linux-hardening - Re: [PATCH] um,ethertap: refactor deprecated strncpy

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Date:   Tue, 12 Sep 2023 09:36:43 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jason Wang <jasowang@...hat.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] um,ethertap: refactor deprecated strncpy

Hi Justin,

Thanks for your patch!

On Mon, Sep 11, 2023 at 7:53 PM Justin Stitt <justinstitt@...gle.com> wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
>
> `gate_buf` should always be NUL-terminated and does not require
> NUL-padding. It is used as a string arg inside an argv array given to

Can you please explain why it does not require NUL-padding?
It looks like this buffer is passed eventually to a user space
application, thus possibly leaking uninitialized stack data.

> `run_helper()`. Due to this, let's use `strscpy` as it guarantees
> NUL-terminated on the destination buffer preventing potential buffer
> overreads [2].
>
> This exact invocation was changed from `strcpy` to `strncpy` in commit
> 7879b1d94badb ("um,ethertap: use strncpy") back in 2015. Let's continue
> hardening our `str*cpy` apis and use the newer and safer `strscpy`!
>
> Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
>  arch/um/os-Linux/drivers/ethertap_user.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/um/os-Linux/drivers/ethertap_user.c b/arch/um/os-Linux/drivers/ethertap_user.c
> index 9483021d86dd..3363851a4ae8 100644
> --- a/arch/um/os-Linux/drivers/ethertap_user.c
> +++ b/arch/um/os-Linux/drivers/ethertap_user.c
> @@ -105,7 +105,7 @@ static int etap_tramp(char *dev, char *gate, int control_me,
>         sprintf(data_fd_buf, "%d", data_remote);
>         sprintf(version_buf, "%d", UML_NET_VERSION);
>         if (gate != NULL) {
> -               strncpy(gate_buf, gate, 15);
> +               strscpy(gate_buf, gate, sizeof(gate_buf));
>                 args = setup_args;
>         }
>         else args = nosetup_args;
>
> ---
> base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
> change-id: 20230911-strncpy-arch-um-os-linux-drivers-ethertap_user-c-859160d13f59

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds