mac80211: Use memset_after() to clear tx status

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.

Additionally fix the common helper, ieee80211_tx_info_clear_status(),
which was not clearing ack_signal, but the open-coded versions
did. Johannes Berg points out this bug was introduced by commit
e3e1a0bc ("mac80211: reduce IEEE80211_TX_MAX_RATES") but was harmless.

Also drops the associated unneeded BUILD_BUG_ON()s, and adds a note to
carl9170 about usage.
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Christian Lamparter <chunkeey@gmail.com> [both CARL9170+P54USB on real HW]
Link: https://lore.kernel.org/r/20211118203839.1289276-1-keescook@chromium.orgSigned-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/ath/carl9170/tx.c

@@ -275,12 +275,12 @@ static void carl9170_tx_release(struct kref *ref)
 	if (WARN_ON_ONCE(!ar))
 		return;
 
-	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));
+	/*
+	 * This does not call ieee80211_tx_info_clear_status() because
+	 * carl9170_tx_fill_rateinfo() has filled the rate information
+	 * before we get to this point.
+	 */
+	memset_after(&txinfo->status, 0, rates);
 
 	if (atomic_read(&ar->tx_total_queued))
 		ar->tx_schedule = true;

drivers/net/wireless/intersil/p54/txrx.c

@@ -431,11 +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));
-	BUILD_BUG_ON(offsetof(struct ieee80211_tx_info,
-			      status.ack_signal) != 20);
+	memset_after(&info->status, 0, rates);
 
 	if (entry_hdr->flags & cpu_to_le16(P54_HDR_FLAG_DATA_ALIGN))
 		pad = entry_data->align[0];

include/net/mac80211.h

@@ -1205,12 +1205,7 @@ ieee80211_tx_info_clear_status(struct ieee80211_tx_info *info)
 	/* clear the rate counts */
 	for (i = 0; i < IEEE80211_TX_MAX_RATES; i++)
 		info->status.rates[i].count = 0;
-
-	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, rates);
 }