Re: [PATCH] net: dsa: realtek: rtl8365mb: replace deprecated strncpy with ethtool_sprintf

[Kees Cook <keescook@chromium.org>]

On Tue, Oct 10, 2023 at 02:07:17PM +0300, Vladimir Oltean wrote:
> Hello Justin,
> 
> On Mon, Oct 09, 2023 at 10:43:59PM +0000, Justin Stitt wrote:
> > `strncpy` is deprecated for use on NUL-terminated destination strings
> > [1] and as such we should prefer more robust and less ambiguous string
> > interfaces.
> > 
> > ethtool_sprintf() is designed specifically for get_strings() usage.
> > Let's replace strncpy in favor of this more robust and easier to
> > understand interface.
> > 
> > Link: https://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@vger.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@google.com>
> > ---
> > Note: build-tested only.
> > ---
> >  drivers/net/dsa/realtek/rtl8365mb.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/dsa/realtek/rtl8365mb.c b/drivers/net/dsa/realtek/rtl8365mb.c
> > index 41ea3b5a42b1..d171c18dd354 100644
> > --- a/drivers/net/dsa/realtek/rtl8365mb.c
> > +++ b/drivers/net/dsa/realtek/rtl8365mb.c
> > @@ -1303,8 +1303,7 @@ static void rtl8365mb_get_strings(struct dsa_switch *ds, int port, u32 stringset
> >  
> >  	for (i = 0; i < RTL8365MB_MIB_END; i++) {
> >  		struct rtl8365mb_mib_counter *mib = &rtl8365mb_mib_counters[i];
> > -
> > -		strncpy(data + i * ETH_GSTRING_LEN, mib->name, ETH_GSTRING_LEN);
> > +		ethtool_sprintf(&data, "%s", mib->name);
> 
> Is there any particular reason why you opted for the "%s" printf format
> specifier when you could have simply given mib->name as the single
> argument? This comment applies to all the ethtool_sprintf() patches
> you've submitted.

The primary reason is that without the "%s", any format specifiers in
mib->name will be processed by sprintf(), which could lead to very
unexpected results. One never wants to just pass a string directly to
the sprintf-family of functions for this reason. "%s" is needed to keep
things safe.

-Kees

-- 
Kees Cook