srv_session *srv_sess,
>   {
>   	char *full_path;
>   	char *a, *b;
> +	int len;
>   
>   	full_path = kmalloc(PATH_MAX, GFP_KERNEL);
>   	if (!full_path)
> @@ -596,19 +597,19 @@ static char *rnbd_srv_get_full_path(struct rnbd_srv_session *srv_sess,
>   	 */
>   	a = strnstr(dev_search_path, "%SESSNAME%", sizeof(dev_search_path));
>   	if (a) {
> -		int len = a - dev_search_path;
> +		len = a - dev_search_path;
>   
>   		len = snprintf(full_path, PATH_MAX, "%.*s/%s/%s", len,
>   			       dev_search_path, srv_sess->sessname, dev_name);
> -		if (len >= PATH_MAX) {
> -			pr_err("Too long path: %s, %s, %s\n",
> -			       dev_search_path, srv_sess->sessname, dev_name);
> -			kfree(full_path);
> -			return ERR_PTR(-EINVAL);
> -		}
>   	} else {
> -		snprintf(full_path, PATH_MAX, "%s/%s",
> -			 dev_search_path, dev_name);
> +		len = snprintf(full_path, PATH_MAX, "%s/%s",
> +			       dev_search_path, dev_name);
> +	}
> +	if (len >= PATH_MAX) {
> +		pr_err("Too long path: %s, %s, %s\n",
> +		       dev_search_path, srv_sess->sessname, dev_name);
> +		kfree(full_path);
> +		return ERR_PTR(-EINVAL);
>   	}
>   
>   	/* eliminitate duplicated slashes */

Looks good.

Acked-by: Guoqing Jiang <guoqing.jiang@linux.dev>