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

[Jinpu Wang]

On Tue, Dec 12, 2023 at 10:47 PM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202312100355.lHoJPgKy-lkp@xxxxxxxxx/
> Cc: "Md. Haris Iqbal" <haris.iqbal@xxxxxxxxx>
> Cc: Jack Wang <jinpu.wang@xxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxxxxx>
> Cc: linux-block@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
lgtm.
Acked-by: Jack Wang <jinpu.wang@xxxxxxxxx>
> ---
>  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 */
> --
> 2.34.1
>