[PATCH] um,ethertap: refactor deprecated strncpy

[Justin Stitt]

On Tue, Sep 12, 2023 at 12:36 AM Geert Uytterhoeven
<geert at linux-m68k.org> wrote:
>
> Hi Justin,
>
> Thanks for your patch!
>
> On Mon, Sep 11, 2023 at 7:53 PM Justin Stitt <justinstitt at google.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.

It looks like it's being passed as a list of command-line arguments in
`run_helper()`. Should this be NUL-padded due to its eventual use in
user space? If we think yes I can send a v2. Thanks for pointing this
out.


>
> > `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 at vger.kernel.org
> > Cc: Kees Cook <keescook at chromium.org>
> > Signed-off-by: Justin Stitt <justinstitt at google.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 at linux-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