linux-kernel - [PATCH AUTOSEL 6.0 02/44] wifi: cfg80211: Fix bitrates overflow issue

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Message-Id: <20221119021124.1773699-2-sashal@kernel.org>
Date:   Fri, 18 Nov 2022 21:10:42 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Paul Zhang <quic_paulz@...cinc.com>,
        Johannes Berg <johannes.berg@...el.com>,
        Sasha Levin <sashal@...nel.org>, johannes@...solutions.net,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org
Subject: [PATCH AUTOSEL 6.0 02/44] wifi: cfg80211: Fix bitrates overflow issue

From: Paul Zhang <quic_paulz@...cinc.com>

[ Upstream commit 18429c51c7ff6e6bfd627316c54670230967a7e5 ]

When invoking function cfg80211_calculate_bitrate_eht about
(320 MHz, EHT-MCS 13, EHT-NSS 2, EHT-GI 0), which means the
parameters as flags: 0x80, bw: 7, mcs: 13, eht_gi: 0, nss: 2,
this formula (result * rate->nss) will overflow and causes
the returned bitrate to be 3959 when it should be 57646.

Here is the explanation:
 u64 tmp;
 u32 result;
 …
 /* tmp = result = 4 * rates_996[0]
  *     = 4 * 480388888 = 0x72889c60
  */
 tmp = result;

 /* tmp = 0x72889c60 * 6144 = 0xabccea90000 */
 tmp *= SCALE;

 /* tmp = 0xabccea90000 / mcs_divisors[13]
  *     = 0xabccea90000 / 5120 = 0x8970bba6
  */
 do_div(tmp, mcs_divisors[rate->mcs]);

 /* result = 0x8970bba6 */
 result = tmp;

 /* normally (result * rate->nss) = 0x8970bba6 * 2 = 0x112e1774c,
  * but since result is u32, (result * rate->nss) = 0x12e1774c,
  * overflow happens and it loses the highest bit.
  * Then result =  0x12e1774c / 8 = 39595753,
  */
 result = (result * rate->nss) / 8;

Signed-off-by: Paul Zhang <quic_paulz@...cinc.com>
Signed-off-by: Johannes Berg <johannes.berg@...el.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 net/wireless/util.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/wireless/util.c b/net/wireless/util.c
index 775836f6785a..450d609b512a 100644
--- a/net/wireless/util.c
+++ b/net/wireless/util.c
@@ -1555,10 +1555,12 @@ static u32 cfg80211_calculate_bitrate_eht(struct rate_info *rate)
 	tmp = result;
 	tmp *= SCALE;
 	do_div(tmp, mcs_divisors[rate->mcs]);
-	result = tmp;
 
 	/* and take NSS */
-	result = (result * rate->nss) / 8;
+	tmp *= rate->nss;
+	do_div(tmp, 8);
+
+	result = tmp;
 
 	return result / 10000;
 }
-- 
2.35.1