linux-hardening - [PATCH 1/2] [RESEND] wifi: carl9170: re-fix fortified-memset warning

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Message-Id: <20240328135509.3755090-2-arnd@kernel.org>
Date: Thu, 28 Mar 2024 14:55:04 +0100
From: Arnd Bergmann <arnd@...nel.org>
To: Kees Cook <keescook@...omium.org>,
	Christian Lamparter <chunkeey@...glemail.com>,
	Kalle Valo <kvalo@...nel.org>,
	Johannes Berg <johannes.berg@...el.com>
Cc: linux-hardening@...r.kernel.org,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	linux-wireless@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	Colin Ian King <colin.i.king@...il.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] [RESEND] wifi: carl9170: re-fix fortified-memset warning

From: Arnd Bergmann <arnd@...db.de>

The carl9170_tx_release() function sometimes triggers a fortified-memset
warning in my randconfig builds:

In file included from include/linux/string.h:254,
                 from drivers/net/wireless/ath/carl9170/tx.c:40:
In function 'fortify_memset_chk',
    inlined from 'carl9170_tx_release' at drivers/net/wireless/ath/carl9170/tx.c:283:2,
    inlined from 'kref_put' at include/linux/kref.h:65:3,
    inlined from 'carl9170_tx_put_skb' at drivers/net/wireless/ath/carl9170/tx.c:342:9:
include/linux/fortify-string.h:493:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  493 |                         __write_overflow_field(p_size_field, size);

Kees previously tried to avoid this by using memset_after(), but it seems
this does not fully address the problem. I noticed that the memset_after()
here is done on a different part of the union (status) than the original
cast was from (rate_driver_data), which may confuse the compiler.

Unfortunately, the memset_after() trick does not work on driver_rates[]
because that is part of an anonymous struct, and I could not get
struct_group() to do this either. Using two separate memset() calls
on the two members does address the warning though.

Fixes: fb5f6a0e8063b ("mac80211: Use memset_after() to clear tx status")
Link: https://lore.kernel.org/lkml/20230623152443.2296825-1-arnd@kernel.org/
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
I found this while testing randconfig builds, a .config that shows this
for me is at https://pastebin.com/yWFKvZYu

Sorry I failed to follow up to Kees' request for a reproducer when
I posted this last year.
---
 drivers/net/wireless/ath/carl9170/tx.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/ath/carl9170/tx.c b/drivers/net/wireless/ath/carl9170/tx.c
index e902ca80eba7..0226c31a6cae 100644
--- a/drivers/net/wireless/ath/carl9170/tx.c
+++ b/drivers/net/wireless/ath/carl9170/tx.c
@@ -280,7 +280,8 @@ static void carl9170_tx_release(struct kref *ref)
 	 * carl9170_tx_fill_rateinfo() has filled the rate information
 	 * before we get to this point.
 	 */
-	memset_after(&txinfo->status, 0, rates);
+	memset(&txinfo->pad, 0, sizeof(txinfo->pad));
+	memset(&txinfo->rate_driver_data, 0, sizeof(txinfo->rate_driver_data));
 
 	if (atomic_read(&ar->tx_total_queued))
 		ar->tx_schedule = true;
-- 
2.39.2