[PATCH] wifi: mt76: fix potential tx_retries underflow

[Ryder Lee]

When FIELD_GET returns 0 for the retry count, subtracting 1 causes
an unsigned integer underflow, resulting in tx_retries becoming a
very large value (0xFFFFFFFF for u32 or 255 for u8).

Fix by checking if count is non-zero before subtracting 1.

Signed-off-by: Ryder Lee <ryder.lee at mediatek.com>
---
 drivers/net/wireless/mediatek/mt76/mt7915/mac.c | 10 +++++-----
 drivers/net/wireless/mediatek/mt76/mt7921/mac.c |  5 +++--
 drivers/net/wireless/mediatek/mt76/mt7925/mac.c |  5 +++--
 drivers/net/wireless/mediatek/mt76/mt7996/mac.c |  6 +++---
 4 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt7915/mac.c b/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
index cec2c4208..334c19ab2 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7915/mac.c
@@ -912,16 +912,16 @@ mt7915_mac_tx_free(struct mt7915_dev *dev, void *data, int len)
 		}
 
 		if (!mtk_wed_device_active(&mdev->mmio.wed) && wcid) {
-			u32 tx_retries = 0, tx_failed = 0;
+			u32 tx_retries = 0, tx_failed = 0, count;
 
 			if (v3 && (info & MT_TX_FREE_MPDU_HEADER_V3)) {
-				tx_retries =
-					FIELD_GET(MT_TX_FREE_COUNT_V3, info) - 1;
+				count = FIELD_GET(MT_TX_FREE_COUNT_V3, info);
+				tx_retries = count ? count - 1 : 0;
 				tx_failed = tx_retries +
 					!!FIELD_GET(MT_TX_FREE_STAT_V3, info);
 			} else if (!v3 && (info & MT_TX_FREE_MPDU_HEADER)) {
-				tx_retries =
-					FIELD_GET(MT_TX_FREE_COUNT, info) - 1;
+				count = FIELD_GET(MT_TX_FREE_COUNT, info);
+				tx_retries = count ? count - 1 : 0;
 				tx_failed = tx_retries +
 					!!FIELD_GET(MT_TX_FREE_STAT, info);
 			}
diff --git a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
index 03b4960db..668bfa195 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7921/mac.c
@@ -530,8 +530,9 @@ static void mt7921_mac_tx_free(struct mt792x_dev *dev, void *data, int len)
 		stat = FIELD_GET(MT_TX_FREE_STATUS, info);
 
 		if (wcid) {
-			wcid->stats.tx_retries +=
-				FIELD_GET(MT_TX_FREE_COUNT, info) - 1;
+			u32 count = FIELD_GET(MT_TX_FREE_COUNT, info);
+
+			wcid->stats.tx_retries += count ? count - 1 : 0;
 			wcid->stats.tx_failed += !!stat;
 		}
 
diff --git a/drivers/net/wireless/mediatek/mt76/mt7925/mac.c b/drivers/net/wireless/mediatek/mt76/mt7925/mac.c
index c47bd812b..c56a9e530 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7925/mac.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7925/mac.c
@@ -1141,8 +1141,9 @@ mt7925_mac_tx_free(struct mt792x_dev *dev, void *data, int len)
 
 		if (info & MT_TXFREE_INFO_HEADER) {
 			if (wcid) {
-				wcid->stats.tx_retries +=
-					FIELD_GET(MT_TXFREE_INFO_COUNT, info) - 1;
+				u32 count = FIELD_GET(MT_TXFREE_INFO_COUNT, info);
+
+				wcid->stats.tx_retries += count ? count - 1 : 0;
 				wcid->stats.tx_failed +=
 					!!FIELD_GET(MT_TXFREE_INFO_STAT, info);
 			}
diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
index a59c14c8f..3fad977ba 100644
--- a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
+++ b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
@@ -1361,13 +1361,13 @@ mt7996_mac_tx_free(struct mt7996_dev *dev, void *data, int len)
 				cur_info++;
 			continue;
 		} else if (info & MT_TXFREE_INFO_HEADER) {
-			u32 tx_retries = 0, tx_failed = 0;
+			u32 tx_retries = 0, tx_failed = 0, count;
 
 			if (!wcid)
 				continue;
 
-			tx_retries =
-				FIELD_GET(MT_TXFREE_INFO_COUNT, info) - 1;
+			count = FIELD_GET(MT_TXFREE_INFO_COUNT, info);
+			tx_retries = count ? count - 1 : 0;
 			tx_failed = tx_retries +
 				!!FIELD_GET(MT_TXFREE_INFO_STAT, info);
 
-- 
2.45.2