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

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Date: Wed, 18 Oct 2023 17:03:02 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
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 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?

>  
>  	/*
>  	 * 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...

So, this change results in the same behavior, but is it right?

-Kees

-- 
Kees Cook