linux-kernel - Re: [PATCH v2] wifi: ath10k: replace deprecated strncpy with memcpy

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Message-ID: <b85a4e54-61c7-45fe-b97d-46338b237a64@quicinc.com>
Date:   Tue, 24 Oct 2023 16:19:29 -0700
From:   Jeff Johnson <quic_jjohnson@...cinc.com>
To:     Kees Cook <keescook@...omium.org>,
        Justin Stitt <justinstitt@...gle.com>
CC:     Kalle Valo <kvalo@...nel.org>, <ath10k@...ts.infradead.org>,
        <linux-wireless@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v2] wifi: ath10k: replace deprecated strncpy with memcpy

On 10/24/2023 2:34 PM, Kees Cook wrote:
> On Tue, Oct 24, 2023 at 05:42:16PM +0000, Justin Stitt wrote:
>> strncpy() is deprecated [1] and we should prefer less ambiguous
>> interfaces.
>>
>> In this case, arvif->u.ap.ssid has its length maintained by
>> arvif->u.ap.ssid_len which indicates it may not need to be
>> NUL-terminated. Make this explicit with __nonstring and use a plain old
>> memcpy.
>>
>> This is also consistent with future copies into arvif->u.ap.ssid:
>>
>> 	if (changed & BSS_CHANGED_SSID &&
>> 	    vif->type == NL80211_IFTYPE_AP) {
>> 		arvif->u.ap.ssid_len = vif->cfg.ssid_len;
>> 		if (vif->cfg.ssid_len)
>> 			memcpy(arvif->u.ap.ssid, vif->cfg.ssid,
>> 			       vif->cfg.ssid_len);
>> 		arvif->u.ap.hidden_ssid = info->hidden_ssid;
>> 	}
>>
>> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
>> Link: https://github.com/KSPP/linux/issues/90
>> Cc: linux-hardening@...r.kernel.org
>> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
>> ---
>> Changes in v2:
>> - update subject to include wifi
>> - prefer memcpy() over strtomem() (thanks Kalle, Jeff)
>> - rebase onto 6.6-rc7 @d88520ad73b79e71
>> - Link to v1: https://lore.kernel.org/r/20231013-strncpy-drivers-net-wireless-ath-ath10k-mac-c-v1-1-24e40201afa3@google.com
>> ---
>> Note: build-tested only.
>>
>> Found with: $ rg "strncpy\("
>> ---
>>   drivers/net/wireless/ath/ath10k/core.h | 2 +-
>>   drivers/net/wireless/ath/ath10k/mac.c  | 3 +--
>>   2 files changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/wireless/ath/ath10k/core.h b/drivers/net/wireless/ath/ath10k/core.h
>> index 4b5239de4018..ba9795a8378a 100644
>> --- a/drivers/net/wireless/ath/ath10k/core.h
>> +++ b/drivers/net/wireless/ath/ath10k/core.h
>> @@ -607,7 +607,7 @@ struct ath10k_vif {
>>   			u8 tim_bitmap[64];
>>   			u8 tim_len;
>>   			u32 ssid_len;
>> -			u8 ssid[IEEE80211_MAX_SSID_LEN];
>> +			u8 ssid[IEEE80211_MAX_SSID_LEN] __nonstring;
>>   			bool hidden_ssid;
>>   			/* P2P_IE with NoA attribute for P2P_GO case */
>>   			u32 noa_len;
>> diff --git a/drivers/net/wireless/ath/ath10k/mac.c b/drivers/net/wireless/ath/ath10k/mac.c
>> index 03e7bc5b6c0b..f3f6deb354c6 100644
>> --- a/drivers/net/wireless/ath/ath10k/mac.c
>> +++ b/drivers/net/wireless/ath/ath10k/mac.c
>> @@ -6125,9 +6125,8 @@ static void ath10k_bss_info_changed(struct ieee80211_hw *hw,
>>   
>>   		if (ieee80211_vif_is_mesh(vif)) {
>>   			/* mesh doesn't use SSID but firmware needs it */
>> -			strncpy(arvif->u.ap.ssid, "mesh",
>> -				sizeof(arvif->u.ap.ssid));
>>   			arvif->u.ap.ssid_len = 4;
>> +			memcpy(arvif->u.ap.ssid, "mesh", arvif->u.ap.ssid_len);
> 
> This is a behavior change, isn't it? i.e. arvif->u.ap.ssid is no longer
> zero-padded. Is this actually ok for the driver?

yes, it is safe, and consistent with other uses of this field as noted 
in the commit description.