linux-hardening - Re: [PATCH v3 2/2] wifi: brcmsmac: replace deprecated strncpy with memcpy

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Message-ID: <CAFhGd8oAaVUP3KcXPkpF061T55PfGCKbxMRKay-==RWguMqJWQ@mail.gmail.com>
Date: Thu, 19 Oct 2023 10:37:45 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Arend van Spriel <aspriel@...il.com>, Franky Lin <franky.lin@...adcom.com>, 
	Hante Meuleman <hante.meuleman@...adcom.com>, Kalle Valo <kvalo@...nel.org>, 
	linux-wireless@...r.kernel.org, brcm80211-dev-list.pdl@...adcom.com, 
	SHA-cyfmac-dev-list@...ineon.com, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 2/2] wifi: brcmsmac: replace deprecated strncpy with memcpy

On Wed, Oct 18, 2023 at 5:03 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Tue, Oct 17, 2023 at 08:11:29PM +0000, Justin Stitt wrote:
> > Let's move away from using strncpy and instead use the more obvious
> > interface for this context.
> >
> > For wlc->pub->srom_ccode, we're just copying two bytes from ccode into
> > wlc->pub->srom_ccode with no expectation that srom_ccode be
> > NUL-terminated:
> > wlc->pub->srom_ccode is only used in regulatory_hint():
> > 1193 |       if (wl->pub->srom_ccode[0] &&
> > 1194 |           regulatory_hint(wl->wiphy, wl->pub->srom_ccode))
> > 1195 |               wiphy_err(wl->wiphy, "%s: regulatory hint failed\n", __func__);
> >
> > We can see that only index 0 and index 1 are accessed.
> > 3307 |       int regulatory_hint(struct wiphy *wiphy, const char *alpha2)
> > 3308 |       {
> > ...  |          ...
> > 3322 |          request->alpha2[0] = alpha2[0];
> > 3323 |          request->alpha2[1] = alpha2[1];
> > ...  |          ...
> > 3332 |       }
> >
> > Since this is just a simple byte copy with correct lengths, let's use
> > memcpy(). There should be no functional change.
> >
> > In a similar boat, both wlc->country_default and
> > wlc->autocountry_default are just simple byte copies so let's use
> > memcpy. However, FWICT they aren't used anywhere. (they should be
> > used or removed -- not in scope of my patch, though).
> >
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> >  drivers/net/wireless/broadcom/brcm80211/brcmsmac/channel.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/wireless/broadcom/brcm80211/brcmsmac/channel.c b/drivers/net/wireless/broadcom/brcm80211/brcmsmac/channel.c
> > index 5a6d9c86552a..f6962e558d7c 100644
> > --- a/drivers/net/wireless/broadcom/brcm80211/brcmsmac/channel.c
> > +++ b/drivers/net/wireless/broadcom/brcm80211/brcmsmac/channel.c
> > @@ -341,7 +341,7 @@ struct brcms_cm_info *brcms_c_channel_mgr_attach(struct brcms_c_info *wlc)
> >       /* store the country code for passing up as a regulatory hint */
> >       wlc_cm->world_regd = brcms_world_regd(ccode, ccode_len);
> >       if (brcms_c_country_valid(ccode))
> > -             strncpy(wlc->pub->srom_ccode, ccode, ccode_len);
> > +             memcpy(wlc->pub->srom_ccode, ccode, ccode_len);
>
>         const char *ccode = sprom->alpha2;
>         int ccode_len = sizeof(sprom->alpha2);
>
> struct ssb_sprom {
>         ...
>         char alpha2[2];         /* Country Code as two chars like EU or US */
>
> This should be marked __nonstring, IMO.
>
> struct brcms_pub {
>         ...
>         char srom_ccode[BRCM_CNTRY_BUF_SZ];     /* Country Code in SROM */
>
> #define BRCM_CNTRY_BUF_SZ        4       /* Country string is 3 bytes + NUL */
>
> This, however, is shown as explicitly %NUL terminated.
>
> The old strncpy wasn't %NUL terminating wlc->pub->srom_ccode, though, so
> the memcpy is the same result, but is that actually _correct_ here?

Judging from the usage, we can see that only bytes at offset 0 and 1 are
used. I think the comment "/* Country string is 3 bytes + NUL */" might
be misleading or perhaps there are other uses that I can't find (which
require NUL-termination)?

>
> >
> >       /*
> >        * If no custom world domain is found in the SROM, use the
> > @@ -354,10 +354,10 @@ struct brcms_cm_info *brcms_c_channel_mgr_attach(struct brcms_c_info *wlc)
> >       }
> >
> >       /* save default country for exiting 11d regulatory mode */
> > -     strncpy(wlc->country_default, ccode, ccode_len);
> > +     memcpy(wlc->country_default, ccode, ccode_len);
> >
> >       /* initialize autocountry_default to driver default */
> > -     strncpy(wlc->autocountry_default, ccode, ccode_len);
> > +     memcpy(wlc->autocountry_default, ccode, ccode_len);
>
> struct brcms_c_info {
>         ...
>         char country_default[BRCM_CNTRY_BUF_SZ];
>         char autocountry_default[BRCM_CNTRY_BUF_SZ];
>
> These are similar...

I can't find any uses for these either.

>
> So, this change results in the same behavior, but is it right?
>
> -Kees
>
> --
> Kees Cook

Thanks
Justin