Subject: Re: [PATCH 39/64] mac80211: Use memset_after() to clear tx status

From: Kees Cook <[email-masked]>
Date: Sat, 31 Jul 2021 08:55:14 -0700

On Tue, Jul 27, 2021 at 01:58:30PM -0700, Kees Cook wrote:
> In preparation for FORTIFY_SOURCE performing compile-time and run-time
> field bounds checking for memset(), avoid intentionally writing across
> neighboring fields.
> 
> Use memset_after() so memset() doesn't get confused about writing
> beyond the destination member that is intended to be the starting point
> of zeroing through the end of the struct.
> 
> Note that the common helper, ieee80211_tx_info_clear_status(), does NOT
> clear ack_signal, but the open-coded versions do. All three perform
> checks that the ack_signal position hasn't changed, though.

Quick ping on this question: there is a mismatch between the common
helper and the other places that do this. Is there a bug here?

> 
> Signed-off-by: Kees Cook <[email-masked]>
> ---
> Should these each be clearing the same region? Because they're currently not.
> ---
>  drivers/net/wireless/ath/carl9170/tx.c   | 4 +---
>  drivers/net/wireless/intersil/p54/txrx.c | 4 +---
>  include/net/mac80211.h                   | 4 +---
>  3 files changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/wireless/ath/carl9170/tx.c b/drivers/net/wireless/ath/carl9170/tx.c
> index 88444fe6d1c6..6d2115639434 100644
> --- a/drivers/net/wireless/ath/carl9170/tx.c
> +++ b/drivers/net/wireless/ath/carl9170/tx.c
> @@ -278,9 +278,7 @@ static void carl9170_tx_release(struct kref *ref)
>  	BUILD_BUG_ON(
>  	    offsetof(struct ieee80211_tx_info, status.ack_signal) != 20);
>  
> -	memset(&txinfo->status.ack_signal, 0,
> -	       sizeof(struct ieee80211_tx_info) -
> -	       offsetof(struct ieee80211_tx_info, status.ack_signal));
> +	memset_after(&txinfo->status, 0, rates);
>  
>  	if (atomic_read(&ar->tx_total_queued))
>  		ar->tx_schedule = true;
> diff --git a/drivers/net/wireless/intersil/p54/txrx.c b/drivers/net/wireless/intersil/p54/txrx.c
> index 873fea59894f..f71b355f8583 100644
> --- a/drivers/net/wireless/intersil/p54/txrx.c
> +++ b/drivers/net/wireless/intersil/p54/txrx.c
> @@ -431,9 +431,7 @@ static void p54_rx_frame_sent(struct p54_common *priv, struct sk_buff *skb)
>  	 * Clear manually, ieee80211_tx_info_clear_status would
>  	 * clear the counts too and we need them.
>  	 */
> -	memset(&info->status.ack_signal, 0,
> -	       sizeof(struct ieee80211_tx_info) -
> -	       offsetof(struct ieee80211_tx_info, status.ack_signal));
> +	memset_after(&info->status, 0, rates);
>  	BUILD_BUG_ON(offsetof(struct ieee80211_tx_info,
>  			      status.ack_signal) != 20);
>  
> diff --git a/include/net/mac80211.h b/include/net/mac80211.h
> index d8a1d09a2141..7abc1427aa8c 100644
> --- a/include/net/mac80211.h
> +++ b/include/net/mac80211.h
> @@ -1200,9 +1200,7 @@ ieee80211_tx_info_clear_status(struct ieee80211_tx_info *info)
>  
>  	BUILD_BUG_ON(
>  	    offsetof(struct ieee80211_tx_info, status.ack_signal) != 20);
> -	memset(&info->status.ampdu_ack_len, 0,
> -	       sizeof(struct ieee80211_tx_info) -
> -	       offsetof(struct ieee80211_tx_info, status.ampdu_ack_len));
> +	memset_after(&info->status, 0, ack_signal);
>  }
>  
>  
> -- 
> 2.30.2
> 

-- 
Kees Cook